Do power attic fans help?

[Shophound]

If the major reason for not running a PAV is b/c the average home is not sealed tight enough to thwart conditioned air being sucked from the house and into the attic, could then a argument be made to only run the PAV when the house A/C is NOT on?

In other words, if its warm enough to heat that attic but the homeowner chooses NOT to turn on the AC that day, would turning on the attic fan mitigate some heat transfer from the attic to the living area? Its not like you are pulling conditioned air that you paid to maintain into the attic ...

???

Also, for the life of me I cannot understand how the PAV can create such a pressure differntial as to suck air from the living areas. Between the ridge vent, gables, and eaves of a newly constructed home, how can that large a number of openings to the outside not be sufficient to faciliate a air exchange between the PAV and the outside?

Powered attic ventilation and hot weather coincide. When you need a/c, it's also when the PAV is set up to turn on. If you encounter a warm day where it's pleasant enough to open windows and not run a/c, you could flip on the PAV if you wish...the objection to PAV's is how they affect a conditioned environment. With that said, I'll repeat what I said earlier...PAV use and high a/c demand often coincide, making them poor teammates.

As for the pressure differential issue, remember that air is dumb. It doesn't know it isn't supposed to creep up out of the house through a recessed "can" light stuck in the ceiling adjoining an attic. It just does. If the entire attic is in a negative pressure, and air has a choice to be made up from a can light or a gable end vent....there is no choice. Both will draw if a difference in pressure exists between the attic air space and an adjoining air space at a higher pressure. They won't move the same volume of air due to size, but both will move air. You don't really care if your gable end vent can ingest 200 feet per minute of air, but you should care your can light can suck air conditioned air from your house to provide air for the PAV.

Back to the first point...in my area, the window between needing heat and needing cooling is fairly small. We have a few weeks in spring and fall where the windows can be opened and the house ventilated/cooled naturally. The rest of the time we're either heating or cooling mechanically. In both cases, my efforts of late has been to help my house contain the air I pay to heat and cool. A PAV would countermand that effort. Going around and sealing the crap out of every penetration to the attic in my ceiling has not. The attic is ventilated naturally and has radiant barrier installed, so I expect our cooling load this summer to be managable. This will be our first summer in this house since buying it last fall, so we have no comparison to how it performed prior to these changes, but I expect it would be better.

[printmanjackson]

Where would you mount that rheostat device? In the attic? Rheostats (such as a light dimmer) generate a lot of heat when in operation. They need an adequate heat sink to disperse that heat or they'll cook to death. If the rheostat is installed in the house, room temperature is often sufficient to cool stats that are used for light duty. Have you ever seen commercial installations of rheostats/dimmers, such as in restaurants or banquet facilities? They have finned heat sinks on the surface of the wall, and those sinks get HOT. And that's just for dimming the lights in a restaurant. I couldn't imagine sticking a "dimmer" on a PAV in an attic, even if the thing was rated for it. If I put it in the house, that rheostat is going to add heat to the house...just what I don't need.

If your heart is set on a PAV, get a solar powered one and seal the crap out of your ceiling adjoining attic spaces.

I see your point shophound. I've already yanked out all of my PAV's

[Shophound]

I see your point shophound. I've already yanked out all of my PAV's

Thanks, printman. My last post may have come across a bit preachy...for that I apologize if it was. I was mainly hoping to emphasize the point that slowing a PAV, in addition to the air pressure problems we've discussed, might end up creating more of a net heat gain, offsetting any benefit for slowing the fan.

I wouldn't mind at some point a discussion regarding solar powered PAV's. I remember a study Florida Solar Energy Center did, but I think the emphasis of that study was pertaining to radiant barrier. Solar powered PAV's do present similar problems as their electric grid brethren...if the attic floor is not sealed well, conditioned air may be drawn out of the house and into the attic. It just won't cost as much at the meter due to the PAV running on light beams.

Pertaining to this entire discussion, if much more emphasis was placed on making attics airtight (not necessarily vapor tight) in relation to the interior of a house, many benefits to such an approach are available for the homeowner. Until attics are made airtight, natural draft attic ventilation appears to be the best method for modulating attic temperatures, along with radiant barrier.

[Daltex]

Is there a independent study showing PAV's in a proper installation do more harm than good? I had a huge increase in my TX elec. bill years ago and went in the attic to check the AC (leaks, ducts etc.) and couldn't breath it was so hot. The PAV wasn't running so I replaced motor and the temp dropped 20*. The next bill was 200.00 less than the previous month and the OD temp was higher.

