Wondering about the fireplace, details onp.74.
Since when is it a good idea to pressurize the house to “minimize air infiltration”? Aren’t we encouraging exfiltration? Doesn’t that cause miost warm air to flow out through the insulation, causing condensation or frost in the framing? This would likely lead to rot or at least mildew, etc.
Also, one cord of wood per year cannot be helping significantly to reduce the heating loads. Depends on climate, insulation levels and air sealing, of course. It is a pretty big house (3000 sf).
Some of the other heating arrangements look pretty sexy, but very complicated.
Replies
I am wondering about this also. I am having a new HVAC unit installed. The contractor mentioned it was a good idea to design the system to create a positive pressure in the house. At the time I did not question him, but now think the idea will cause moisture to penetrate the FG insulation from the inside. What is everyone's thoughts. Thanks Jay
I'm with you on not pressurizing the house. You don't want to have all the dampers on the bath fans and range hood blowing conditionned air out of the house all the time while the airhandler is running sucking outside air in.
What you want is enough fresh air to give folks something good to breath distributed through out the house at a rate of 7.5 cuft per minute per occupant (plus one) per ashrae 62.2 but on a per occupant rather than per bedroom basis. Why ventilate the house when your at work or on vacation? Why not ventilate more when you have the boys over for the game.
We use those $25.00 Cooper 6109 occupancy sensors with the 30 minute delay off to operate the Panasonic energy star 110 CFM bath fan-lights whenever anyone visits the bathroom. To let the fresh air in we put a filtered intake damper in the laundry near the dryer and make sure there is plenty of circulation to the kitchen (jumper duct if necessary)
If the owners have big dogs I encourage them to keep a water bowl in the powder room since dogs put out as much humidity as humans.
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"You cannot work hard enough to make up for a sloppy estimate."
"You don't want to have all the dampers on the bath fans and range hood blowing conditionned air out of the house all the time while the airhandler is running sucking outside air in."
That's an exaggeration at best. The pressurization is very, very slight. The dampers on the exhaust fans and the range hood and the clothes dryer, etc., are leaking now. Sometimes they leak in, sometimes they leak out. Which, depends on the wind and other variations. The fact is that all structures not specifically design to develop differential pressure, leak. In a building that has gravity vented appliances (not uncommon in most of the country), a slight positive pressure prevents backdrafting. Untempered outside air might be accepteble in Carolina, but in most of the Midwest, New England and the Pacific Northwest, your scheme would be acceptable only to the attorneys that would sue you and those that would defend you. Point being, you might want to look into this concept, learn about it and try to understand it before you criticize what you do not understand. Any of the books by Joe Lstiburek (sp?) concerning ventilation and presurization issues would be good place to start.
"Point being, you might want to look into this concept, learn about it and try to understand it before you criticize what you do not understand." No point in getting into a pissing contest on this but the fact remains that Lstiburek's "Air Cycler" has been shown to be being disabled by occupants due to owners objecting to cold drafts and I really have an issue with the idea that the ventilation rate should be linked to the amount of time per day that the furnace or AC is running. Higher performance thermostats do have sensors that will open an air intake damper when the outdoor temp is between 40 and 75 degrees and close them otherwise if you want to use passive ventilation through the return air. I've been reading Joe's writing since the airtight drywall approach stuff with the R-2000 program up in Canada back in the 90's and I have profited greatly from his research. But I really believe that a high performance house can benefit by ventilation that responds dynamically to the number of people in it at any given time as indicated by bathroom usage. ------------------
"You cannot work hard enough to make up for a sloppy estimate."
We can discuss the facts about various approaches to ventilation and pressurization of occupied space if you wish. Your characterization of positive pressure control, in my opinion, was exaggerated and inaccurate. I assumed that either you were unfamiliar with the concept or had a personal axe to grind. I'll reserve that judgement for the time being.
I do not propose or support ventilation being based on furnace run time. I don't know where that came from. If my previous post led you to believe that, then I must have worded it poorly. My bad.
I believe that 7.5 cfm per person during occupied times only, in an average residence built today, is far from adequate. In example, a 20,000 cf house with 4 people. 30 cfm of ventilation is 0.09 air changes per hour. Most houses in North America leak more than that. The IMC requires 0.35 ACH or 15 cfm per person. Code minimums in a large portion of the US, in my opinion are insufficient.
