Attic fan OK with Central Air?


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Old 07-16-09, 02:18 PM
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Question Attic fan OK with Central Air?

When a heating tech was here several weeks back, he made a comment in reference to efficiency of a power-chimney on the oil furnace being "... like having central air conditioning with an attic fan, it is sucking out the cool air".

I'm not sure if he was referring to my setup -- I do have central air and an attic fan. This is not a "whole house fan", in that it does not draw intake from the living space -- it is just a thermostatically controlled fan mounted on the attic vent. The attic air is neither heated nor cooled.

Am I really losing much efficiency of the air conditioning by using the attic fan?

Thanks,
JimR
 
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Old 07-16-09, 02:33 PM
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I had that setup in the 70s...the first attempt at reducing energy use. My experience was they get noisier with wear, wobble, etc. There also is some fire danger as they age. I never thouight it made sense to spend money to run a motor all dayand half the night in order to try to save a little electricty use. That aside, more current thought is that most homes have air leakage between the attic and the conditioned space, so the attic fan pulls conditioned air into the attic. That's where he was coming from. To me if the attic breathes good it won't have a chance to get it from below. It doesn't blow ALOT of air, the blades are only like 12" dia or so. 21st centrury solutions include.....ridge vents and soffit vents....outside air runs across the hot decking and out the top...thats what I have. No moving parts, natural ventilation.
 
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Old 07-17-09, 01:55 AM
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Originally Posted by flaflashr View Post
When a heating tech was here several weeks back, he made a comment in reference to efficiency of a power-chimney on the oil furnace being "... like having central air conditioning with an attic fan, it is sucking out the cool air".

I'm not sure if he was referring to my setup -- I do have central air and an attic fan. This is not a "whole house fan", in that it does not draw intake from the living space -- it is just a thermostatically controlled fan mounted on the attic vent. The attic air is neither heated nor cooled.

Am I really losing much efficiency of the air conditioning by using the attic fan?

Thanks,
JimR
The rocket scientists are elsewhere, building rockets. If your attic temperature goes to 120F on a hot afternoon, your ceilings will get proportionally warmer and deliver more heat load to your air-conditioned living space, which your air conditioner will have to remove.

Will that fan also unintentionally suck some of your nice cold air up into the attic and waste it? Maybe, but you should fix those leaks - they're wasting your "cold", fan or not. Paths might include ceiling light fixtures, combustion air for the furnace, etc. The attic fan will mostly be drawing air from attic vents. (Don't block the combustion air path - but you can make sure it stays in the furnace room, not the living space.)

Is it worth running a 1/3 hp motor for several hours a day? In Los Angeles, ours is set for 105F and tends to come on around 1-2 PM and goes off around 6 or 7 PM. It made a very noticeable difference on the aircon duty cycle and I think in the electric bill, but the aircon is old and low SEER.

The attic fan was a ho-hum model from Home Depot, and has been running for at least five years without noticeable increase in vibration. I can tell when it's on if I listen for it. If one day I need to replace it, no big deal and not particularly expensive.

No-motor vent methods? Sure, but cutting one hole in the roof might be so much less trouble that it's the best choice.
 
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Old 07-17-09, 04:08 AM
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Building code requires gas/propane furnaces to get their combustion air from the exterior, so a 4" access to the attic is the common remedy.
 
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Old 07-17-09, 09:29 AM
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Whether you need a attic fan (we call it roof fan) or not depends on the air temperature in your attic and the attic insulation you have. If your attic air is above 120 degree, then you may want one provide you have enough eve vents to provide the air intake. then if you have extra thick attic insulation(say 18 icnhes), even 120 degree, they won't affect your rooms under it. A/C system may be (just a may be) another concern if you have them installed in the attic. SO, several factors need to be considered. no simple answer. But in this part of the country (Texas), lots, lots of houses have roof fans installed, some large houses have 2 or 3 of them on the roof.
 
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Old 07-17-09, 10:56 PM
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Originally Posted by srercrcr View Post
Building code requires gas/propane furnaces to get their combustion air from the exterior, so a 4" access to the attic is the common remedy.
Certainly. But the furnace closet (assuming such an installation) can be quite well sealed from the living space, so that won't be a path for conditioned air to be "sucked out" by the attic fan.
 
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Old 07-18-09, 01:08 AM
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Yes but if it needs to get air it can pull it though the return air duct, which is conditioned. A well ventilated attic though is ideal.
 
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Old 07-18-09, 06:08 AM
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when your driving around and you see those large silver globe shaped vent hoods turning on big warehouse spaces thats the heat build up in the ceilings that is turning then...not the wind.the attic fan ther is minimal to vent the attic more then to do air changes,and if you had a fan setup like the size of your AC supply fan then your talking serious ait changes.i've been in large residential attics and they had fans with blades the size of a small prop plane mounted at one end of the attic on the wall and dampers at the other end....and they couldn't sleep with the noise during the night and the neighbors complainted to boot!the size of an attic fan versus the size of the attic is pulling the heat out but minimal on air changes.most defintly takes a load off the running AC if you can drop the attic 30F from the outside temps hitting the roofs on a sunny day
 
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Old 07-18-09, 09:32 AM
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Originally Posted by srercrcr View Post
Yes but if it needs to get air it can pull it though the return air duct, which is conditioned. A well ventilated attic though is ideal.
I thought it was required that combustion air and return air had to be sealed from one another. Not saying that's true in old houses, but would be something up there on the to-do list when puttering around with attic ventilation.


Someone mentioned more insulation, often a good idea. My first problem with that is I can't find any unfaced insulation to add.
 
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Old 07-29-09, 12:13 PM
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an energy audit with a blower door test will show you where all the air leaks from the attic down to the main space. Except for 2 places i never thought of, mine is sealed pretty well and actually he told me the door to my attic is super tight. Not letting anything through it.
 
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Old 07-29-09, 03:55 PM
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luckydriver, what 2 places didn't you think of?
 
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Old 07-30-09, 06:55 AM
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going from my utility room to my attic is a bunch of wires from my electrical box feeding the rest of the house. This includes one fresh hole made big enough for a 220 spa line. all those holes were even visibly drafty (if you knew to look there and see the dust bunnies moving in the wind) but once the blower was put on you could really feel the air sucked from the attic. However i dont wish to seal that up because my boiler is right there and i believe fresh air is a good thing in this case. No evidence to back this up, just a preference. Though i do have 2 drafty aluminum windows and an old wooden door that probably is providing enough fresh air

2nd place was a rather wide crack (approaching 1/4 inch at some parts) in the plaster in the kitchen, running probably 6 ft across the ceiling. I just never put 2 and 2 together and thought air could escape there. I saw evidence it does. So i do plan on sealling that before winter.

and there was another issue i totally forgot about, all my lightswitches and outlets are very drafty..need to fix those too! thanks for the reminder.
 
 

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