*Since I don't have ridge vents the air was trapped w/o any exhaust. Not a fair comparison with a ridge vent house.

*I added enough soffit vents to eliminate the feeling of air being pulled through the attic access while standing in the attic. Before doing this there was a mild cool breeze of condidioned air being sucked in.

[krd]

I don't want to tell you boys your business but attic fans do help in several ways.For one they keep the attic cooler by pulling outside air through, not inside conditioned air. They also prevent the build-up of mold in the attic. They make your roof shingles last a whole lot longer( ask any roofer). The motors on the fans are usually shaded pole motors and the felt washer that keeps the inside sleeve bearing oiled usually dries up after a couple of seasons, so it needs to be oiled every season. I'll like to see some smoke tests run, showing the air being sucked through the can lights etc and going up into the attic. In fact in my area all the large two story home are putting their ac units for the upper floor in the attic and blowing the conditioned air into the bedrooms. Ridge vents are the easiest way to encourage air flow through the attic, so if all these houses have such extreme leakage into the attic, wouldn't ridge vents also pull conditioned air out of the house?

[platchford]

*Since I don't have ridge vents the air was trapped w/o any exhaust. Not a fair comparison with a ridge vent house.

*I added enough soffit vents to eliminate the feeling of air being pulled through the attic access while standing in the attic. Before doing this there was a mild cool breeze of condidioned air being sucked in.

Those two statements say it all. The reason you have a PAV is you have no ridge vents and, I might assume, no gable vents. The PAV is the only exhaust available. Given the choice, a proper ridge vent would be much preferred to a PAV. Also, you already state that you noticed a difference in the amount of conditioned air being pulled from the house once you added enough soffit vents. There is still conditioned air being pulled from your home, but now it is probably not a real noticeable (but not necessarily insignificant) amount.

[Daltex]

Those two statements say it all. The reason you have a PAV is you have no ridge vents and, I might assume, no gable vents. The PAV is the only exhaust available.....

That's why I said what I said.

.....Given the choice, a proper ridge vent would be much preferred to a PAV. Also, you already state that you noticed a difference in the amount of conditioned air being pulled from the house once you added enough soffit vents. There is still conditioned air being pulled from your home, but now it is probably not a real noticeable (but not necessarily insignificant) amount.

Much preferred? Where do you source this from? I would suggest that without independent verification that the small amount of heat extraction from a ridge vent verses an air turnover rate much greater with a PAV that the PAV "could" be more efficient because it does remove more heat from the attic. You aren't saying an attic 20* hotter is not an issue are you? Are you saying that a PAV doesn't remove more attic heat?

Just looking for documentation. Not saying you are wrong.

[Brian GC]

Is there a independent study showing PAV's in a proper installation do more harm than good? I had a huge increase in my TX elec. bill years ago and went in the attic to check the AC (leaks, ducts etc.) and couldn't breath it was so hot. The PAV wasn't running so I replaced motor and the temp dropped 20*. The next bill was 200.00 less than the previous month and the OD temp was higher.

Daltex,

You’ve made an outstanding point with impressive data to back it up. I would also like to see documentation showing that a PAV would impair the function of an AC when adequate soffet and gable vents are present. Though I am partial to the free and consistent operation of turbine fans, I can’t see how a small loss of conditioned air, if any, would counter the benefits of a 20* cooler attic. Ridgevents, IMO, will not cool like a PAV.

It would be like saying an AC would be significantly hampered with a bath or kitchen exhaust fan on, which are at least double the CFM of an attic leak. The miniscule loss of air closest to the ceiling is a loss of the hottest air in the house. And, if recessed lights are leaking, it would cool the light fixture and surroundings.

Brian

[Joe Mascitti]

Did you not have any ridge vents? Was your attic totally sealed?

[Shophound]

Daltex,

You’ve made an outstanding point with impressive data to back it up. I would also like to see documentation showing that a PAV would impair the function of an AC when adequate soffet and gable vents are present. Though I am partial to the free and consistent operation of turbine fans, I can’t see how a small loss of conditioned air, if any, would counter the benefits of a 20* cooler attic. Ridgevents, IMO, will not cool like a PAV.

With how I've seen many homes built, tract or custom, that "small loss of conditioned air" isn't so small. If the PAV is thermostatically controlled and set to come on when attic temps reach, say, 100 degrees, and no RB is installed, that PAV is pulling air through an attic for a healthy portion of a hot day, continuing to do so after sunset until attic temps drop below 100.

Additionally, air movement through insulation nukes the R value. Air coming up from the house, air moving over the top of the insulation via PAV induced velocity...two sources of air movement working against the insulation's role to retard heat transfer into the house.