I believe untempered makeup air being dumped into any occupied space is poor design. In a mechanical-only space, its marginal design. If the weather conditions permit natural ventilation, then its a non-issue. In extreme weather conditions, which our designs must address to be adequate, it is a bad idea. This only my opinion, but your statement that many have disabled a cold draft producing system, leads me to believe that I am not alone in this understanding.
These are related to, but not the issue I raised. That is positive pressure control of inleakage.
Any system that is based on the exhaust fans pulling in outside air as needed, is a negatively pressurized space system. Negatively pressurized building envelopes suffer from uncontrolled inleakage at various contruction joints. Do you disagree?
In a positively pressurizred system, the leakage is out, not in.
The issue is, which is better and why.
TimThe common approach I see to positive pressurization of houses is to install a 6" duct from the return air duct directly to the outside air so that fresh air is pulled into the house whenever the air handler is operating. Maximum ventilation happens during the hottest and coldest days of the year regardless of building occupancy. If I have an ax to grind it is with this practice which seems to have grown out of ASHRAE 62.2 and the 7.5 CFM per bedroom plus one rule. I build tight houses that are energy star certifying at 32% to 63% more efficient than code. I'm really not worrying about bulk air penetration of my walls causing humidity issues. I do worry about excessive interior humidity and bad air quality and deal with that through source control and ventilation, bringing unconditioned air into the laundry adjacent to the dryer and operating bath fans on timers for aiming at 7.5 cfm per occupant 24/7. I used timer controlled energy recovery ventilators in the 90's but have stopped using them in the last five years or so. We generally pulled the exhaust out of the bathrooms and dumped the supply into the return air duct. Should I assume that this is the option you are supporting?I don't think this discussion needs to be so passionate. There are a number of different approaches that fit a number of different situations and there is more than enough gray area to go around. If I were building in Montana I would undoubtedly use a system that conditioned the incoming air and it might be some version of a smart intake air damper on the return air vent for positive pressurization. Since I build in NC it's a minor inconvenience to occasionally let the laundry get a little chilly or stuffy, certainly nothing to incur a lawsuit. Where are you building and what kind of system design are you using to maintain fresh air in the homes you are working on?------------------
"You cannot work hard enough to make up for a sloppy estimate."
My original issue with you was not with your approach to ventilation (which given your location, I agree is not a bad way to ventilate), but with your exaggerative "diss" of building pressurization. You have yet to either justify or clarify your that.
"operating bath fans on timers for aiming at 7.5 cfm per occupant 24/7"
You'll have to explain how ventilation occurs "24/7" when the source of the air movement, i.e. the bath exhaust fans, operates on timers. How is the space ventilated when the exhaust fans are not operating?
"I used timer controlled energy recovery ventilators in the 90's but have stopped using them in the last five years or so. We generally pulled the exhaust out of the bathrooms and dumped the supply into the return air duct. Should I assume that this is the option you are supporting?"
Using energy recovery in the manner you describe (less the timer) is a well proven practice, with some limitations. You may assume whatever you wish.
Take the OA into the AH return with some simple barometric controls and you have a well functioning pressure control system. With some experimentation and adjustment, a barometric damper can be adjusted to only provide ventilation on demand, otherwise a slight positve pressure keeps it closed. If air is consumed (indicating occupancy indirectly, but accurately) by an exhaust, a dryer, or a non-sealed combustion appliance, the damper opens. This approach only works properly with a variable speed air handler with constant low speed ventilation operation. A two-stage gas furnace or hydro-air with a minimum discharge air temperature controls work the best. Stupid (on-off) HVAC controls work as you describe, more or less, but then again, they are stupid.
I design HVAC systems for all manner of structures in the upper Midwest. Single family residential being a personal interest, not a vocation. Like I explained to another poster on this issue, positive pressure control of buildings other than houses is nothing new. Simplifying the concept to the point that (an average) builder and/or the low priced HVAC hack can understand and implement, that's a new challenge.
