Why do we suck air out of our moist rooms? Why not blow dry?
We do not suck dry our hair or suck out candles!
air seal the door and provide an intentional flap to the outside??
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Because usually when an exhaust fan is used there's something that we specifically want to exhaust. (Why do you think some folks call it a "fart fan".)
I agree we need to "change" the air and also make some noise!
Seriously All, Shelternerd has thought about this and I think he has a wise solution
http://www.chandlerdesignbuild.com/files/ventingStandardsComplete.pdf
I am not sure I agree with the intake location in the laundry room. Our building code considers the laundry room a source of moisture. Does anyone know how much moisture is generated by a modern well vented dryer and an EnergyStar Washer?
A well-vented dryer should be a net suck. A washer is roughly equivalent to a tub bath (or two or three), I'd think. Neither should be a major source of moisture.Of course, there are laundry rooms and there are laundry rooms. Some have just the washer/dryer, in others folks string clotheslines and dry clothes in the winter, make major use of a laundry tub, spill water on the floor while washing the dog, etc.I frankly would prefer the furnace air return scheme -- cheap, simple, nothing much additional to maintain.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
When I designed and built our house I put the laundry in the pantry. I added an exhaust fan to get rid of excess heat, not humidity. Helps when the cats have revolted and not covered up in the cat box too.
Back to Homedesign's post, blowing in would only distribute the moist air unless there was some exit path for it. In winter would not be a bad thing but I can sure tell the difference in the summer if I do not let shower fan run for a few minutes.
I wired so fan would be on when light was on, if I had to do again would wire differently, probably a threeway arangement or something.
We just use a timer switch -- have trained ourselves to turn it on when we start the shower, and set it for an extra 15 minutes or so past when we'll get out.You can get (I'm told -- have never seen them) humidistat switches for the fan, to turn it on when humidity exceeds a certain level.Would make sense to have a 2-speed fan, I suppose. Come on at low speed when the light is on, higher speed when the humidistat calls.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
"A well-vented dryer should be a net suck. "Net to the dryer, but not to the house." A washer is roughly equivalent to a tub bath (or two or three), I'd think. Neither should be a major source of moisture."the comment was not about removing moisture from the laundry room. It has nothing to do with the washer.The comment was about WHERE to induce makeup air. And his comment was that the dryer has a relatively high exhaust rate and is used longer than a stove exhaust so that is the area that he brings in his makeup air..
.
A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Almost all the fart fans are cheap disposable things. It's seems like there could be a market for ones that were a step up. But not the ones that have a remote inline motor. To expensive in my mind for the application.
I think the most important application is moisture removal and that "fart" mitigation comes as a bonus. Extra money spent on proper moisture removal would be well spent.
I've heard about moisture problems associated with bathrooms but have never personally experienced it. I've experience to little moisture in the house though.
Maybe it's a lifestyle thing. The air in our house is dry.
Popawheelie, I am from a hot/mixed humid climate (North Texas) and internally generated moisture is a problem here. The moisture generated in a bathroom,kitchen or laundry can also create problems thru-out the house.
See the previous post.
The design conditions in North Texas (100/76) are similar to those I refer to in my design (95/75). In your case, exhausting the moist air from a bathroom during a shower makes even less sense. The main caveat being that an appropriately located supply and an operational forced air cooling system are required.
Exhausting, and introducing that hot humid outside air to make up the volume exhausted, will introduce more moisture inside you house than any water shower will.
Tim, I finally had a little time to consider your post. I like it. Do you have a way to start and time your central fan when you take a shower? Do you have a return in or near your bathroom? Conventional practice is not to have a return in the bath the theory being not to return an unpleasant smell. Do you have a conventional exhaust fan for those other moments? If so..when the exhaust comes on...how is the air replaced? Can you keep your house slightly positive night and day? Have you looked at ShelterNerd's set-up? What do you think about his plan?
John
"Do you have a way to start and time your central fan when you take a shower?" - I have a programmable thermostat, programmed to assure the unit is blowing and cooling at two critical times during the day (for me) when I go to bed at night, and when I'm getting ready for work in the morning; unless cooling is not required and then windows are open as much as possible.
The whole idea of sealing and insulating your house, cooling and dehumidifying the air inside and then dumping it outside beacuse of poor planning and a little moisture, is not ideal, to say the least.
"Do you have a return in or near your bathroom?" - No, the supply into the (master) bathroom has nowhere to go but through the walk-in (walk-through?) closet and my bedroom, all the while mixing with dryer air, before getting to a return register.
"Do you have a conventional exhaust fan for those other moments?" In some bathrooms, not all.
"...how is the air replaced? Can you keep your house slightly positive night and day?" I have two forced air furnace/air conditioners, one of which is a variable speed unit and operates with constant low fan. Both units have a Skuttle connected to the return that provides fresh air to the house and make-up air for all the air "consumers" in the house (exhaust fans, water heater, clothes dryer). The house runs more or less close to neutral except during high demand on the furnace/ac systems. The Skuttles react to differential pressure (they are a barametric damper, specifically designed and intended for providing ventilation to a small air handling systems), so during times of increased high speed fan operation, they provide more air to the house and positively pressurize the envelope. During periods of limited or no demand for either heating or cooling (in my area this is 4 solid months of the year), the windows are usually open. To answer the question: yes I can keep my house positive day and night. A Skuttle, properly installed, includes a volume damper in the ductwork to allow for balancing of the system.
"Have you looked at ShelterNerd's set-up?" - No, I haven't.
Tim, thank you for all the great detail, You guys have given me a lot to think about.
If you're running the AC it's generally more efficient to run the exhaust fan during a shower than not. Saves the AC having to expend energy to remove the moisture.And, of course, in a kitchen one may want to remove cooking odors with a vent hood, or exhaust heat during AC season.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
"If you're running the AC it's generally more efficient to run the exhaust fan during a shower than not. "
I disagree.
Based on an energy analysis of a simple system (see previous posts), it costs 3 to 5 times more in energy added to the house to exhaust/makeup, than to absorb, at summer design conditions. More in the hotter, more humid regions. In mild conditions, maybe so, but then, thats what windows are for, and the AC wouldn't be running in milder conditions>
I do agree that exhausting cooking heat, vapor, and/or odors is prefered, as are direct removal of "personal" odors.
The air in our house is dry.
