Do power attic fans help?

[printmanjackson]

Do power attic fans do a better job of venting the heat out? I have heard some people say they are a waist of money because the motors won't last long running in the heated condition they run in. I have 3 in my attic and all 3 motors are done. I want to replace them with either a higher quality moter or take them out completly and use something else. I live in Tennessee so our arttics here get pretty hot.

[lentz]

They seam to help a little. I have to change them out about every three years. A ridge vent works much better. You do have to have opening in the boxing or in the eaves for the air to circulate.

[adrianf]

http://www.fsec.ucf.edu/en/publicati...sec-gp-171-00/

Go to conclusions for summary and this is with a solar/battery fan.
HTH

[grumpy20716]

If home has ridge vent system we will not install or repair fans.The fan will not allow the ridge vent to work as designed

[deejoe]

IF an attic space has the proper amount of soffit and ridge (or other high ,near the peak ) vents, then no other type of venting is ever needed.
Gravity air flow, entering the soffits and exiting high near or right at the peak will do the trick.
No electrical powered or high priced turbine fans are required.

[platchford]

If you have a ridge vent then don't bother replacing the fans. If you don't have a ridge vent then you may consider adding a ridge vent rather than replacing several fans.

[mark beiser]

Powered attic vents, aka PAVs, pull conditioned air up out of the house through light fixtures, wiring penetrations through the top plates in walls, the attic access, and any other path it may find.
The more passive ventilation from outside, the less air it will pull up out of the house, but it will always be pulling air out of the house.

I have been in a number of homes where the difference between not cooling to the set point on a 85-95º day, and maintaining the set point up through our 100º outdoor design condition, was simply to disconnect the power to the PAV.

See the 2nd part of my signature.

[printmanjackson]

Powered attic vents, aka PAVs, pull conditioned air up out of the house through light fixtures, wiring penetrations through the top plates in walls, the attic access, and any other path it may find.
The more passive ventilation from outside, the less air it will pull up out of the house, but it will always be pulling air out of the house.

I have been in a number of homes where the difference between not cooling to the set point on a 85-95º day, and maintaining the set point up through our 100º outdoor design condition, was simply to disconnect the power to the PAV.

See the 2nd part of my signature.

This has been one of my major conserns about using power attic fans, sucking the conditioned air out of the house. That said, this house does have plenty of soffit vents.

Switching to a ridge vent system would seem to be very labor intensive unless a new roof was being installed.

[Brian GC]

This has been one of my major conserns about using power attic fans, sucking the conditioned air out of the house. That said, this house does have plenty of soffit vents.

Switching to a ridge vent system would seem to be very labor intensive unless a new roof was being installed.

PAV’s may very well be too powerful to be used in an attic, especially multiple units. I’ve used inexpensive turbine fans for years with good results. With a slower and more consistent exhaust, they will draw only through the existing eave vents, not affecting interior air.

[hvaclover]

Powered attic vents, aka PAVs, pull conditioned air up out of the house through light fixtures, wiring penetrations through the top plates in walls, the attic access, and any other path it may find.
The more passive ventilation from outside, the less air it will pull up out of the house, but it will always be pulling air out of the house.

I have been in a number of homes where the difference between not cooling to the set point on a 85-95º day, and maintaining the set point up through our 100º outdoor design condition, was simply to disconnect the power to the PAV.

See the 2nd part of my signature.

Amen to that. If your using any attic fans at all you might as well have a whole house attic fan going while running your a/c. It would be just as bad as opening all your windows and trying to cool the house with a/c. Ain't gonna happen cause you are pulling in hot air at a higher rate then the ac can handle.

[wolfstrike]

i guess every house needs to have the attic inspected to make a good decision.

i was at a customer's house last month, the outdoor temperature was just warm enough to turn on the AC, but when i went into the attic, the attic was well over a hundred and something degrees.

the attic was completly sealed and i think they could have really profited from an attic fan,
but i wouldn't place one there without installing a vent on the other side.



my mom's house could use one too, her attic has openings under the roof overhang, and its still too hot.

[printmanjackson]

I found this on another site about venting:

>The rule with attic ventilation is 1 : 300 with a vapor barrier and 1 : 150 without a vapor barrier. So if you had insulation in the attic with a vapor barrier and the attic was 900 sq. ft., you would need 3 sq. ft. of free venting. If it was blown in insulation without a vapor barrier, you would need 6 sq. ft. of free venting. This is when you have an equal amount of square footage of high and low vents ( gable and soffit).

