Do power attic fans help?

[Shophound]

Do the calcs! Removing 70,000 btus/hour to maintain <110^F requires 4,000 cfm of air flow! The fan power is more than the a/c and the attic is still 110^F. I may want to continue selling attic fans because I make money doing it. OK- now I understand. Regards TB

LOL!

So, at 4,000 CFM and 70,000 BTUH gain, you gotta bring in air from outdoors that is 16 degrees cooler than 110^F, or 94^F. Wow. Once the temp goes above 95, you either increase airflow or attic temp creeps over 110.

At 1,500 CFM, a common rating for a PAV, to hold 110^F in an attic at 70,000 BTUH gain would require make-up air that is 44 degrees below 110,^F or 66 degrees, to keep the attic at 110.

So...with typical summer conditions, the best one can hope for by forced ventilation is to cool an attic off from, say, 130 - 140 to 110 - 120. That's not bad, but we're still not at the ideal state of an attic being as near to ambient as possible. With RB and good natural ventilation, it might be an equivalent reduction vs. a non RB attic with a PAV. The difference being the passive solutions don't draw conditioned air from the house at the same rate a PAV would.

It seems to keep coming back to my earlier bullet points...do all of it, and the solar PAV seems a viable option if the ceiling adjoining an attic is well sealed. I would LOVE to see study data that reflects such an approach, as to how effective it might be.

[Daltex]

LOL!

.....but we're still not at the ideal state of an attic being as near to ambient as possible. With RB and good natural ventilation, it might be an equivalent reduction vs. a non RB attic with a PAV. The difference being the passive solutions don't draw conditioned air from the house at the same rate a PAV would.

It seems to keep coming back to my earlier bullet points...do all of it, and the solar PAV seems a viable option if the ceiling adjoining an attic is well sealed. I would LOVE to see study data that reflects such an approach, as to how effective it might be.

This is too funny. I love the fact that someone can calculate my attics heat gain without even seeing, much less measuring the attic space or covering material.

The gain would be less on a day with a strong wind cooling the shingles. How is that factored in? The gain would also be exponentialy less as the heat level rises in the attic. A totally sealed attic with a black tar shingle wouldn't reach 300* on a hot summer day.

Does a ridge vent remove more cfm than a properly designed PAV? Come on.

Calculating the equipment and design of an HVAC system is relatively complex but it seems that the attic ventilation is often overlooked or "un-calculated". Without doing the minimal math to figure intake air to match the cfm of the PAV could lead to a big problem. Having a leaky ceiling could do great harm as the previous link regarding the commercial drop ceiling showed.

I think the answer for me is to stick with the PAV due to my roof design. If someone asked me what they should get, I would certainly say to try and use ridge vents if possible and if not then just turbines if possible. Try and seal all ceiling penitrations and ductwork. Use a RB. You can't over supply the intake air (soffits or gables). Thats my 2 cents.

[Joe Mascitti]

I just remembered my crude test I performed last summer after reading all the negatives about the PAV's. This is what made me shut it off. Don't laugh...I shut the upstairs AC unit off and sprayed a strong air-freshner throughout. After about 30 minutes when the scent was gone, I pulled down my attic steps and went into the attic (Pav is running)...the smell of air-freshner was strong. Flipped the switch off on the PAV.....

Does anyone have an idea how to temp seal the fireplace flue (bat-insulation)...I know the downstairs unit has to be pulling air from the flue...it can't seal any better than the attic steps.

I installed my attic tent last Monday...Took 30 minutes...great product, we will see what happens come summer...

[Shophound]

This is too funny. I love the fact that someone can calculate my attics heat gain without even seeing, much less measuring the attic space or covering material.

The gain would be less on a day with a strong wind cooling the shingles. How is that factored in? The gain would also be exponentialy less as the heat level rises in the attic. A totally sealed attic with a black tar shingle wouldn't reach 300* on a hot summer day.

Does a ridge vent remove more cfm than a properly designed PAV? Come on.

Calculating the equipment and design of an HVAC system is relatively complex but it seems that the attic ventilation is often overlooked or "un-calculated". Without doing the minimal math to figure intake air to match the cfm of the PAV could lead to a big problem. Having a leaky ceiling could do great harm as the previous link regarding the commercial drop ceiling showed.

I think the answer for me is to stick with the PAV due to my roof design. If someone asked me what they should get, I would certainly say to try and use ridge vents if possible and if not then just turbines if possible. Try and seal all ceiling penitrations and ductwork. Use a RB. You can't over supply the intake air (soffits or gables). Thats my 2 cents.