And, I'm from even more humid than Homedesign--down here in SE central Texas, it rarely gets below 40% humidity (not uncommon to have RH go from 95+% in the morning, then drop to a mere 58% or so).
Be glad you live in a dry climate.
The seasons "spring" and "fall" are pretty much defined locally as those brief times of the year when we go through the dewpoint temperature twice a day instead of just the once.Occupational hazard of my occupation not being around (sorry Bubba)
Fantec in line vent fan with light ( GU 10 50 Watt halogen) in my area for the kit is $190. The "decent" ones you can pick from the home centers are incandescent (which may be outlawed [COUGH!! Orwellian COUGH!!] soon enough anyway,sound like an AC compressor in relation to the in line models @ $100.
These models are much smaller in profile and provide better light.
... Anyone up for a book burning tonight?
- the Hearth and the Salamander
To expensive in my mind for the application.
Small Fantec inline is only $130 plus $20 or so in flex duct.
Beer was created so carpenters wouldn't rule the world.
I'll look into it if it comes up. I thought they were more $.
We use them all the time and have a couple sitting around still in the box. With shipping the 100 cfm model shouldn't be more than $150 for the grill and motor.
Beer was created so carpenters wouldn't rule the world.
sucking does have it's place...
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming
WOW!!! What a Ride!
Forget the primal scream, just ROAR!!!
IMERC,
I am not saying that blowing is better. I am just raising the question.
they both have their place...Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming<!----><!----><!---->
WOW!!! What a Ride!Forget the primal scream, just ROAR!!!
To quote Splintie:
"naughty, naughty..."
Only dead fish swim with the stream. Author Unknown
IIRC, at least some HRV (heat recovery ventilation systems) use fans to both suck air out of the house and blow air into the house. I am guessing that this system may help keep the system from developing either a positive or negative pressure in the house and thus minimize the chances for pushing or drawing air through any unsealed areas of the house. Of course I could certainly be wrong on my suppositions - maybe it is just a way of increasing the complexity of the system and thus the chances to get a high dollar service call for the vendor...
One of the reasons for the two fans is that there are filters on both air streams, and also the heat exchanger core (such as it is), which means a pressure drop.
I planned my air system to have different sized fans to pressurize the house. As I condition the air on the way in, best to have leaks all go out. No vendor as I DIYed and keep a pair of spare fans on hand, $20 worth.
Very simple, with tiny fans. Our 20k cu ft requires only 167 cfm to provide a total air change every 2 hrs. Replaced a malfunctioning dehumidifier (I buy used ones) yesterday that dries the air on the way in. The heat exchanger then takes the large majority of added heat back outside. BTW, we run an exhaust fan when showering, keeps the rh lower as makeup air isn't unconditioned outside air (very humid here).
Non-stop filtered, dried, fresh air supplied everywhere, including closets.
Commercial units that I've seen all had same sized fans PAHS Designer/Builder- Bury it!
call it what you want, sucking or blowing.
I'll take it either way, anytime
DITTO!!!!Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming<!----><!----><!---->
WOW!!! What a Ride!Forget the primal scream, just ROAR!!!
here the humidity and temps are fairly low...
internal surges of moist air are not desired....
a bath exhaust is all that is required as an open window or two fill the requirements for AC....
also if the structure is closed up too tight and the air artifically treated that's not so good either...
there are efficient quiet fans available... be cost effective...
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming<!----><!----><!---->
WOW!!! What a Ride!Forget the primal scream, just ROAR!!!
Tell me more about AC and open windows, never had AC before.
it's all in the thermostat...
too cold... close the window...
too hot ... open the window....
way too hot... open a second window... opposing is preferrable...Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming<!----><!----><!---->
WOW!!! What a Ride!Forget the primal scream, just ROAR!!!
Sucking is more efficent than blowing--blowing air is like herding cats--wants to curve back on itself and not go where you are trying to blow it. Sucking draws it in relatively straight line to the fan. Imagine trying to pump water by blowing it to where you want it to go and you have a similar idea.
Edited 8/10/2007 7:47 am ET by Danno
Why do we suck air out of our moist rooms? Why not blow dry?
I assume you're being a bit facetious.
But just in case you're not, blowing and sucking are really the same thing. You can't have one without the other. It's just a question of where the fan is located.
DCD,
"It's just a question of where the fan is located"
Exactly.. why not blow "not so humid" air toward the wet shower wall and or glass.
When a home has been flooded..out come the fans...not the suckers
You have to step back and think about what you want to accomplish.
Exhaust fans are there to EXHAUST air to the outside; therefore, the fan is located at the downstream part of the airflow, and pulls the air from the house. It's not important to the system where the replacement air comes from.
On the other hand, if you want FRESH AIR into the house, you put in a fan that will pressurize the house with fresh air; in this case, you don't care how the air gets out of the house, you're simply wanting to blow fresh air into the house.
It's all in what you want the system to do.
<It's not important to the system where the replacement air comes from>
<you don't care how the air gets out of the house>
HammerHarry,I disagree and I have exactly the opposite opinion.
We need to be in control of how and where air enters and leaves each space as well as the house itself. We do not want to suck air in from a musty or contaminated unconditioned crawl space/attic or garage. We do not want to exhaust air near the fresh air intake. We do not want to pump moist air into unintentional holes and wet the structure.
I also think VaTom has a good point about circulating air in every part of the house.
If you pressurize the house with fresh air it will leave through all the exhaust dampers such as bath fan and hoods and general leaks. The concern is that in the winter it will drive humidity into your walls and ceilings where it will condense and create a mold problem. Depending on your climate this may not be a big deal. Tyeing it into the return air of your HVAC system means it runs more when the weather is extreme. Do you need to ventilate more on the hottest and coldest days of the year? Probably not. Better to ventilate based on the number of human beings in the house (and dogs and house plants) This is why timers on bath fans (high quality Panasonic type energy star rated fans) makes so much sense. Letting air in via the laundry just keeps you from over ventilating on wash day, a dryer can pull 300 CFM from your house, why heat and cool the air you are using to dry your clothes? M------------------
"You cannot work hard enough to make up for a sloppy estimate."
ShelterNerd, I guess I did not really undertsand this before. Are you saying that a dryer is like an exhaust fan and is exhausting more moisture than it contributes?