Your concern about attic fans is correct. The problem is not the fan but the over sizing of attic fans. Most attics can only handle about 1500 cfm of ventilation because of the vents. Your fan that you have probably moves about 2000 to 5000 cfm. In which case it could cause a problem with drawing air from the home and causing the problems you mentioned.

Properly sizing the fans is important because of this. You could increase the size of the vents but you're looking at more than doubling the amount. You're much better off putting in the proper size fan. The volume of air in the attic remains constant. So if the vents only allow 1500 cfm and the fan moves 2500 cfm, it will seek to get the air from somewhere, like the home. If the fan couldn't get the air from anywhere else, you would just burn out the fan.<

[platchford]

This has been one of my major conserns about using power attic fans, sucking the conditioned air out of the house. That said, this house does have plenty of soffit vents.

Switching to a ridge vent system would seem to be very labor intensive unless a new roof was being installed.

Not really assuming you have a typical shingled roof. Consider this, you install a shingles starting at the bottom and working your way up. Since the ridge vent goes at the top you don't actually have to disturb any of the lower shingles. Generally, one would simply use a circular saw to cut a parallel line about 6-12 inches from the peak of the roof. Remove the shingles and OSB or underlayment that was cut away. Install the ridge vent material. Re-install shingles at the peak of the roof over the ridge vent. A new roof by comparison takes considerably longer.

[captube]

With ridge vent just make sure you have enough ridge to provide the adequate free opening required for proper ventilation.
I see a lot of cut up roof lines that obviously don't provide enough ridge to vent properly.

[mark beiser]

I found this on another site about venting:

>The rule with attic ventilation is 1 : 300 with a vapor barrier and 1 : 150 without a vapor barrier. So if you had insulation in the attic with a vapor barrier and the attic was 900 sq. ft., you would need 3 sq. ft. of free venting. If it was blown in insulation without a vapor barrier, you would need 6 sq. ft. of free venting. This is when you have an equal amount of square footage of high and low vents ( gable and soffit).

Your concern about attic fans is correct. The problem is not the fan but the over sizing of attic fans. Most attics can only handle about 1500 cfm of ventilation because of the vents. Your fan that you have probably moves about 2000 to 5000 cfm. In which case it could cause a problem with drawing air from the home and causing the problems you mentioned.

Properly sizing the fans is important because of this. You could increase the size of the vents but you're looking at more than doubling the amount. You're much better off putting in the proper size fan. The volume of air in the attic remains constant. So if the vents only allow 1500 cfm and the fan moves 2500 cfm, it will seek to get the air from somewhere, like the home. If the fan couldn't get the air from anywhere else, you would just burn out the fan.<

The fan doesn't know it is only supposed to draw air through the vents provided for it.
If you have a fan, regardless of size, drawing air from the attic, it will create a pressure in the attic that is lower than that in the house.
Where there is a pressure difference, and a path for air to flow, the air will follow it.
Putting in a smaller fan only reduces the CFM that will be drawn from the house.

Then there is the whole thing about pulling hot air out of the attic having very little impact on the total amount of heat in the attic.
Attic ventilation does absolutely nothing at all to reduce the radiant heat transfer in the attic, and the hot air pulled out represents only a relatively small portion of the total heat that is moving through the attic.

Block the heat from coming in, ideally with a roof system that does so, but more practically with a radiant barrier if your ductwork and/or air handling equipment are in the attic, and provide adequate passive ventilation.

If your ductwork and/or air handling equipment are up there, it is more cost effective to just blow in more insulation, and not worry about how hot the attic gets, unless you are in an area where winter ice damns are an issue.

[printmanjackson]

>The fan doesn't know it is only supposed to draw air through the vents provided for it.
If you have a fan, regardless of size, drawing air from the attic, it will create a pressure in the attic that is lower than that in the house.
Where there is a pressure difference, and a path for air to flow, the air will follow it.
Putting in a smaller fan only reduces the CFM that will be drawn from the house.<

Please explain .... how can the pressure in the attic be lower than inside the house when the same amount of air is entering the attic that is leaving? I would think with the proper amount of soffit vents it would offer less restriction than anything else in the house.

