Correct temp setting for attic fan

[wyliesdiesels]

Did a search and didnt find much.

Im wondering what i should set my attic fan thermastat to. It was set to 95* F by whoever last touched it. I turned it down to about 85* to see if it makes a difference. I just moved in so I want to get everything setup before the summer heat arrives!

It was also unplugged. ly Some ***** also did a redneck splice where they cut the hardwire going to the fan, installed an outlet and then poorly spliced in a computer cord onto the end of the Romex

Don't know why they didnt either hardwire the fan into the back of the outlet or install a switch so they could disconnect the fan....but thats a topic for a different discussion...i will be fixing that in the coming weeks....

I live in the scorching central valley where weve had temps as HIGH as 108* in the summer.

I dont want it running all the time but also dont want the heat load in the attic to affect the A/C

Insulation(blown in cellulose) is decent in most places- about 8" and the house was built in 1996....about 1100 sq ft and it looks like the AC condenser is a bit oversized @ 3tons!

 

[Zeke]

I set mine at 110º otherwise it would run all the time.

 

[dw1]

I set mine at 110º otherwise it would run all the time.

That is what I also set them at when I hook them up

 

[mpire]

From what I have read, there is great debate on attic fans.

Mostly, because they create negative pressure in your attic that ***** the conditioned air out of your house and ends up costing you more.

I keep thinking I might want to put a fan over the garage, but I am not sure about cutting holes in my roofing.

 

[dw1]

Attic fans have come a long way, they now can have built in Humidistats to help remove humidity as well as circulating and removing heat

 

[larry_g]

I don't have an attic fan but common sense would tell me that I would want the thermostat to be set above the ambient temperature outside otherwise you could be removing cooler (85*) air and and replacing it with hot (108*) outside air. It would even make more sense to have a differential thermostat that senses both inside and outside air and if the outside was cooler then turn on the fan.

lg
no neat sig line

 

[wyliesdiesels]

When I got home last night the fan was on and the sun was down. I had to crawl back up there and turn the T-stat backup otherwise it wouldve ran most the night....guess its a touch and go type thing...

I don't have an attic fan but common sense would tell me that I would want the thermostat to be set above the ambient temperature outside otherwise you could be removing cooler (85*) air and and replacing it with hot (108*) outside air. It would even make more sense to have a differential thermostat that senses both inside and outside air and if the outside was cooler then turn on the fan.

lg
no neat sig line

As i understand it, negative air pressure would be created IF there wasnt enough vents. My attic has quite a few soffit vents as well as another gable vent at the other end so i dont think negative air pressure will be an issue!

I do like your differential tstat idea!

 

[soob]

You're going to have negative air pressure in the attic anyway, because your a/c is forcing air into rooms inside, and it has to go somewhere. If the attic is sucking air with its own fans more air will go that way, especially if the return is blocked by things like closed doors.

 

[pseudorealityx]

You're going to have negative air pressure in the attic anyway, because your a/c is forcing air into rooms inside, and it has to go somewhere. If the attic is sucking air with its own fans more air will go that way, especially if the return is blocked by things like closed doors.

1) The mass of air in and out of an HVAC system is equal, no matter what.

2) Air being forced into rooms will follow the low pressure, which is typically the return side of the HVAC system. There's very little forcing it into the attic, and anytime you're ventilating the attic, you've typically got ample path for incoming air without it trying to pull through the ceiling of your house.

 

[dfiler2]

I'm assuming your fan is pushing air into the attic and out the vents. If that is the case you would never have "negative" air pressure in the attic, you could have negative air pressure in the home. I use an attic fan but only when I am not using the AC, it does a great job of keeping the upstairs and the whole house cooler before it gets really hot out.

 

[67carl]

I put an gable fan in my house and it does help. I live just a bit north of the OP so I can relate to the scorching summers! It is located in a gable end and pulls air out of the attic. I also have a gable vent on the other side of the house, ridge vents and I installed soffit vents (there weren't any before), so when the fan comes on it doesn't try to pull air from inside the house. Plenty of openings to pull outside air through. If you google "gable fan" you will see raging debates about negative pressure, sucking conditioned air into the attic, etc... As long as you have enough venting (ridge/gable/soffit) it won't be an issue. From my experience it makes a difference and I wouldn't let naysayers talk you out of it. Look at whole house fans as well. Take advantage of the delta breeze in the summer. Got mine on a timer so it comes on for a few hours during the coolest time of night. Works great.