I think what has thrown me off is fact that we are required by code to provide exhaust fans in our laundry rooms.
Yes a dryer pulls approx 300 CFM from the room, heats it blows it through a drum of spinning clothes and then exhausts it through the dryer vent. Some moisture may make its way into the room but in general you get more humidity from the washer (esp if you use hot water) than with the dryer. BUT the dryer needs to get that 300 CFM from somewhere, cracks under doors and windows, the chimney flue, ceiling cans and the gap around the plumbing under the bath tub are all likely candidates for make-up air. That's why I put an air intake next to the dryer- to keep it from pulling in the air it needs from other locations. If you are running the range hood the air can make it from the laundry to the range hood in most cases if you plan for it. I hope this helps.M------------------
"You cannot work hard enough to make up for a sloppy estimate."
Thanks, Shelternerd. I just did not "get it" before. Do you install the code exhaust fan in the laundry? We have one in our laundry, but we never use use it. I wonder how many people ever use theirs. Do you undercut your bathroom doors?
John
We do undercut doors about an inch and also use jumper ducts between the MBR and the living area if there is no return in the master bedroom suite. We are not required to install exhaust fans in our laundry rooms here in NC so we don't install them, just the supply damper.All the bestM------------------
"You cannot work hard enough to make up for a sloppy estimate."
SehterNerd,
Doh!, I assumed that it was IRC code. Maybe our local code has it wrong and should require a supply damper like your North Carolina code. We(North Texas) are sorta similar climate. I did notice IRC requires bathroom ventilation to be exhausted "directly" outside (unless vented by window). If there were only some way to combine part of what Tim is talking about (saving BTU's)with your strategy.The beauty of your plan is that the homeowner does not have to think about it.
John
Providing supply air is not yet required by code but is a requirement of Energy Star at 7.5 CFM and the new NAHB-ICC National Green Building Standard is going to give points for (but not require) 15 CFM per occupant. As I see it Tim's strategy is to circulate conditioned air from his house into the bath to drive the humid air into the return air vent where the humidity is removed by his AC coil. This may be a good efficiency strategy but it is not providing make-up air. Living in a sealed box creates indoor air quality problems esp if there are sources of pollution inside the box such as humid carpet, pets, teenage boys, etc. I personally think the 7.5 CFM per person ought to be plenty and that the natural air change rate of the completed home should be taken into consideration in the calculation. (Which assumes you are blower door testing your houses to determine the ACH Nat.) Basically I'm looking for the best middle path between too much ventilation (lost BTUs) and not enough (bad air quality.)M------------------
"You cannot work hard enough to make up for a sloppy estimate."
ShelterNerd, What do you think Lstiburek is trying to tell us (southerners) with the Number 2 Dumbest thing to do in the South?
http://www.buildingscience.com/bsc/topten/south.htm
I know Joe can't be right about everything. Just curious what you think.
Good point. The ASHRAE standard he's referring to is actually a little more flexible than 15 cfm/occupant but his point about over-ventilating to meet an arbitrary and bogus minimum standard is very well taken. I do still think that many of the extremely energy efficient houses we are building need some additional ventilation. I walked into one of our houses this winter to respond to a complaint about condensation on the (low-E, Argon-filled) windows and my glasses fogged up. They had two dogs, a fair number of house plants, and a home office and the place was just plain stuffy. I had them put timers on their bath fans and haven't heard from them since other than friendly waves when I see them in town. I like being able to design to hit a number and the 7.5 cfm number "feels" reasonable to me. But Joe's point about indoor air pollutants is right on. A house with no carpet, green building systems, managed humidity, detached garage, and right-sized HVAC is likely to have less of a need for ventilation than the standard tract house in America and finding a way to calculate that is beyond the scope of any building code. Homeowner habits have a huge effect and some contaminates (Benzene from attached garages for one) move so slowly that you need a six month sampling frequency to see the movement. I think the only safe thing to say is that the jury is still out and that we need to come up with some plan of action in the interim. My plan is as laid out on my site but I can't say it won't change as more data comes in over the next year or two or if a lower cost Energy Recovery Ventilator were to hit the market. At this point, even if I add no mark-up to an ERV it will cost my customer $1,500 to $2,000 to have installed. Many of the projects I'm involved in don't have enough money to cover that up-charge. Up grading the bath fans and adding an air intake in the laundry is cheap and stands a pretty good chance of getting enough ventilation done to avoid most problems. I hope this is helpfull------------------
"You cannot work hard enough to make up for a sloppy estimate."
Thanks for the thoughtful response
"a dryer pulls approx 300 CFM from the room"
Not a residential dryer. Most, if not all res dryers are vented through a 4" vent, that at the static pressure available at the discharge of the unit, might pass 100 cfm when its new and clean, if the run is short and straight enough.
"Do you need to ventilate more on the hottest and coldest days of the year? Probably not."
I disagree. Exactly the opposite is true. When the weather is at an extreme, that is when you're going to have things buttoned-up the most. That is when no widows are open, and everyone is in the house instead of outside enjoying the fine weather. And this IS precisley when ventilations is needed the most.
"If you pressurize the house with fresh air it will leave through all the exhaust dampers such as bath fan and hoods and general leaks." Most residential ventilation equipment is incapable of providing anything other than the very slightest amount of relative pressurization.
"The concern is that in the winter it will drive humidity into your walls and ceilings where it will condense and create a mold problem." Not a legitimate concern. You're going to grow mold in bathroom fans and kitchen exhaust hoods? Not too likelky. The leaks in construction are there and will be problems, possibly, eventually. Conditioned air leaking out is much better than unconditioned air leaking in.
Wind driven differential pressure causes most of the leakage into and out of buildings. A slight positive pressure is the best alternative to zero leaks and perfect balance (neither of which are reasonably possible). Fresh air introduced into the return of a forced air system, whether it be pre-treated by an erv/eru or not, that gets tempered and mixed before being "supplied" to the space, is the best way to accomplish that.