[deejoe]

Printman; installing a continuous ridge vent is quite simple. Remove the existing ridge shingle caps ,cut a 1" piece (strip) from each side of the sheathing at the peak and then fasten the lengths of ridge vent along the peak.
A continuous ridge vent is best, but you could install a few properly placed "mushroom" type vents close to the peak instead. They will do a good job at exiting air.
For every 300 sq ft of attic floor area you require a minimum of 1 sq ft of vent ,divided 50/50 between the soffit and ridge vent.
Example: if attic floor area is 900 sq ft, then you would need a minimum of 3 sq ft of vent. 1.5 for the soffit and 1.5 for the ridge.
144 sq in = 1 sq ft. (easy math,eh?)
Remember, the more vent area, the better, and it will also take inanyrestriction that "insect screening" in the vents will create.

[Shophound]

Please explain .... how can the pressure in the attic be lower than inside the house when the same amount of air is entering the attic that is leaving? I would think with the proper amount of soffit vents it would offer less restriction than anything else in the house.

As Mark said, the fan doesn't care where the air comes from. The entire attic will be in a negative with a PAV in use. If the typical gaps in construction found in typical American house ceiling are extant, there isn't much resistance for interior air to move into the attic. If you want a visual reference, if your house has ceiling mounted supply air grills for the a/c, pull one or more of those grills off and note the gap between the drywall and the supply duct boot. Nine times out of ten it's significant, and the most overlooked area to consider when tightening up the building envelope.

Need even more of a visual reference? Check this out, from my own house prior to my efforts to seal it:

[mark beiser]

>The fan doesn't know it is only supposed to draw air through the vents provided for it.
If you have a fan, regardless of size, drawing air from the attic, it will create a pressure in the attic that is lower than that in the house.
Where there is a pressure difference, and a path for air to flow, the air will follow it.
Putting in a smaller fan only reduces the CFM that will be drawn from the house.<

Please explain .... how can the pressure in the attic be lower than inside the house when the same amount of air is entering the attic that is leaving? I would think with the proper amount of soffit vents it would offer less restriction than anything else in the house.

1000 small holes, that collectively add up to 1 square foot, will collectively allow the same quantity of air through that a single 1 square foot hole would.

Pretty much every bit of independent 3rd party research I have seen on the subject comes to the conclusion that PAVs are not worth the investment in the up front cost, maintenance and operating cost, even in attics that are very well sealed off from the conditioned space.

Since I have gotten into measuring house leakage, I have had the opportunity to actually measure exactly how much air PAV's were pulling out of a number of homes.
The worst one I found was a 6,000ish square foot house with three 1600 CFM PAVs very little passive ventilation, and a large number of non air tight recessed light fixtures and poorly sealed thermal bypasses. With all of the fans that roofers installed last spring, about 2800 cfm was being pulled out of the house. the main part of the downstairs couldn't maintain a 78º set point on a 85º day, with an AC system that was about 20% oversized for keeping it 75º on a 100º day with normal infiltration.

The best one I have tested was pulling about 150 cfm out of a 1800 square foot house. They had a 1200 cfm PAV, with 2 gable vents and lots of soffit vents, 8' ceilings with no recessed light fixtures or attic access within the conditioned space. The PAV was causing a little over half an air change per hour, mostly from pulling air in through electrical outlets and light switches through the wire penetrations in the top plates of the walls.

[cem-bsee]

I think that too many make lots of 'blanket' statements which assumes lots--

I have posted results for my house during July 20th --
my PAV seem to help --
--at least the attic does not get overly hot --
-- HOWEVER, I do not have the test instrumentation to do a definitive test
-- 'it is hard to have one's cake and eat it too'
----- I have tried to have the PAV on | off on simular 'hot' days [95+]
-------but, this involves getting onto the roof or into a cramped attic & unplugging the PAV.

-- btw, I HAVE plugged my penetrations, replaced my regular screening for the gable vents with 0.25" hardware cloth & replaced all of my eve vents.

be aware, ridge vents should not be used with this house since the roof pitch is too low -- wind would probably drive rain into the attic --

be aware, here we have high RH [>85%] overnight until 10am, most days! --
--- during ALL seasons.

the power consumption of my fan is but pennies per day! so that is not an issue!

each household has their own lifestyle -- what works for me is probably unacceptable for most -- [my elec billing last 31d = $60 = total elec, = resistance heating. 4y ave = $70/mo.

this house has LOTS of shade= no spot on my dark asphalt singled roof gets full sunlight more than 2h/d during the summer.

I have lots of cellulose on the ceiling, covering ~80% of the 8" ceiling joists; a trilevel with a Gambrel roof over the bdrms;2133sf,8ft ceilings. I have tracked my utility billings for 8+y-- can recite btu/DD/cf.