What model fan are you using? Mine will come on at the temp I set (currently 115f), but will stay on until the temp gets to 110f, so it won't keep cycling on and off. Likely yours does that as well. The best thing I did and highly recommend is running the power through a single pole switch, located in a convenient location, so you can shut power off to the fan when you want to. I installed my switch in the garage so if I don't want the fan running I just flip the switch (it blows over the deck in the backyard so if we're sitting out there I can turn it off).

 

[soob]

1) The mass of air in and out of an HVAC system is equal, no matter what.

2) Air being forced into rooms will follow the low pressure, which is typically the return side of the HVAC system. There's very little forcing it into the attic, and anytime you're ventilating the attic, you've typically got ample path for incoming air without it trying to pull through the ceiling of your house.

The first part is true, for what it's worth, which isn't all that much. The question is how the balance is achieved (whether it's by air circulating in the house or circulating through the attic). Your second proposition just begs that question.

If you have fiberglass insulation, go up in the attic and look at it around penetrations in the ceiling. You'll find it blackened with dust. That dust got there because of air movement through those penetrations.

 

[pseudorealityx]

Yes, but it's typically not enough to worry about. If your 3 ton, 1200 cfm HVAC unit serving the house is losing 10 cfm to the attic, it's in the noise.

 

[soob]

I'm hardly the first person to have come up with this. Here's some actual data from a study:

http://www.homeenergy.org/show/article/year/1995/id/1165

Tooley and Davis took measurements from July through September 1993 at eight homes in Research Triangle Park, North Carolina. Each attic contained passive ventilation in addition to a powered ventilator. All the houses had some depressurization when the ventilator operated, with pressures ranging from -0.5 to -2.5 Pascals (Pa). The tests showed that the powered attic ventilators, on average, drew 231 cubic feet per minute (CFM) of conditioned air out of each house and caused, by themselves, 0.72 air changes per hour (ACH). This flow from the house to the attic ranged from a low of 104 CFM to a high of 646 CFM, and ACH ranged from 0.38 to 1.2.

231 cubic feet per minute is a lot. The fans sucked 3/4 of the air out of the house every hour.

 

[pseudorealityx]

Context is key.

"The high figures above were for the largest house in the study, which had two ventilators and noticeable air leakage paths between the house and the attic."

And

"Tooley and Davis concluded that if a powered attic ventilator is to be used, ... and ensure that adequate net free vent area is provided in the attic. The amount of air the fan moves must also be considered so that it can be properly sized (a bigger fan is definitely not better in this case), and pressure measurements should always be used to confirm safe operation."


Shocking... Some homeowner or lowest paid contractor threw in a/some ventilator(s) without adequate inlet area, and it made the whole house go negative.

 

[pseudorealityx]

1) The mass of air in and out of an HVAC system is equal, no matter what.

The first part is true, for what it's worth, which isn't all that much.

By the way... a 3 ton air handler moves over 5000 POUNDS of air in an hour, or almost 90 lbs of air per minute.

And for what it's worth, cooling supply air is ~5% denser than room air. It's about a 6% difference between hot supply air and room air.

 

[soob]

Context is key.

"The high figures above were for the largest house in the study, which had two ventilators and noticeable air leakage paths between the house and the attic."

And

"Tooley and Davis concluded that if a powered attic ventilator is to be used, ... and ensure that adequate net free vent area is provided in the attic. The amount of air the fan moves must also be considered so that it can be properly sized (a bigger fan is definitely not better in this case), and pressure measurements should always be used to confirm safe operation."


Shocking... Some homeowner or lowest paid contractor threw in a/some ventilator(s) without adequate inlet area, and it made the whole house go negative.

I'm sure your house is exceptional and the builders took their time, filling every hole they drilled in the top plates and the ceilings, etc., and that none of this applies to you. But, you know, maybe that's not true of everyone, people who had their houses built by ordinary contractors. Every house I've ever seen with power ventilators also had awful air sealing. Having adequate attic ventilation doesn't do any good if there are holes in the ceiling.