TimYou make some really good points here but I'll still stick to linking ventilation to occupancy as measured by bathroom usage per my discussion on my website. Last week the temp was 100 degrees with near 90 percent humidity so my family kicked the HVAC up to 80 and took off for the mountains. I'm glad weren't bringing that air into the house while we were on vacation. Even when we're home I don't want any more of that air than absolutely necessary in the house. Likewise in the middle of the winter when I'm in the office and the kids are in school during the day I see no need for extra ventilation. But I sure need it when I have two kids home sick from school on a freezing day. I agree that positive ventilation can only provide the barest minimum of pressurization precisely because of all the exhaust dampers that allow the pressure to escape. And I agree that there is little worry about growing mildew in exhaust ducts. It's the window cracks and so on that I am concerned about but I agree that the pressure of a supply fan is a negligible issue on that score. But leaks are a problem precisely for the reason you state. Pressure differentials set up by wind and forced air HVAC systems are the driving forces for infiltration. The slight positive pressurization of a supply fan is not sufficient to resist the pressure of a ten mile an hour breeze or the pressure of a closed door on a master suite with 200 cfm trying to make it under a one inch undercut back to the return air vent. The best solution to these kinds of pressure are really good envelope and duct sealing and good flashings (and jumper ducts where appropriate). This extreme air tightening is what is setting up the stuffy indoor air problem. From my perspective linking the intake air to occupancy levels is the best solution but there in undeniably a good argument for just linking it to the operation of the air handler. Both solutions are better than just buttoning the house up and hoping for the best. I've been building solar homes for thirty years and I've seen way too much mildew on window frames and skylights due to excessive airtightness. The key is to ventilate. Your strategy is a good one esp in the north. We have more of a moisture and bad outdoor air issue down here in the south so I think my strategy makes more sense down here. I may be proven wrong tomorrow, it's thought provoking discussions like this that shake our preconceptions loose. Thanks for moving the dialog forward.------------------
"You cannot work hard enough to make up for a sloppy estimate."
I am not a builder. I am a mechanical engineer that deals primarily with commercial HVAC systems and controls, my own house being the primary exception. I design many systems with demand controlled ventilation and highly recommend such for complex systems of any size. I do not consider residential systems to be complex, and any amount of ventilation added is a positive, so long as we avoid extremes.
Mold and/or mildew grows in areas of poor air circulation, seldom where moist air is present near surfaces that are below the dewpoint of the air, and a suitable food source exists (we know that fungi, including molds and mildews, required but three basics - food, water, moderate temperatures). Theses conditions are not due to the quality of the air that would be alleviated by ventilating. But let's define the term specifcally: ventilation is fresh, outside air, introduced into an occupied space for the purpose of diluting contaminants. Air quality would have to include other factors such as velocity and moisture content.
While I'll agree that ventilating an unoccupied space is wasteful and unnecessary, I disagree that ventilation will prevent mold or mildew. I believe that if the window frames and skylights fall below the dewpoint of the inside air on a such a regular basis as to cause mildew, humidity control (and/or poor installation/product quality) is the problem. Ventilation may help or hurt that problem, depending on the quality of the outside air (OA).
Note that ventillation as a means of humidity control was unheard of (outside of special situations) until houses in the snow belt started getting "tight" in the 70s. Prior to that houses got enough "natural ventillation" (leakage) to prevent humidity buildup due to human activities when closed up. (And in the summer you opened the windows or ran six window AC units full-tilt, effectively handling the moisture problem.)So ventillation has never been intended to reduce humidity below outside levels, but merely to prevent buildup to unhealthy levels, and then usually only in colder weather. Similarly, in a closed building other air contaminants can build up -- mold spores & formaldehyde, eg -- along with odors.Thus, ventillation specs were originally produced with this "northern home" scenario in mind. As a result, they may not be appropriate for a "southern home" with a more porous envelope -- the target air change rate is likely to already be occurring naturally, at least on windy days.What this means is that when we talk about ventillation we need to be specific about climate, building practices, and the habits of the humans occupying the buildings. Otherwise we're not all on the same page.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
"they may not be appropriate for a "southern home" with a more porous envelope"
Dan, we need tight homes in the south also and we are starting to figure out how tho do it. We are standing on the shoulders of some pretty wise people from Minnesota and Canada.
For the purpose of this discussion I would selfishly like to talk about humid climates and tight homes(less than 1/3 ACH)
Edit to add...I don't think anyone has mentioned the word dehumidifier yet. One of our local experts (Austin)thinks that the best solution for us (north texas)is a stand alone dehumidifier.
Edited 8/15/2007 5:51 pm by homedesign
Yeah, if you get a home sufficiently well insulated and energy-efficient, you can get to a point where the moisture buildup occurs faster than an AC can effectively remove it.Problem is, most conventional dehumidifiers probably have an effective SEER of around 5. It might be more efficient to operate the AC and furnace in tandem vs run a conventional dehumidifier. What is needed longer term is some extra valving and piping to be able to switch from using the condenser coil to using a coil that follows the evaporator in the air handler.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
<< Yeah, if you get a home sufficiently well insulated and energy-efficient, you can get to a point where the moisture buildup occurs faster than an AC can effectively remove it.Problem is, most conventional dehumidifiers probably have an effective SEER of around 5. It might be more efficient to operate the AC and furnace in tandem vs run a conventional dehumidifier. What is needed longer term is some extra valving and piping to be able to switch from using the condenser coil to using a coil that follows the evaporator in the air handler.>>We're experimenting with Zoned Bypass Heat Pumps. When zone A stops calling for cool a motorized damper shuts it down and another damper opens to a bypass that sends the air that would have gone to zone A back through the cooling coil. It keeps the velocity at the coil and at the zone B registers constant but double drys the air by doubling half of it back over the coil. We've run them on a variable speed heat pump but I've been hearing that a 14 SEER variable speed unit only achieves 14 SEER at high speed and drops in efficiency as the speed drops off and since they rarely run at full speed that actual SEER is less than rated. In view of this I'll be switching to fixed speed 14 SEER units on the zoned bypass.------------------
"You cannot work hard enough to make up for a sloppy estimate."
We're experimenting with Zoned Bypass
Shelternerd, I have been trying to learn more about hvac and came across a warning about zoned bypass in a video. It is in segment 3 by Armin Rudd from the online training link that I sent you. I will provide the link if anyone else is interested.
Go ahead and re-send me the link. I was up at the NAHB national in July working on the National Green Building Standard with a bunch of building scientists and building inspectors and scepticism amoung them about the real world performance of variable speed HVAC systems is what drove me back towards using the zoned bypass with a fixed speed 15 SEER system. Most HVAC guys I know are very impressed with the variable speed units. One of the best scoring houses we ever did by energy star standards had a variable speed zoned bypass and that system was designed by the engineer at my energy star certifier's office. But doing both variable speed and zoned bypass seemed a little like belt and suspenders design to me.
