Solar Attic Fans
 

Scam or needed? Finding articles leaning both ways and need help deciding if this is waste of money and possibly harmful or is it a great idea and everyone should have them. Thanks for any help,

There are a few "independent" assessments.

Here is one.

Are Solar Attic Fans Worth It? - Waypoint Inspection Property Inspection

"maybe a five year payback"

:popcorn::popcorn:

I saw that one. Not so worried about cost as much as are they really doing anything helpful too.

If that is where the bar is set, "doing anything helpful", the answer is yes.

That's an extremely low bar...........no one says they do nothing.

Buy one. :shrug:

Some say they pull air from inside house, but with such low CFM, and all the soffit vents that’s not a concern. But pulling from the ridge vent in, well that would be counterproductive, possibly harmful. Cost at about $2k for two large ones from Solar Guys installed, I misspoke if I say cost for reverse airflow would not be a problem.

In my opinion, they are not worth buying. Even if you use the cost saving figures in the cited article, the payback period would be 15 years, based on a $2,000 cost versus $750 used on the article. But, I also doubt that you will save as much electricity as they claim.

Here's the alternative that I used. I had Solar Guys install an electric (not solar) garage fan at a cost (10 years ago) of approximately $100. It is installed to pull air through garage door vents, into the garage, and out through the fan pulling the air into the attic.

It keeps the garage quite cooler, shaded air temperature, which is fine for us.
I'd say it also keeps my whole house cooler since the attic gets ventilated too.
Electrical cost is negligible and I think I save on A/C costs since my utility bills seems pretty reasonable.

Energy-Saving Solar Garage Fan Install | The Villages & Central FL

Does it have a fire damper?

Not needed, no payoff period.

Let's look at actual facts for attic fans.
I had a 120 volt, thermostatically controlled fan and the means to track what time of the day it came on and which days. The fan came on late morning, typically 10:30 to 11:00. More telling is the shut off. From 10:30 PM to 2:00 AM.

Next, consider solar panels on the fan are not binary. They generate power based upon the position of the sun, the energy generated determines the efficiency (speed) of the fan.

So, this means as you need a fan to expell the heat from your attic, which is heated by the hot roof long after solar noon your fan's speed is slowing and finally shutting down.

Finally, consider where the heat from your home comes from. It is a box with six sides, only one is impacted by a cool attic. Actually this has been measured. An UNINSULATED ceiling is responsible for 25 - 30% of your heat load. That is one reason your attic is the best insulated of all six sides of the box you call home.

For illustration let's say that your ceiling is not insulated and let's use a overly optimistic assumption that the solar fan will reduce your attic temp by 10% lower than it is already, that every month is August and your electric bill is 100% for electricity (no tax, absolutely no fees).
We can make this even more ridiculous and say that every bill is 250.00, that you need two solar fans to pull off this feat (@900.00 ea). All of this with the realization that about 55% of your bill is HVAC looks like this:

$250 (bill) x .55 (% of HVAC) x .3 (heat gained from ceiling) x .1 (savings from lower attic temp = 4.13 savings per month.

$1,800 (cost of fans) / $4.125 (monthly fan savings) = 436 months pay back. That's 36 years, four months after you remove all your insulation and devise a day for the sun to shine longer.

Any questions?

Any fan the function is to move air. In a closed system, such as your attic, at best a partly closed system it is like sucking air out of a coke bottle. It is impossible unless air gets in. In you attic, you use the fact that hot air rises, reason you have vents on the bottom taking in cooler outside air and vests on the top venting the hottest air.

All of these sales pitches use numbers to SELL a product that are if all is ideal. Things in the real world are never ideal. Installing fans in your attic creates a slight negative pressure. You should add extra intakes more risk of animal entry or the fans will suck
air you paid to cool into your hot attic.

Will; it truly pay off?

I believe your electric company will GIVE you an energy audit. What to do to save money and estimate of what you will save is likely a good place to start.

they pull the hot air out of the attic.. the only issue with solar is I can't hear if it's working. I have one and have no idea if it's working or not..

In theory the fans (solar or 120v) pull air in from all sources (ridge, Soffit, and a/c cooled home via leaks) equally, this theory doesn't consider friction. In other words the fans pull air predominantly from the closest source as that air moves easily to the fan.

I've already illustrated that there is no HVAC savings to justify solar fans there are two other aspects that are worthy of discussion. Will your attic be cooler and will your roof last longer?

More ventilation would lower attic temp and this extend shingle life but only if there is enough of it. I can go though the math of it but frankly it's more effort than I'm willing to spend here. Suffice to say two fans aren't enough to make a measurable difference.

But additional static vents could make a difference as they are so inexpensive that you can put six to ten more on a designer home with ridge vents and effectively double your roof peak ventilation.

That's exactly what I did.

EXACTLY what I was about to write. I sent my drone up to see if I could tell, nope. Stuck my head in the attic, nope, couldn't tell because my entry is in the garage nowhere near the fan. Finally, I went up on the roof and found it was spinning. No idea if it's doing any good or if I wasted $500 (or whatever it was ~6yrs ago). Didn't get it to SAVE MONEY just hoping it would make the A/C work more efficiently with an (allegedly) cooler attic. Probably could have done without it. Up north we had one that could suck you out of the house through the attic entry when the steps were down. :D Sounded like a P-51 when it was running but it definitely dropped the attic temp noticeably. It was 110AC obviously not solar.

We also have garage exhaust fans put in by Solar Guys. It obviously won't get the garage below ambient outdoor temps, but it helps and helps the attic.

One thing I have found is that the fan they put in creates a serious pressure drop in the garage (the vents are not large enough) and so, when we open the door from the house to the garage the door is sucked closed. If you stand there with the door open, the cool air in the house is sucked out. So, at the moment I open the garage doors about 3 inches which mostly moves that problem. I am considering putting more vents into the garage doors to reduce the pressure drop in the garage.

That's interesting about the pressure. Added vents should solve the issue. I've not had that problem but I have a fairly small 1-1/2 car garage. I think I read that you should have 1 square foot of fan / venting for each 100 square feet of garage and I've got a tad over that, maybe why I've not seen the pressure.