My own house can serve as an example. Last fall we had RB sprayed on the roof deck underside, and the fiberglass insulation increased to a claimed R30 value. It looks like a blizzard hit our attic if you go up there and look around. We found, even after both improvements, that the house felt chilly on cold nights and days. At one point I had a four day off work stretch at home, and I removed every single supply diffuser grill in the ceiling in the house. Every one had glaring gaps between drywall and supply register boot. Every one of them got sealed. I also went to most of the electrical outlets and switchboxes and installed gaskets behind the plates.

My comfort level in the house on cold days improved after that, particularly in one room I occupy frequently that used to be a bit chilly beforehand...the office. With all that insulation in the attic just by itself, the thinking going in was we should've been snug as a bug in a rug. We weren't. Why? Too much air exchange in the house, too much air exhausting the house via penetrations in the ceiling adjacent to the attic. I could literally blow insulation into that attic until it reached the roof deck at the ridgeline...without sealing those gaps we'd still have a drafty house.

For summertime behavior, an attic with an unsealed ceiling, PAV or no PAV, will either suck air from a house, or if the attic becomes sufficiently pressurized via wind or superheated air with poor ventilation, that super hot air will push INTO the house via the ceiling penetrations vs. vent conditioned air to the attic. This is known as reverse stack effect. Either way, air exchange between house and attic is not a good thing. Period.

And, if recessed lights are leaking, it would cool the light fixture and surroundings.

At the expense of make-up air via infiltration into the house to replace the conditioned air being used to cool can lights and attic insulation. Sounds like a silly way to spend comfort cooling dollars, yes?

As for fart fans and kitchen exhaust fans, they typically have short run times. For that reason they are not considered when performing a Manual J load calculation as for affecting air exchange in a house. However, when the fans run they do create loss of conditioned air via increased infiltration to make up for air lost to the outdoors via the exhaust openings.

[Daltex]

Did you not have any ridge vents? Was your attic totally sealed?

If your refering to me- As I stated, no other ventilation other than a PAV that the motor went out. Yes, other than the soffit vents and bad PAV, it was totally sealed.

This is why I wanted to know if there was hard data on the PAV vs. Ridge Vents. I only have knowledge of sealed vs. PAV.

[Daltex]

......Additionally, air movement through insulation nukes the R value. Air coming up from the house, air moving over the top of the insulation via PAV induced velocity...two sources of air movement working against the insulation's role to retard heat transfer into the house.

Air moving over the insulation via PAV induce velocity? A mouse farting would move more air over the insulation.

......I could literally blow insulation into that attic until it reached the roof deck at the ridgeline...without sealing those gaps we'd still have a drafty house.

For summertime behavior, an attic with an unsealed ceiling, PAV or no PAV, will either suck air from a house, or if the attic becomes sufficiently pressurized via wind or superheated air with poor ventilation, that super hot air will push INTO the house via the ceiling penetrations vs. vent conditioned air to the attic. This is known as reverse stack effect. Either way, air exchange between house and attic is not a good thing. Period.

I agree 100% that gaps are the #1 priority when considering heat gain/loss in the conditioned space. This is why closed cell foam is such a huge bang for the buck long term.


At the expense of make-up air via infiltration into the house to replace the conditioned air being used to cool can lights and attic insulation. Sounds like a silly way to spend comfort cooling dollars, yes?

The air in my cans is way hotter than the attic air so- Yes, loss of small amount of conditioned (hottest conditioned) air into the attic to remove the hotter can heat is a great way to spend conditioned $. If using fluorecent bulbs in the can then Yes, you are correct. I don't use fluorecent in the cans. The makeup air is only a problem if it's coming from the attic. Doors and windows wouldn't make near as much heat gain vs. attic. Thus seal attic tight!

The PAV's need to be chosen like an AC/Heating system. You must calculate the flow rates and have an adequite amount of soffit/gable incoming air to match it. 1000-3000 cfm is pretty passive regarding the entire volume of an attic. What make it problematic is when you don't have adequite intake and you create to much negative pressure.

Still want documentation on properly designed PAV vs. Ridge Vent.

[Joe Mascitti]

I would bet the farm that the contractors have no info on how many soffet vents to put, nor the size of them. They just slap a 4X12 every 4 feet down the soffet line and that's it. There's no way it is matched to the PAV, and therefore in 99% of the cases, the PAV creates a negative pressure effect in the attic and that draws conditioned air thru the attic stair case, can lights, cracks, etc. thru the attic and inside while pulling in hot air from the outside thru drafty doors, windows and fireplace vents.