< My original issue with you was not with your approach to ventilation (which given your location, I agree is not a bad way to ventilate), but with your exaggerative "diss" of building pressurization.>Tim, my exaggerative "diss" of building pressurization was this statement.<I'm with you on not pressurizing the house. You don't want to have all the dampers on the bath fans and range hood blowing conditioned air out of the house all the time while the air handler is running sucking outside air in.> Not really all that harsh, sorry if I touched a nerve there. I know that you feel strongly about this and you live in a cold climate where my approach would not work. <You have yet to either justify or clarify your that.>The way I attempted to clarify that was by saying the following.<The common approach I see to positive pressurization of houses is to install a 6" duct from the return air duct directly to the outside air so that fresh air is pulled into the house whenever the air handler is operating. Maximum ventilation happens during the hottest and coldest days of the year regardless of building occupancy. If I have an ax to grind it is with this practice which seems to have grown out of ASHRAE 62.2 and the 7.5 CFM per bedroom plus one rule.> <You'll have to explain how ventilation occurs "24/7" when the source of the air movement, i.e. the bath exhaust fans, operates on timers. How is the space ventilated when the exhaust fans are not operating?>Lets say that a person wants to move 7.5 cfm on a 24/7 basis with a bath fan that moves 100 cfm. For every 100 minutes that person is in the home the bath fan needs to run 7.5 minutes. (7.5 cfm X 100 minutes = 750 cubic feet = 100 cfm X 7.5 minutes.) if a person visits the bath room once every eight hours then 8 x 60 = 480 minutes x 7.5 cfm =3,600 divides by 100 cfm = 36 minutes runtime on the fan per visit to the bathroom. So we set our occupancy sensor to operate the fan while the occupant is in the bathroom PLUS 30 minutes after he leaves and we achieve 7.5 cfm but we don't ventilate while he's at work or on vacation and we increase ventilation when he has visitors. This may not be as elegant as the system you describe but it is a lot cheaper and it's far better than a straight pipe through the wall to the return air duct even if it has a damper to throttle it down when the outside temp is colder than 40 or hotter than 75. I'm sorry you feel dissed and that you feel that my approach is unprofessional and likely to get me sued. Mainly I'm sorry I pissed you off. My goal here is to share the systems that work for me here in NC and keep the conversation open while expressing my reservations with the ASHRAE 62.2 standard. It sounds like you have a really elegant and sophisticated system that works well for your climate. Please keep communicating and share what is working for you. Some elaboration would be a great contribution to the discourse. <Take the OA into the AH return with some simple barometric controls and you have a well functioning pressure control system. With some experimentation and adjustment, a barometric damper can be adjusted to only provide ventilation on demand, otherwise a slight positive pressure keeps it closed. If air is consumed (indicating occupancy indirectly, but accurately) by an exhaust, a dryer, or a non-sealed combustion appliance, the damper opens. This approach only works properly with a variable speed air handler with constant low speed ventilation operation. A two-stage gas furnace or hydro-air with a minimum discharge air temperature controls work the best.> Fill this out, let us know what this looks like and how it results in better air quality and less energy consumption. It sounds like a great solution especially for a very cold climate.Merry ChristmasM------------------
"You cannot work hard enough to make up for a sloppy estimate."
Shucks. This had all the makings of a decent slap fight.Ever wonder why "holistic" doesn't start with "w"?
I pressurize, leaks all go out. Incoming air is heat exchanged with outgoing. Works extremely well. Allows me to easily control the quality of air we breathe.
BUT- I don't have framing or insulation to get wet. Nor do I need an active heating system.
The American Lung Assoc. houses here pressurize, and have standard framing. Made me wonder...
Did have a long conversation with the HVAC guy for them. Pretty much all he wanted to talk about was how much rotten wall/wet insulation he's seen with standard housing. That was after he realized his sales pitch wasn't going anywhere with me.
PAHS Designer/Builder- Bury it!
Edited 12/11/2007 8:46 am ET by VaTom
"Since when is it a good idea to pressurize the house to "minimize air infiltration"? "
I've been involved in HVAC design for over 20 years, and this has been a tenet of good practice that entire time and was well established when I started.
Since when has the residential construction industry ever cared about what counts as good HVAC design? As far as I can tell, this is not a widespread reality, today and has never been.
While the merits are debatable (not necessarily bad or indisputably good, but truly debatable), the concept has been around for some time. The realtive moisture of the inside air to the outside air and the ability of the materials of construction to withstand wetting are among the many variables that can affect whether this a good practice or a bad practice.
The basic premise being, however, that somewhat controlled outleakage is better somewhat uncontrolled inleakage.