I spent some time talking with my HVAC guy (a fellow nerd) today about the AirCycler and AirCycler thermostat combo and he suggested that the thermostat he has been using has a circuit that works with an exterior mounted thermometer and controls an air intake damper such that the damper allows air into the house when the fan runs so long as the outdoor temp is over 40 degrees or under 80 degrees, owner adjustable. He said it could also set the electric back-up strip to only come on if the outdoor temp was under 30 degrees. I asked him about the deal with cycling the blower for ten to twenty minutes every hour unless it had already cycled due to normal operation and he seemed more sceptical but offered to check into it.
I've printed out the files you sent me and plan to show them to him next week. The house we're building right now is very tight (spray foam and sealed, tented, crawl) and only sizes out at 2.5 tons total by manual J so we decided to stick with the 15 SEER Variable speed and the regular ventilation system I usually use. But we're still tinkering with the final design.
Thanks for all the thought provoking discussion.
M------------------
"You cannot work hard enough to make up for a sloppy estimate."
Shelternerd,The video that mentions the zoned bypass is segment 3 by Armin Rudd. The site is hosted by The State of Oklahoma Energy Officehttp://www.bestofbuildingscience.com/
(if you do not see "videos" as a choice..make your window larger)
John Proctor (HVAC guru) videos are good too.I think I just got the rough draft of the HBA paper you are talking about (all 90 pages!)
"Keep the rain out. Keep the outside air out. Keep the inside air in. Let the moisture out in both directions" per Lstiburek
Edited 8/24/2007 7:48 am by homedesign
I'm still liking the zoned bypass after watching the video but I really like the idea of putting a freeze stat on the cooling coil to prevent ice formation. This is an interesting rabbit hole.Most of the research these guys are doing are with production homes at the energy star level. The problems we are seeing are directly related to the extreme insulation levels and air tightness of the high performance homes we are building. When you need so little heat and cooling you run into humidity problems and comfort problems due to the infrequency of system operation. (Or variable speed systems that run on lowest speed all the time) Thus the fan cycler and 7.5 CFM fresh air intake standard. I'm having a lot of fun getting my head around all the trade offs. Thanks for sending me all this good (if overwhelming) information. M------------------
"We don't build them like they used to."
" It might be more efficient to operate the AC and furnace in tandem"
Dan, running the ac and the furnace at the same time?
that sounds ridiculous! (at first) but isn't that what we do with our car defroster?
and speaking of car defrosters....do they suck or blow?
Actually, some commercial systems do operate in tandem like that, specifically to control humidity.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
I'm enjoying this thread.
You are right that ventilation only serves to control mold if it is used as part of a humidity control solution as in adding cold dry air to a home in the winter to reduce interior humidity.
The condensation on windows and skylights is only happening during cold winter conditions when adding some dry outdoor air would be a way to reduce interior humidity.
Ventilation increases interior humidity in the summer and so increases the chance of mold in a hot humid environment when the AC is running and increases the cost of that AC due to the cost of removing latent humidity. This is exactly why I am opposed to automatically ventilating a house when the AC is running.
Ventilation to control humidity is only effective in the winter months. The use of ventilation to dilute contaminates is not as effective as just designing homes to control humidity and minimize VOCs, benzene, and formaldehyde.
I'm beginning to question the 7.5 CFM/occupant ventilation rate required by Energy Star but it seems like a fair middle road. The 15 CFM rate referenced by the ANSI/ASHRAE 62.2 and adopted by the NAHB-ICC National Green Building Standard seems excessive and un-necessary to me and apparently also to Joe Lstiburek.
I'm just trying to get a better handle on the ventilation issue as we tighten our houses up and I guess I'll continue to shoot for 7.5 CFM until something better comes along. I think I'll pass on green points available for venting to 15 cfm/occupant until I get a better explanation. And the practice of just venting whenever the HVAC is running still makes no sense to me.
------------------
"You cannot work hard enough to make up for a sloppy estimate."
"Ventilation increases interior humidity in the summer and so increases the chance of mold in a hot humid environment when the AC is running and increases the cost of that AC due to the cost of removing latent humidity. This is exactly why I am opposed to automatically ventilating a house when the AC is running."It seems unlikely to me that ventillation would significantly increase the chance of mold. Yes, you're introducing (relatively) moist air into the house, but, if properly introduced through the return air ductwork, it's pretty thoroughly mixed with interior air before it reaches any susceptable surfaces. If humidity is under reasonable control to begin with (via AC) then ventillation should not seriously upset that control.Interior mold in an AC environment is almost inevitably linked to out-and-out moisture leakage, or to outside air invading the envelope and condensing on the outside of cooling ductwork.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
<<Interior mold in an AC environment is almost inevitably linked to out-and-out moisture leakage, or to outside air invading the envelope and condensing on the outside of cooling ductwork.>>We're seeing mold on sheetrock inside walls and on shoes in closets but only in other peoples houses with over-sized AC systems, not mine. My main focus is not preventing mold but trying to tweak every ounce of energy efficiency I can out of my homes. We were at 72% better than code under the old energy star but they changed the reference code and the rules on us last July and now we're only 32% better than code with the same construction systems. I've been holding off on ERV's but may have to bite the bullet. I will not go to adding outside air every time the AC kicks on though. Mixing in wet air and stirring it around the house only to burn electricity to dry it out when it makes it back to the ac coil just doesn't make sense to me.------------------
"You cannot work hard enough to make up for a sloppy estimate."
Interesting discussion guys! Allergies are a big concern of mine
so I've used filtered, positive air pressure to try to mitigate the
outside pollen etc. Dan H, your point about humidistats is bang
on for about $50 you can get simple bathroom moisture removal
without having to train your kids or your granny.
"I will not go to adding outside air every time the AC kicks on though"ShelterNerd, it does not have to be every time... remember this system we talked about once?
