Do power attic fans help?

[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!

[Joe Mascitti]

IF those are 5 PAV's...the clothes would get sucked off your back while walking down the hallway...

[adrianf]

The House Is A System
•Research at the Advanced Energy Corporation showed that 40%of the air exhausted by a PAV is conditioned inside air!

The House Is A System
•Convection loops move only 20% of the heat from the roof deck to the attic floor!
•Radiant heat transfer is responsible for 80%+ of the heat transfer.
•So, you could create Hurricane Katrina in your attic and only affect 20% of the heat transfer.

The House Is A System The House Is A System
•Lessons:
3.“Improve”attic ventilation and run the risk of making the house more humid, more likely to grow mold, cause the electric bills to go up and possibly cause a fire or CO poisoning!

[Shophound]

If "The House Is A System" was more widely understood, we'd be well on our way to seeing not only greater energy savings, but higher comfort level satisfaction within our homes. We've been trained to seek active (read: energy consuming) solutions for problems caused by passive sources that have been inadequately considered. Concurrently we'll always need some level of mechanical climate control due to the design of most homes and the various climates we all live in. It just makes sense to place focus on the dwelling structure itself to do more of what it is supposed to do...give us shelter. Shelter not only from rain and sun, heat and cold, but the vagaries of climate that we all must live with.

[adrianf]

If "The House Is A System" was more widely understood, we'd be well on our way to seeing not only greater energy savings, but higher comfort level satisfaction within our homes. We've been trained to seek active (read: energy consuming) solutions for problems caused by passive sources that have been inadequately considered. Concurrently we'll always need some level of mechanical climate control due to the design of most homes and the various climates we all live in. It just makes sense to place focus on the dwelling structure itself to do more of what it is supposed to do...give us shelter. Shelter not only from rain and sun, heat and cold, but the vagaries of climate that we all must live with.

I have personnaly invited and paid for GCs to attend building envelop seminars. Of the ones that I have invited very few have accepted.

[mark beiser]

I passed by a house last week that had 8 PAVs visible from where I was. I didn't have time to turn around to go take a picture, but I'll try to remember to get one soon.
It was a large 6000+ sq ft home.
They were large PAVs too, probably rated for 1200+ CFM each...

The House Is A System

Amen to that!

[printmanjackson]

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.

looks like someone will be greasing the meter

I wonder how loud they are. I bet you can heard them from the street!

[steve3871m]

Some info:
http://www.energystar.gov/index.cfm?...ic_ventilation

http://www.eere.energy.gov/buildings...pdfs/26450.pdf

http://search.nrel.gov/query.html?qp...ic+ventilation

[texasmax]

Thanks all for the excellent information and discussion

[energy_rater_La]

I think it would be a wonderful thing if more people would understand the
house as a system.
One hvac owner I deal with has an ongoing argument with me about pav's.
Until I 'prove' to him that they don't work he will keep installing them.
Unfortunately..any 'proof' is by quote college educated idiots' therefore
not to be believed.
I avoid the topic as much as possible with folks with that mindset, they will
never chabge their minds.

good discussion folks. Long..but good!

[adrianf]

Two quick qualifing questions.

Is the earth flat or round?

Is smoking 2 packs of unfiltered cigarettes a day unhealthy?

But for the record the college educated idiots can be just as dense. My example: Measured air flow through a 5 ton system was 1260 in and 1260 out the recommendation was to clean the coil. The degreed individual didn't think there was anything wrong because you were getting out of the unit what went in

"In my experience, logic did not convince anybody at any point of time. As I read in a book the other day, only when the spoken word appeals to the other person’s self-interest, will or thought process, will logic be of any use. Logic primarily is to be used for rational beings, or inanimate objects, say with computers. Logic works perfectly in such a system because they are devoid of their own thought process and neither do they possess a sense of individuality and hence self-interest. Human beings are primarily emotional creatures. In an emotional atmosphere, there is no place for logic." Kiran

[Brian GC]

One hvac owner I deal with has an ongoing argument with me about pav's.
Until I 'prove' to him that they don't work he will keep installing them.


good discussion folks. Long..but good!

Hasn’t it been determined here that not all PAVs are bad…only the ones used with inadequate soffet and gable vents? In less than ideal situations, reducing motor rpm to 200 – 300cfm would make for a perfectly operating PAV. Low power with low cfms. Disconnecting a PAV should never be the first choice, it’s illogical.

Logic keep us out of trouble, emotions get us into it. Look at the animal kingdom, they’re 90% logic and do very well.

Brian

[mark beiser]

Hasn’t it been determined here that not all PAVs are bad…only the ones used with inadequate soffet and gable vents? In less than ideal situations, reducing motor rpm to 200 – 300cfm would make for a perfectly operating PAV. Low power with low cfms. Disconnecting a PAV should never be the first choice, it’s illogical.