And, again, that's not one house that was studied, it was eight, and they found significant air was being pulled from every single one. The lowest was 38% of its air per hour. That's a lot.

 

[pseudorealityx]

I'm sure your house is exceptional and the builders took their time, filling every hole they drilled in the top plates and the ceilings, etc., and that none of this applies to you. But, you know, maybe that's not true of everyone, people who had their houses built by ordinary contractors. Every house I've ever seen with power ventilators also had awful air sealing. Having adequate attic ventilation doesn't do any good if there are holes in the ceiling.

And, again, that's not one house that was studied, it was eight, and they found significant air was being pulled from every single one. The lowest was 38% of its air per hour. That's a lot.

Still, context is key... how much air leakage does a normal house get from stack effect and/or just a ridge vent? That's effectively doing the same thing. Hot air is drawn up and out, requiring cooler air into the soffits/gables.

How much air do you lose when there's a 15 mph constant breeze through drafty windows and doors?

How much additional heat load does the HVAC system have to offset when the attic space is 130 degrees as opposed to 105 so your entire top floor ceiling is a big radiant panel?

I think you're making this sound much more dramatic than it is, or at least blaming everything on the powered ventilator.

 

[soob]

Your ceiling is insulated. The insulation... insulates the conditioned space from the attic's heat. But insulation doesn't do any good if the air is being exchanged. Your attic is being brought down to 105 degrees by cold air sucked out of your house. If you want to pay to condition the air in your attic, be my guest. Just remember that the roof deck is a lot hotter than the ceiling.

There's a reason nobody uses power ventilators anymore.

 

[rharman]

Like you said, it' a touch & go thing. Just tweak it as you get more experience.

I've had them for 23 years and have seen no ill effects.

 

[pseudorealityx]

Your ceiling is insulated. The insulation... insulates the conditioned space from the attic's heat. But insulation doesn't do any good if the air is being exchanged. Your attic is being brought down to 105 degrees by cold air sucked out of your house. If you want to pay to condition the air in your attic, be my guest. Just remember that the roof deck is a lot hotter than the ceiling.

There's a reason nobody uses power ventilators anymore.

Passive vents are the main reason you don't see powered ventilators anymore. Attic ventilation is still required by code. But the relative maintenance free approach of soffit vents + ridge vent is more cost effective than adding powered ventilators...

And those passive vents are still causing negative pressure against your ceiling. You keep avoiding all the other issues at play here.

 

[kamlung]

115F is what mine is at

 

[soob]

And those passive vents are still causing negative pressure against your ceiling. You keep avoiding all the other issues at play here.

They might, but it's not the same kind of pressure. It takes more pressure to push air through a small hole for a power vent than a big one for a ridge vent. More pressure pulls more air through small holes in your ceiling. You're just plain wrong about this.

 

[wyliesdiesels]

Wow! I knew this was a heated(pun intended ) topic but I didnt think i would get this many comments...

Great discussion keep it going

I'm assuming your fan is pushing air into the attic and out the vents. If that is the case you would never have "negative" air pressure in the attic, you could have negative air pressure in the home. I use an attic fan but only when I am not using the AC, it does a great job of keeping the upstairs and the whole house cooler before it gets really hot out.

Nope! Its sucking air out. I have 3-4 soffit vents in the front of the house, 2-3 in the back as well as a gable vent at the other end...

I put an gable fan in my house and it does help. I live just a bit north of the OP so I can relate to the scorching summers! It is located in a gable end and pulls air out of the attic. I also have a gable vent on the other side of the house, ridge vents and I installed soffit vents (there weren't any before), so when the fan comes on it doesn't try to pull air from inside the house. Plenty of openings to pull outside air through. If you google "gable fan" you will see raging debates about negative pressure, sucking conditioned air into the attic, etc... As long as you have enough venting (ridge/gable/soffit) it won't be an issue. From my experience it makes a difference and I wouldn't let naysayers talk you out of it. Look at whole house fans as well. Take advantage of the delta breeze in the summer. Got mine on a timer so it comes on for a few hours during the coolest time of night. Works great.