Central-fan-integrated supply ventilation - this system is simple, effective, andeconomical. It provides fresh, filtered, outside air in a controlled amount using the existingHVAC delivery system for even distribution and mixing. Set-up intermittent central-fanintegratedsupply, designed to ASHRAE 62.2P rate, with fan cycling control set tooperate the central air handler as much as 33% of the time, but not less than 25% of thetime, occurring within at least every three hours to provide ventilation air distribution andwhole-house averaging of air quality and comfort conditions ($125 to $150). Include anormally closed motorized damper in the outside air duct with the AirCycler™ FRVcontrol (+$50 to $60). See http://www.buildingscience.com/resources/mechanical/air_distribution.pdf for more detailed information.
edit to say :sorry the link does not seem to work try this one
http://www.buildingscienceconsulting.com/resources/mechanical/fancycling/air_distribution.pdf
Edited 8/15/2007 8:23 pm by homedesign
Edited 8/15/2007 8:28 pm by homedesign
Okay, I've printed that thing out and I'm going to go pour myself a shot of Dewars and study it. Thanks for sticking with the conversation. Back soonMichael------------------
"You cannot work hard enough to make up for a sloppy estimate."
<<"I will not go to adding outside air every time the AC kicks on though"
ShelterNerd, it does not have to be every time... remember this system we talked about once? http://www.buildingscienceconsulting.com/resources/mechanical/fancycling/air_distribution.pdf>>I spent some time digesting that article. Can't say I understand it all but I'm intrigued enough that I'll give it a shot on the next house and see how it goes. Thanks for the info, and for your persistence.M------------------
"You cannot work hard enough to make up for a sloppy estimate."
ShelterNerd,
I don't perfectly understand it either...that's why I ask so many questions.
I have no experience with mechanical systems. I just think that if Lstiburek(Steebrook) thinks its a good idea...then I should try to understand.
I would like to know what Tim, Dan and Tom think and learn more about the barometric dampers that Tim talks about.
Don't forget the number one dumbest thing to do in the South!
<<Don't forget the number one dumbest thing to do in the South!>>That would be me moving down from Maine to marry a sweet southern girl and teach these yahoos how to build homes? ------------------
"You cannot work hard enough to make up for a sloppy estimate."
For the most part, all of these issues have been rigorously explored over the past two decades in commercial applications. The term "sick buiding syndrome" was coined in the eighties when, after the energy crises of the late seventies, building practices began to include increased levels of envelope sealing and/or greater levels of insulation, without increased ventilation rates. I can't tell you how many fresh air intakes I've seen completely blocked-off over the years (usually in course of investigating a sick building), done so in the narrow-sighted attempt to reduce energy consumption at all costs. ASHRAE 62 got revised. Additionally, as a response (knee-jerk) to the SBS, the pendulum reached the other end of the arc and I would see systems overwhelmed with more ventilation than could be properly conditioned and more than was necessary. ASHRAE 62 was revised again. Somewhere around the end of the eighties/early nineties, chicken-little discovered mold and anounce the sky was falling and was covered in "toxic mold". I have spent a fair portion of my career investgating and resolving these issues. In houses there three primary causes of excessive mold propogation: oversized AC units, poor envelope construction attention to detail (a poorly sealed house is usually better than a house that is 90% well sealed!), and water intrustion (i.e. water leaks).
I deal with primarily with mechanical codes (very few home owners and/or builders will pay for the engineering required to deveolp a comprehensive, site and occupant specific HVAC system design) that specify ventilation rates for non-residential occupied spaces. Most of these codes are based on the ASHRAE standard, and prescribe a per occupant or per sq.ft. ventilation rate or a combination of the two.
Different spaces should have ventilation rates determined based on used and occupancy. A flat rate or any sort, is usually the wrong approach. Houses are typically very lightly occupied, produce very little levels of contaminants. Point exhaust for specifc need are used intermittantly. I prefer to ventilate single family residential buidings (i.e. house) based on air changes, not by occupant. A contant, minimum rate of ventilation will maintain good quality most of the time and in the current trend of larger houses/smaller families, can absorb the larger, intermittant loads as they occur.
In terms of a whole house sytem, I like the ducted dehumidifier (like a DesertAire) in lieu of a furnace/ac. When the load is light, hot gas reheat provides dry "space neutral" temperature, low dewpoint air. When cooling is required, an outside condenser (with heat exchanger and fans only) or refrigerant cooler rejects heat and make the system an "air conditioner". I wish this is what I had done in my home. I live in heavy woods in N.IL, have double exterior walls and r-50 plus in the attic, and have an AC system sized at 1250 sf/ton. On a 100 degree day, it runs 23 hours, and maintains 77 degrees and 50% rh. On an 80 degree day, I still struggle with humidity control. Its cool enough at night not to require cooling and temps us to open windows. But it will be hot and humid enough in late aternoon and evening, that cooling/dehumidification is desired.
Tim, this is an excellent discussion.
Yes, this is a good discussion.
I am a native on North Texas, BTW. Grew up in Richardson. I know the climate and the area well.
<<I like the ducted dehumidifier (like a DesertAire) in lieu of a furnace/ac.>> I took a look at that and it looks like it's only available in industrial size, How would you do something like this in a house that only needs a 3 ton heat pump per Manual J ? ------------------
"You cannot work hard enough to make up for a sloppy estimate."
The one manufacturer that I listed makes units from as small as 1 ton. I'm sure that others provide units for smaller applications as well. It is common for small residential pool rooms, indoor whirlpool areas and small hotel pool rooms to have loads in the smaller tonnages.
Hi Tim,
I saw your post on ventilation that you dealt with code. I am wondering if I may ask you a Code/technical question? I have researched pretty hard to find the answer in my state code (TN)and the IRCC (2003) But I can't find a satisfacory answer.
My question is what code addresses, (if one does) not venting bathroom air in to the attic. I know it is forbidden but I can not find a reference. The only thing I found was in my state code and it said that if the bath required a mechanical vent then it had to be in comliance with ASHRAD 62. When I chased that down I could not find it. Any chance you could point me where I need to go? I know what is right but I just can't reference it.
Thank you in advance!
KD
2006 IRC section R303.3 "the minimum ventilation rate shall be 50 CFM ... air from the space shall be exhausted directly to the outside."------------------
"You cannot work hard enough to make up for a sloppy estimate."
Thank you Shelt! I am on it like a Hobo on a ham "sammich"!
KD
Perhaps an HRV system is the ideal answer. It sucks and blows at the same time.StewieAlways a new way to screw up.
It sucks and blows at the same time.
