What Ventillation System “type”?
I am purchasing a 2 story modular house. 2000 sq ft downstairs, 1050 unfinished sq ft upstairs. House will be placed on a crawlspace type foundation. Planning on placing 2″ thick flowable fill over 1″foam and a 6mm vapor barrier.
The house has 2″ x 12″ perimeter floor supply registers preplaced at the factory; and a large return in the master bedroom and the air handler closet in the utility/laundry room.
I am located near Aiken SC and wish to wisely spend my $$$ allocated towards HVAC. My previous house had a late 90’s model Carrier dual fuel heat pump that gave lackluster performance. I replaced it with a goodman gas pack that is ok.
I do plan on having an energy audit performed when the house arrives. I have 2 estimates with Man J calcs performed, but the recommended system size varies.
I am trying to decide what is the most cost effective options are from an initial cost and operational costs.
I “think” I need to start by deciding on “what type” of central ventillation system will perform best for me.
Attached are 4 types of central vent systems I have found.
Thanks for the valuable input in advance.
John
Replies
1st one - no because it doesn't duct the exhaust to the outside. I've always understood that exhausting air from the house through the attic isn't the best idea ... but I come from a colder climate and I have heard throughout the midwest and east that people exhaust through the attic.
2nd one - NO! It tends to pressurize the house and ultimately force air into the construction.
3rd one - a reasonable alternative. Keep the exhaust slightly larger than supply.
4th - good choice - expensive, though. Requires maintenance and may not give you a lot of energy performance (in your climate).
It's better to have positive pressure in the house than negative pressure, especially with combustion appliances present, but the goal should be to have nearly neutral pressure.
Corporation: n. An ingenious device for obtaining individual profit without individual responsibility. --Ambrose Bierce
Assuming you aren't drawing such a negative pressure to affect a vented combustion appliance and/or that those appliances get combustion air from dedicated combustion air sources (which is actually required in some cases), I disagree. Forcing moisture into construction with positive pressure I've always thought to be a bad idea.
Maybe it would be different in the southeast ... high humidity and high temps. They do lots of things a little different. My experience has primarily been with colder/temperate climates that are a little on the arid side.
My experience has been with colder climates too. Most positive ventillation schemes I've seen seek to maintain a slight positive pressure. This prevents drafts (a comfort issue) and assures that combustion appliance will "draw" sufficiently and not backdraft.
Corporation: n. An ingenious device for obtaining individual profit without individual responsibility. --Ambrose Bierce
A combustion appliance that backdrafts (or has that opportunity) should have combustion air makeup ... which if the appliance is in a confined space or in tight construction in general, deserves it's own combustion air (which has been required in some energy codes and by the mechanical codes).
Lots of points of view and discussions about these topics. Lots of examples of successes and failures seemingly on both sides of the fence. While I'm not disagreeing with your comments, I do present other valid points of science. I've seen positive pressure push moisture into a cold space where it then condenses on surfaces with undesireable results.
Neutral pressure can be tough to guarantee (although CAN be easy if you do it right). I've always erred to vent negatively if you don't affect the combustion appliance ... in most situations. There is a time and place for everything, though.
It's all food for thought. Hopefully we give a poster enough to really digest and make the most informed decision for themselves. The beauty of many opinions and points.
Positive pressure control is a better idea in all regions. Ideally, no leakage would be best, but that's not realistic. I can think think of no benefit to control building pressure to a relative negative. Air leaking out is almost always conditioned to some consistent level (if not, then what difference does it make). At times when the air inside is warmer and more humid, any condensation that might occur as it leaks out to the colder, dryer outside will dry more easily in the outer portions of structural elements. My HVAC design and problem remediation experience is exclusively in the US midwest. Heating is easy, but effective humidty control without high sensiblle loads is more difficult. Uncontrolled inleakage is a constant source of problems and creates as much poor IAQ as most other system maladays, that I have had the personal experience to deal with in this region. Natural draft issues aside (which is another very good reason for positve pressure control), moisture migration and subsequent structural damage and/or mold growth is always worse with inleakage than out leakage. Both occur, but the lesser of two evils is what we attempt to acheive.
I'm not sure what you mean when saying air leakage in poses an IAQ problem. I'm also not sure how leakage in will pose a structural problem. I'm not disagreeing, just asking for clarification on your point of view.