What model fan are you using? Mine will come on at the temp I set (currently 115f), but will stay on until the temp gets to 110f, so it won't keep cycling on and off. Likely yours does that as well. The best thing I did and highly recommend is running the power through a single pole switch, located in a convenient location, so you can shut power off to the fan when you want to. I installed my switch in the garage so if I don't want the fan running I just flip the switch (it blows over the deck in the backyard so if we're sitting out there I can turn it off).

Im not sure what model it is. It was hot in the attic when i went up there so i didnt stay long enough to bother reading the label...

 

[pseudorealityx]

They might, but it's not the same kind of pressure. It takes more pressure to push air through a small hole for a power vent than a big one for a ridge vent. More pressure pulls more air through small holes in your ceiling. You're just plain wrong about this.

The only two 'kinds' of pressure are static pressure and velocity pressure. A negative attic is a case of static pressure.

Negative static pressure in your attic is caused by the air escaping your attic. If you lose 200 cfm through a ridge vent or a powered ventilator, the attic is 200 cfm negative, and it doesn't care where the air went, or what force it to leave the attic. And that means it's going to pull 200 cfm from 'somewhere' to equalize that pressure. That 'somewhere' is a mixture of penetrations to your house, soffit vents, gable vents, every little crack that wasn't caulked perfectly, etc.

You're not actually positively pressurizing your house if you have a 100% recirculating HVAC system. You're pressurizing the ductwork but the house itself remains basically neutral.


If you really think I'm wrong on this, you need to explain yourself better.

 

[BadgerBoilerMN]

Soob has it right.

The key is to keep your conditioned air, conditioned. The cooling system should have a neutral effect on the envelope pressure. Even a window shaker simply moves inside air through the coil condensing moisture and reducing rH in the space.

The problem with attics, even my super-foamed attic, is primarily radiant energy. Convection has little effect in the middle of summer, in most climates. If the low night temperature is relatively high there is little relief. Drawing moist exterior air into an attic dried out by the elevated heat does nothing for the latent heat--the part that really makes a body uncomfortable.

Better to fan you than the attic.

http://www.energyvanguard.com/blog-...e-1-Reason-Power-Attic-Ventilators-Don-t-Help

http://www.greenbuildingadvisor.com...ons-powered-attic-ventilators-are-waste-money

 

[pseudorealityx]

Soob has it right.

Disagree.

The key is to keep your conditioned air, conditioned. The cooling system should have a neutral effect on the envelope pressure. Even a window shaker simply moves inside air through the coil condensing moisture and reducing rH in the space.

This agrees what *I* said, not with what soob said.

The problem with attics, even my super-foamed attic, is primarily radiant energy. Convection has little effect in the middle of summer, in most climates. If the low night temperature is relatively high there is little relief. Drawing moist exterior air into an attic dried out by the elevated heat does nothing for the latent heat--the part that really makes a body uncomfortable.

You just contradicted yourself. If the problem is radiant, then latent heat has nothing to do with it. Moist air doesn't radiant any more/less than dry air at the same temperature. Is the problem a radiant heat issue, or a humidity issue?

Better to fan you than the attic.

This I agree with. But it wasn't the OP's question.

http://www.energyvanguard.com/blog-...e-1-Reason-Power-Attic-Ventilators-Don-t-Help

http://www.greenbuildingadvisor.com...ons-powered-attic-ventilators-are-waste-money

That's the same article, copied to 2 different websites.

And the original article, linked here: http://www.energyvanguard.com/blog-...c-****-Power-Attic-Ventilators-Are-a-Bad-Idea
.... is garbage. The author uses extremes as anecdotal evidence.

They do reference this report: http://www.fsec.ucf.edu/en/publications/pdf/FSEC-CR-1496-05.pdf

However, if you read it, it makes many conclusions based on old research. For instance, at one point on page 6( of PDF), it's comparing forced ventilation vs. passive, saying that a cooling load reduction of 140 watts vs a consumption of 284 watts. 284 watts is 0.38 hp. Today, you can easily find roof fans that consume 1/10th that power, not even looking at solar powered fans.

If a powered ventilator is backdrafting a water heater or pulling crawl space air up in significant quantities, then there's 1 of 2 things happening.

1) not enough venting within the attic itself. That's a design issue.
2) the attic ventilator is sized way too large. That's also a design issue.