Now that's talent!
Stewie, I believe there are climates where ERVs and HRVs may be the best solution.
I am not sure about the mixed/humid climate. Besides the pricey price I have heard of a study (I think Armin Rudd-Build America) did case studies for different systems in a mixed humid climate and found that ERV was the least effective at managing humidity. Has anyone else heard this?
That's what we have, with a humidity contol on it, and timed switches in each bathroom.
Where do you get the "not so humid air" from? And not so humid as compared to what?
Bryan, My original "odd" thought was to duct the air from a hallway or perhaps any nearby space that was not as humid as the bathroom. Using an exhaust type fan and directing the air toward the shower area. So instead of sucking air under the bathroom door and thru the traditional exhaust fan...It would be blowing air into the room. Of course there would need to be some kind of a one way "flap" that leads to the outside so that this air would have a place to go....or this air could be ducted to an hrv or erv and then outside.
My scheme still has the same problem as the traditional exhaust fan....if air is to exit the house then how do we control where the make-up air enters the house.
I think a lot of good ideas have been posted. I think I (we) have a lot to think about.
It is easy to say "build tight and ventilate right" we just need to figure out what right is.
I think we may need 2 ways to ventilate a bathroom. One to expel odors and humid air when we don't want humid air...and One to capture the humid air and use it when we need it.
> One of our local experts (Austin)thinks that the best solution for
> us (north texas)is a stand alone dehumidifier.I doubt it, unless you can figure out how to put the exhaust air outside the house. Those things generate a lot of heat.George Patterson
That's what I do (keep the dehumidifier heat out of the house), but the best bet is a heat pump water heater. Dehumidifies while adding the heat where you actually want it.PAHS Designer/Builder- Bury it!
VaTom, I am not familiar with heat pump water heaters. Do you have a link or time to explain? Would this work well in North Texas? In an a conventional house on a small lot ?Are you talking about part of a ground source system?
Edited 8/15/2007 10:10 pm by homedesign
A heat-pump water heater is like a ground-source AC, only the "source" is a water heater tank. Popular in some commercial/industrial situations where there's some use for the hot water. In fact, the original units were designed for use in dairy barns where the cooling was needed for the milk and the hot water for cleaning.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
Dan,
is there a residential version?
Dunno -- don't keep track.But, BTW, there's also the heat-pumping clothes dryer -- uses a dehumidifier to get the moisture out, then pumps the hot air in to dry further.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
heat pump water heaters -
http://www.nyletherm.com/waterheating.htm
http://www.aers.com/etech_residential_water_heating.html
thanks for the link Paul, are you going to use one in your house?
A heat pump water heater is part of the plan.I will have a very large insulated crawl space with a concrete floor. About 1,600 sq. ft. and a ceiling height between 5 and 10 feet. The heat pump water heater will be cooling and dehumidifying the crawl space while heating the water.
Paul you are very brave to take on the 500 year wall. Thanks for the nudge on the reform wall. I am still going thru the archives. Ray Moore deserves some respect.
am not familiar with heat pump water heaters. Do you have a link or time to explain? Would this work well in North Texas? In an a conventional house on a small lot ?Are you talking about part of a ground source system?
Nothing to do with outside or your ground. Been around for decades, small audience. Air source, which puts the heat into your water. Roughly 3x the efficiency of elec resistance tank heaters. The interesting part is the byproduct, cool dry air which is generally ignored.
http://www.energy.wsu.edu/ftp-ep/pubs/building/res/ht_pmp_water_htrs.pdf
We run ours seasonally, whenever the house is naturally 70º or higher. Layout of the house is critical, however. Where's your water heater? Our standard tank heater got company from a box 16x24x10" hanging on the wall. Uses indoor air. Removes heat and dehumidifies. Sound a little like ac? Ours paid for itself in just over 1 seasons' use, 5 yrs ago. Uncomplicated, just good utilization.PAHS Designer/Builder- Bury it!
Ok, give me a tip on that. I have a dehumidifier in my shop to keep the tools from rusting. I can't put the unit itself outside, unless I want to shut it off during the winter. What do you do - run ductwork to the outside?George Patterson
They make a unit that vents hot moist air to the outside through a 4" dryer vent pipe. I've seen them for $300 at the box stores. Very slick units. You can set them to cool or just to dry which is how I have mine set up. Another option for a shop is a "Vertical PTAC unit" which is a little hotel heat pump that can heat or cool but doesn't hang out of the wall, instead it has a grille that sucks in outside air, runs it over a coil and blows it back outside. $1,400 and perfect for work shops and studio spaces, available with resistance heat or heatpump heat with resistance back up 220 V 30 amp.------------------
"You cannot work hard enough to make up for a sloppy estimate."
Next time you see one, can you get a brand and model?Thanks,
George Patterson
<<Next time you see one, can you get a brand and model?>>
Try this, I think with all the sleeves and t-stat it can be had delivered for about 1,400.00
http://63.99.248.238/Default.aspx?tabid=521------------------
"You cannot work hard enough to make up for a sloppy estimate."
Sorry to be unclear. What I meant was ...> They make a unit that vents hot moist air to the outside through
> a 4" dryer vent pipe. ..... that one.George Patterson
This might be a similar product to what he's referring to? Portable AC that exhausts warm, moist air through an external vent.
http://www.haieramerica.com/en/product/HPR10XC6
I can't put the unit itself outside, unless I want to shut it off during the winter. What do you do - run ductwork to the outside?
George, do you need to dehumidify winters in NJ? Can't imagine.
My dehumidifier sits quietly on a covered shelf outside. I isolate the heat generating parts from the airflow, which removes a large part of the heat. Of the remainder, the vast majority is picked up when the airflow goes through my heat exchanger, part of the overall air system. Condensate waters the garden.
I always buy used ($50-100), keep a spare on hand. Most recently a Whirlpool low temp 50 pint, which puts out considerably less heat than the Sears' models I'm accustomed to. Haven't taken it apart to see why, but it's very noticeable. Less than 50 pint I don't bring home. PAHS Designer/Builder- Bury it!