Not sure what you mean by "but effective humidty control without high sensiblle loads is more difficult. "
Leakage in usually guarantees there will be no moisture pumped into the construction and what leakage does occur, the RH drops quickly as it gets warmer.
Leakage out is why attic ventilation is required by code ... to help mitigate the problem. It is also why they invented Tyvek ... to help control moisture from leakage out while blocking air leakage.
The vast majority of houses have never been designed to operate in positive pressure ... new or old. They've historically had exhaust fans with no way to make up that air (other than natural air leakage in through the construction) and in spite of any combustion appliance and they haven't had rampant problems (barring the usual problems with the exhaust fans not working properly).
In the Northewest, the energy codes have required ways to allow fresh air in from exhaust fan systems and also have required combustion air for appliances. This pretty much balances the systems, but they still run negative as the incoming air relies on the negative pressure from the fans and appliances to draw air in.
I've seen baseball size ice on roof nails because of uncontrolled leakage out ... it collects in cold weather and then when it warms up ... all hell breaks loose. An extreme example, but allowing condensation to collect in the outer or any part of construction regularly isn't a good thing. That is why they require positive connection of exhaust ducts to the outside of the structure (not just the attic).
Maybe you can clarify some of your points for us.
You should read some of the publications of Joe Lstiburek (sp?). He has done a great deal of reasearch and written about most of it. He also does a far better job of describing the details of what I alluded to.
Typically, inleakage is due to (other simple wind induced with is infiltration, couple with exfiltration as well) a specific point souce, a poorly connected exhaust duct, a poorly detailed panned joist, things along thse lines. In the colder months, the cold air will cool inside surfaces, cavities, etc and causes condensation in a location that will not be easliy removed. Moisture in wood structures causes mold and rot, leading to structural damage and poor indoor air quality. This is something I have observed in the field and made me a believer in the positive pressure control approach. At a slight positive pressure, in the absence of gross point leakage, a problem in its own regard, outleakage is very slight, and MOL spreadout across most/all of the construction joints.
Attics are ventilated for other reasons, not part of this discussion.
As far as humidity control in low sensible load environmets goes, the "difficulty" lies in the manner in which a cooling based air conditioner system is operated. Thermostats control to a sensible setpoint only. We'll use my house/property as an example. I have a double-walled, heavily insulated home on a densly wooded, north facing/sloping lot. Heavy shade and a lot of humidity, but little direct sun, and with the envelope I have, little transmision from outside air. I have a 4 ton AC unit, that serves 1350 sf/ton. Except when the outsie air is over 90, the unit will run very little and the house gets over cooled and/or too humid. I have no reheat in place and will simply over cool to compensate, at times. While I intended to undersize this system, I still have runtime issues. Without air moving over a cold evaporator coil, dehumidifcation does not occur. IF my house were subject to higher outside air temps and/or greater solar radiation, the sensible load on my system would increase resulting in greater run times without over cooling and therefore, more effective dehumidification of my home. I have a separate humidity meter and run my system more so to control humidity than temperature, unless it gets really hot. On a 100 degree day, it will run for 23.5 hours. Systems can be set up (and hopefully design/selected) to address this (see below), if done so intentionally by a knowledgeable individual. The average HVAC conhackster cannot/will not do so.
Let's say to make matters worse, I have inleakge of warm, moist air. Greater latent load with little increase in the sensible load, make for more of a problem than if the cooled, dryer air leaked out. If indoors it is 75 degF and 50% rh (what I design to most of the time) and outside, like most of the summer in N.IL, its 85/85. I will not condense any moisture via outleakage, but I would with inleakage (that OA has a dewpoint of 82 degrees). Eventually that will see the evap coil, but not before the damage (discomfort) is done.
Making a house with a forced-air system slightly positive is, BTW, very easy. I am a fan of, and use in my home, a device called a "Skuttle". This is barometric outside air damper, installed in an insulted duct with a manual volume damper, to the return side of the system near the upstream side of the system filter. Mine is set to crack open in the ventilation (i.e. low speed fan operation of my varibale speed furnace) mode of my system with my water heater drawing air from the basement for combustion and flue dilution/tempering. When the fan steps up to either stage of heating of full speed for cooling, the volume damper limits the amount of ventilation induced. I probably introduce 30 to 40 cfm at max fan speed. Compared to the 1200 of so in the system, a small increase in the coils loads. (I do run my AC at 300 cfm/ton to increase the latent capacity of the coil, while dereasing the sensible)
I'm familiar with Mr. Lstiburek's work. Looks like you are focused on the cooling side in midwest climates with hot summers and high humidity ... which I admitedly have limited exposure to. My hot summers have historically been 15-30% RH ... and as low as 1% RH where I'm living now.