Logic keep us out of trouble, emotions get us into it. Look at the animal kingdom, they’re 90% logic and do very well.

Brian

I disconnect them every chance I get, but I have the training and test instruments demonstrate and quantify the negative impact they have on a particular house.

At least in my area, the cost to fix the average existing home in my area, so that a PAV doesn't negatively impact the building envelope, would be far higher than any alleged potential savings/benefit from having it running.
It would not be economically feasible, if even physically possible, to seal many of the homes I see every day to a level that would prevent a PAV from causing unintended negative consequences.

Then there is the issue that virtually all of the homes in my area with gas furnaces and water heaters draw their combustion air from a ventilated attic, most of the time the appliance is physically located in the attic.
You can bet your a** I'm disconnecting any PAVs I find in those homes.

If you are building a new house, I'm a big fan of moving the the insulation to the roof line, and sealing the attic. It makes one hell of a lot more sense than having to deal with super heated air in an unconditioned attic.
Ventilated unconditioned attics in cooling dominated climates, with the air handling equipment and ductwork in the attic, are a pretty dumb idea, but it has been the norm for decades.

[Brian GC]

I disconnect them every chance I get, but I have the training and test instruments demonstrate and quantify the negative impact they have on a particular house.

At least in my area, the cost to fix the average existing home in my area, so that a PAV doesn't negatively impact the building envelope, would be far higher than any alleged potential savings/benefit from having it running.

Though my post was quoted you did not respond to it.

I'll pose the question again. Isn't reducing the PAV motor speed better in most cases than disconnecting it?

Brian

[paul42]

Though my post was quoted you did not respond to it.

I'll pose the question again. Isn't reducing the PAV motor speed better in most cases than disconnecting it?

Brian

In most cases, the answer is NO. Disconnect it.

Put in a radiant heat barrier, add more soffit vents, add insulation, seal all of your outlet and switch boxes (with the breaker off!), etc... But - disconnect the PAV.

[Shophound]

Hasn’t it been determined here that not all PAVs are bad…only the ones used with inadequate soffet and gable vents? In less than ideal situations, reducing motor rpm to 200 – 300cfm would make for a perfectly operating PAV. Low power with low cfms. Disconnecting a PAV should never be the first choice, it’s illogical.

Logic keep us out of trouble, emotions get us into it. Look at the animal kingdom, they’re 90% logic and do very well.

Brian

Keeping the term "logic" in mind, which approach is more logical?

• Install radiant barrier on underside of roof deck to minimize radiant heat gain to an attic. Radiant heat is said to be over 70% responsible for heating the insulation laying on the attic floor, which then transfers that heat to the conditioned spaces below via conduction and radiation
• Do not install radiant barrier, but rather rely on a powered attic ventilator to reduce attic air temperatures. Heated attic air is said to be under 30% responsible for heating the insulation laying on the attic floor, which then transfers that heat to conditioned spaces below via conduction and radiation

Logic would dicate the radiant barrier solution provides higher return on investment. It is a once and done proposition. The PAV needs an ongoing power source to be useful. When it is not running, it is useless. The radiant barrier never shuts down, it is always available to work, with no ongoing consumption of energy.

Appealing to logic, does reducing a PAV's CFM output from 1,200 to 300, either with radiant barrier or without it installed, make sense for increasing the PAV's effectiveness at reducing that ~30% heat gain from superheated attic air? Earlier in this discussion either I or someone else crunched numbers at what volume of air would be needed to keep an attic at outdoor temperatures on a, say, 100 degree day. The amount of air needed was tremendous. Reducing the airflow to reduce air removal from the conditioned space would be countered by the increased heat gain the attic would experience with less ventilation, resulting in greater heat gain to the attic floor insulation and structural members.

Emotions do not always get us into trouble. Combined with logic, they give us the push to see a matter through, and to do it well. Animal kingdom logic is eat or be eaten...as humans, we can do better than that.

[Shophound]

If you are building a new house, I'm a big fan of moving the the insulation to the roof line, and sealing the attic. It makes one hell of a lot more sense than having to deal with super heated air in an unconditioned attic.
Ventilated unconditioned attics in cooling dominated climates, with the air handling equipment and ductwork in the attic, are a pretty dumb idea, but it has been the norm for decades.

Since when did tract home builders in Texas ever think of doing anything smart when it comes to HVAC and building thermal envelope design?

Granted, most builders came of age in an era of cheap energy. Throw the ducts and equipment up into a superheated attic. Skimpy overhangs on south facing facades. A building envelope punctured like swiss cheese. Lofty interior spaces difficult to heat and cool due to gross stack effect. A roofline more complex than college algebra. An entry that could allow a giraffe to comfortably fit in. All done so when the potential buyer rolls up to the curb, he says, "Wow! I want that house!"

Gotta love it!

/rant