> George, do you need to dehumidify winters in NJ? Can't imagine. Yeah, I'm using it in my garage shop to keep the rust problem down. I have a small gas heater, but the dehumidifer will produce all winter long. My problem is that I've run the drain line through the wall, and it freezes up at times. And, of course, it heats the place up in the summer.George Patterson
That's why it would make more sense to add the extra coil to a standard split AC system.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
I think I'm not in a position to do that, but I would be interested in learning more about it. One factor is that the shop is at the other end of the house from the AC compressor, and there's a section of the house through which you can't run anything without tearing the sheet rock off a section of the outside wall. Which I have done twice before, but am loath to do again.George Patterson
"Why not blow dry?"
I, more or less, do exactly that.
An air supply provides cooled, dry(er) air to the space (shower/bathroom). The supply air mixes with the hot humid air developed during a shower and leaves the space via the normal return path for air in my forced air system. Keeps the space fairly dry during and diries up quickley after showers.
While this does add to the latent load of my evaporator coil, it is insignificant compared with the total and when compared with the volume of hot humid makeup air the would have to come into the house by some means, intentional or otherwise, were I to exhaust the bathroom during showers. At design conditions, without any pre-treatment like an via a heat or energy recovery system, outside air at summer design conditions "costs" 50 btuh/cfm. Using a 100 cfm exhaust fan causes an "extra" 5000 btu/hr or approximately 1/2 ton, load on my system. A little bit of moisture released through taking a shower is small in comparison, less than 1000 btuh for latent energy gain. Since showering (at least in my experience) creates no objectionable odors, as cooking or other bathroom activities might, recirculation of the air is a much better alternative than exhaust, if a suitable system with good air flow and turnover is in place.
A blower will typically increase the pressure in the house and some of the heated, not quite clean and/or moist air will then leak into the walls and can over time lead to problems. This would typically be a greater problem in areas with colder climate where the moisture will condense in the wall.
My thought was to "bring" the "not so humid" air from a "dryer" part of the house not from the outside. Much the same as the common practice. The difference would be that the air would be blown in instead of sucked out. An exhaust flap to the outside or a connection back to an HRV/ERV. Of course if air is expelled from the house then air would need to enter from some other place just like in the more traditional situation.
To All, I realize that my idea may be(is) hairbrain and may not really pan out.
I am just glad that people are discussing ventilation. When an exhaust fan is turned on.... that air has to come from somewhere. With the traditional exhaust fan system the house will go "negative" and that is not a good thing in my climate.
Have you ever looked at Joe's top 10 dumb things to do in the South?
Ventilation is most important.
Maybe I'm not getting your meaning, but a blower will cause a positive pressure in the room it's blowing into and that will cause a seepage of air out of the room into walls, through doors, vents and so on - no matter where the blower pulls the air from. Blowing indoors air would to an extent cause a short circuit of air that will be almost complete when the door to the room is opened.
Blowing air effects the whole house the same. Sucking air from a specific point allows for more effective removal of moisture or smells from a small area. It simply makes better sense.
Beer was created so carpenters wouldn't rule the world.
Not really a bad question.
I expect it's just easier to do well .... whatever you do, you still need a way for fresh air to replace the air you remove.
It wasn't long back that homes leaked enough air that supplying fresh air was not a concern. Today, the #1 cause of fan noise is not enough fresh air coming in to feed the fan blades.
Let's look at your idea ... to push in fresh air ... I expect that eliminating drafts would be a challenge. Who wants cold air blowing into the shower? Then, what air will you exhaust?
After all, the air we need to exhaust is that extra moist, warmer air near the ceiling. We won't help things much by removing air down by the floor. (Which is why windows don't work as well as they could ... the opening needs to be near the ceiling to work best).
Now, as to fan noise ... I'd say the first issue is proper installation. The longer and less straight the duct run, the less air is going to exhaust- and the more noise moving air will make.
Then, as mentioned, you need an adequate supply of fresh air. The idea of blowing in fresh air might not be such a bad idea...
Finally, the type of fan matters. "Propeller" fans can't help but be a lot noisier than "squirrel cage" fans. A large fan, spinning slowly, is quieter than a fast, small fan.
The biggest issue in fan noise, besides simple quality, is the depth of the fan housing -- deeper housings (which place the fan higher up into the ceiling) are quieter. (And, obviously, making the fan remote is quieter still.)
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
Dan,
Next thing you know people will be complaining that the exhaust fan is too quiet!
for the same reason as the courtesy toilet "preflush"
John
> Why do we suck air out of our moist rooms? Why not blow dry?
When I put a fan in a window and open another window at the other end of the house, everything works a lot better if I set the fan to exhaust air.
"When I put a fan in a window and open another window at the other end of the house, everything works a lot better if I set the fan to exhaust air."
That is true if you put the fan in the correct window. Otherwise you may be fiighting the natural pressures. I think the fact that you have 2 windows open is the key concept. Like a supply and a return.
If someone wants to chug a beer really fast all they have to do is make an opening in the other end of the can and they will win every time.
Edited 8/15/2007 6:46 am by homedesign
It really wouldn't make a difference in terms of the amount of air moved - however, the main reason I can figure is that you wish to send the moist air outside of the building.
This would be difficult to accomplish with an intake fan. The fan would draw air from the conditioned space outside the bathroom (since bringing hot humid air from the outside would not remove moisture from the bathroom & bringing in cold air from the outside would simply cause the moisture in the bathroom to condense). In order to vent moist bathroom air to the outside, the bathroom would have to be sealed to the rest of the house & have an exhaust port to the outside.
"In order to vent moist bathroom air to the outside, the bathroom would have to be sealed to the rest of the house & have an exhaust port to the outside."
Soultrain,
look at my original post that is pretty much what I said. I gave up on my quacky idea a while back. I am more interested in finding the best ventilation solution.
Another thing regarding exhaust vs intake fans:
In general, an exhaust fan is best for addressing general environment issues for a particular enclosure. (such as moisture in a bathroom, heat inside the casing of various electronics - DVD players, computers, etc).
Intake fans are good for addressing environment issues related to a particular item.
Using the electronics example, an exhaust fan is useful for keeping the temperature down inside the case as a whole since the air inside the case is hotter than the air outside. However if there is a specific item inside the case that needs cooling - the CPU for example or a high performance video card, it is helpful to have an intake fan blowing directly on that component.
"the CPU for example or a high performance video card, it is helpful to have an intake fan blowing directly on that component."
Soultrain, I thought it might be a good idea to blow the shower walls dry thus making the bathroom dry faster.