Also the systems you describe sound like they are carefully designed for the proper installation and balance/tuning of the final product. I wasn't saying it ain't easy ... just dealing with most mech. contractors (residential), they tend to slap things in as fast as they can and don't pay attention to the details ... and design a system? hold their thumb out and let'er fly.
No offense to you contractors who do things right, but in my experience, you are the exception, not the rule and it shouldn't be that way. I've seen it so many times ... flex duct everywhere and sealing ductwork? heck no! ... Design is by gosh and by golly. Hats off to anyone who raises that bar and begins to provide a higher level of service.
Edited 10/21/2008 10:11 pm ET by Clewless1
I think we may have covered this in the past, but my experience, with a very few notable exceptions is non-residential. Commercial contractors are held to a higher standard, though not much higher.
Yes, my HVAC engineering experience is exclusively in the midwest. Rockford, Chicago , Madison, Milwaukee, Minneapolis, and all points in beween.
I find it much more of a challenge, psychrometrically at least, for simple systems to mitigate mid temperatures with high humidity than vice versa. Like treating a basement well. Very difficult to do well and seldom attempted. Cool, damp and musty.
Point is, though, positive pressurization, in my experience is the better alternative. Now, we're guilty of a highjack.
Hopefully the OP found value in the discussion albeit slightly derailed ... sometimes it triggers other thoughts and creativity in solving problems and answering questions.
Do/have you ever switched pressurizations seasonally?
As Much as possible, I like windows open, so my pressurization in my residence really only applies through the dead of winter.
To answer the question: No, I have never intentionally operated my home in a negative relative pressure. In reality, it is probably more close to neutral than actually positve. I have experimented with the skuttle to vary the amount of ventilation and probably ran negative unintentionally from time to time.
I can tell when everything is closed up how close I get, the fire place won't draft as well.
Understand that the Ventilation systems in your attachment is only the "V" in HVAC, and has nothing (directly) to do with heating and cooling systems. The first answer is: none of the above.
There are a few questions to be answered before the "best" system can be approached. "Best" is of course a very subjective term and depends on what is important to you. That leads in to the first REAL important questions: What's most important to you (in an HVAC system)? Economy? Comfort? Reliability? Sound? Heating? Cooling? First cost?
To determine some of the cost issues, you must what the cost of LP/natural gas and elctricity are in the area your house will be sited. There are trade-offs in initial cost vs operational cost, and in a mild environment like SC, the very most efficient systems have payback periods longer than they can be expected to provide reliable service.
Features cost money. Some are worth it, some or not. Variable speed on a furnace/air handler is a worthwhile cost adder. In SC, 90+% gas efficiency is not going to provide a signicant savings. AC with 19 SEER is a complete waste outside of the tropics. In most moderate areas, little savings can been seen going from 10 to 13.
The 4 systems you posted pics of would, I think, have to be pre-installed, unless I am missing something . You identify 2x12 supplies, a central return, and an air handler closet.
Are the ducts also in place?
Same question re the "fart fan(s)". Are they already in place? And what about kitchen exhaust fan/hood?
With the advent of "super" insulated construction there has been a push towards the type of system you show in pic 4, the theory being that houses are so tight that indoor air quality suffers, so you need engineered fresh air, and you need a heat exchanger for energy efficiency.
My personal feeling is that there are very few houses that are really that tight, that in a lived in house there is enough coming & going & use (i.e. the fart fans & hoods) that ture air quality issues aren't significant, and that that type of engineered system violates the KISS rule, as well as being another energy sink. If the house is stuffy, open the window.
In your location I would consider a whole house attic fan (which, given that your attic is unfinished should be an easy job, as opposed to adding lots of ducting in the whole house). I think they are an energy smart alternative to cooling with AC in a good portion of the cooling season.
If you really want to go out on the cutting edge, there are water heaters that use the waste heat from de-humidifying as heat source. So, you keep the house closed up during the hot day (with good windows, siting, etc.), the water heater dehumidifies it, and at night as it cools, open the windows & run the attic fan. It probably requires too much input (i.e. YOU have to open & close the windows) for most people, but the $$ saved vs. set & forget HVAC can be pretty substantial.