I was just playing with Teddy Bear's numbers. It wasn't meant to apply to your attic, or anyone else's in particular. The discussion may have taken this turn as...how often do folks stop long enough to think about this kind of stuff (once you get past the "why should I give a crap" factor)? We may not hit dead on with our analysis first pass...seldom does that happen with consistency. But I see nothing wrong with thinking out loud, as long as it isn't baseless gibberish. If I've strayed toward that end, it wasn't my intent.

If you want to stick with a PAV, go for it. It's still (at least somewhat) a free country.

[brian in mass]

http://www.masscertifiedhomeinspecti...ndex.php?D=116

[texasmax]

It if it had been a full month since the last post, I would feel bad resurrecting this one, but...

So let me ask this. Most homeowners, including myself, don't know how tight their house/attic boundary is. Regardless of what that tightness is, as long as there's some flow through, there's going to be a point of attic exhaust ventilation (assuming matched intake/soffit ventilation) at which you're maximizing getting excess heat out of the attic while not sucking up any (or a minimal amount) of cooled air from the house. Without conducting a through experiment as to what this point of exhasut ventilation would be, I'm figuring you have the choice of purposefully:
-erring on the side of too little exhaust (leaving some excess heat in the attic), or
-erring on the side of too much (expelling all the excess heat, but also pulling cooled air up out of the house and into the attic).

In terms of $ and comfort, I certainly couldn't say whether 100cfm on the too little side vastly outweighs 100cfm on the too much side, or vice-versa. Even if 100cfm of too much hurts a lot more than 100cfm of too little, couldn't someone play it "safer" by upping their exhaust ventilation with a PAV of reasonable capacity (or even just a number of non-powered turbines), and adding in 2x the corresponding intake soffit venting?

[captube]

Saw this driving by.
If these are PAVs as they appear from the distace i'd like to be there when they fire them off.

[Daltex]

Saw this driving by.
If these are PAVs as they appear from the distace i'd like to be there when they fire them off.

Hope they didn't reverse wire them. The house may lift off.

[Kevin O'Neill]

Hope they didn't reverse wire them. The house may lift off.

I think it is an alien spaceship in disguise.

[dan sw fl]

I think it is an alien spaceship in disguise.

Absolutely, it has the same B-2 Stealth look.
So, maybe it is not so alien afterall.

... You mean PAVs are high tech. L.O.L.

[energy_rater_La]

attic temp is constant 80 degrees..even if it is 100+ outside,
now inside the house..must need a bigger unit...right??
can't keep mold growth down & its always hot..
what a concept!

[Brian GC]

Absolutely, it has the same B-2 Stealth look.
So, maybe it is not so alien afterall.

You mean the F-117 Stealth Fighter?

Brian

[Shophound]

It if it had been a full month since the last post, I would feel bad resurrecting this one, but...

So let me ask this. Most homeowners, including myself, don't know how tight their house/attic boundary is. Regardless of what that tightness is, as long as there's some flow through, there's going to be a point of attic exhaust ventilation (assuming matched intake/soffit ventilation) at which you're maximizing getting excess heat out of the attic while not sucking up any (or a minimal amount) of cooled air from the house. Without conducting a through experiment as to what this point of exhasut ventilation would be, I'm figuring you have the choice of purposefully:
-erring on the side of too little exhaust (leaving some excess heat in the attic), or
-erring on the side of too much (expelling all the excess heat, but also pulling cooled air up out of the house and into the attic).

In terms of $ and comfort, I certainly couldn't say whether 100cfm on the too little side vastly outweighs 100cfm on the too much side, or vice-versa. Even if 100cfm of too much hurts a lot more than 100cfm of too little, couldn't someone play it "safer" by upping their exhaust ventilation with a PAV of reasonable capacity (or even just a number of non-powered turbines), and adding in 2x the corresponding intake soffit venting?

Reducing the amount of heat reaching the attic to begin with is the best approach, hands down. Radiant barrier, insulation between roof deck and attic...whichever method chosen to keep an attic cooler (even the spray-on RB's, with the right quality paints, are considerably more effective than doing nothing or relying only on a PAV) via reducing initial heat gain, you're on your way. PAV's had their day...energy costs were lower and no alternatives such as radiant barriers were widely known or readily available. I now consider the PAV approach merely less desirable and less effective than the passive alternatives.

As for the house in the photo with what appears to be an invasion of landed UFO's near the ridge of the roof...sometimes those are passive ventilators, an alternative to ridge venting and/or whirlybirds. Usually they are square in appearance, but could be round. If they indeed are PAV's, that is HUGE overkill...I wouldn't want to be near that electric meter when it comes flying off the wall!