attic fan/thermostat

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Old 01-11-05, 07:01 PM
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attic fan/thermostat

We have an attic fan that comes on when the thermostat reaches a certain temperature--what that temperature is, I don't know. Anyhow it made a huge racket when it started & stopped this summer and we waited until now to address the problem. We have not looked at the attic physically (it is not easily accessible) but before we attempt it would like to know if we can even turn on the fan if the temperature isn't hot enough--that is do these fans usually have an on/off switch as well? Also do they really remove the hot air from the attic sufficiently enough to keep the house cooler? Is it worth repairing/replacing? We do have central A/C. Also we notice that when the fan works, the louvres seem to open magically--is it forced open by the fan? Any suggestions would be appreciated.
 
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Old 01-11-05, 07:43 PM
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Is this a fan that blows air from the attic to the outside, or a fan that blows air from the house to the attic?

In any event, it is possible to wire in a switch that can turn the fan on even when under the thermostat temperature, and/or wire in a switch that can turn the fan off even when over the thermostat temperature. But I can't say that it is necessarily typical to do either one of these, so you may or may not have one or both of these switches.

If the fan blows air from the attic to the outside, then it is probably installed (an in your case, replaced) from the gable or roof (whereever it is). Whether it is worth it probably depends on your climate and other factors. But from what I've read, fans that blow air out of the attic usually save no more energy than they use.

Yes, the loovers probably are pushed or pulled open by the fan.
 
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Old 01-11-05, 08:59 PM
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It blows air from the attic to the outside. If what you say is true that it doesn't save energy, that may explain why our utility company (PG&E) only gives rebates for whole house fans (blow air from house to attic). Why would whole house fans be better--wouldn't that just put hot air in the attic? how does the hot air leave the attic or does it matter? In any case it looks like we should just leave the fan alone. which is ok with me since it, tho not especially noisy when running, can be heard when it is running. Thanks for your input.
 
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Old 01-12-05, 07:42 PM
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I installed a whole house fan last year, and I really like it. You turn it on only after the sun goes down on summer days, when the outside air is cooler than the air in the house. And of course you wouldn't want to use it and the air conditioner at the same time. Although some people use a thermostat with them, I don't like a thermostat on a whole-house fan for multiple reasons.

It benefits you in at least three ways:
  1. It draws in the cooler outside air to lower the temperature in the house.
  2. It creates a breeze in your house, making you feel better (and freshing up the inside air at the same time).
  3. It blows cool air into the attic, forcing the hot air out of the attic at the same time through the attic vents. This only works well of course if you have good attic ventilation. I added a bunch of new eave vents in addition to the existing five roof vents before I installed the fan. Good attic ventilation has other advantages too--it prevents ice dams in the winter and extends the life of the shingles. The minimum ventilation required by building codes is generally not enough.
Whole house fans are best in climates where the house picks up a lot of solar heat during the day, and the temperature goes down after the sun sets. A whole house fan uses about 5 or 6 amps of 120-volt power, much less than an air conditioner. And an hour or two in the evening is all you need. Of course, it won't do much for you at 2 in the afternoon, so you might also need that air conditioner.
 
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Old 01-12-05, 09:04 PM
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A whole house fan sounds good--where do you install it?
 
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Old 01-12-05, 09:14 PM
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A whole-house fan is usually installed in the ceiling in the center of the highest floor of the house. Often an upstairs hallway.
 
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Old 01-13-05, 01:47 PM
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Where do you live? Nobody below the Mason-Dixon line has a whole-house fan, up north they are very popular. They work best where you can quickly cool the house down after the sun goes down, and then because of cool morning temperatures, the house will stay cool well into the day.

Attic ventilators cool the attic temperature and create air flow in the attic to keep it from stagnating. They definitely help cooling efficiency, because pulling the hot air out of the attic allows the warm air in the house to rise into the attic. They also prolong the life of your roof and roof covering by helping your attic breath.
 
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Old 01-13-05, 11:58 PM
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We live in NJ and we have both a whole house fan and 2 "mushroom" fans that are thermostatically controlled. The mushrooms are supposed to work during hot days to lower ambient attic temp to lower the burden on the AC. We use the whole house fan in the evening and it does everything noted in the replies here. Our thermostats have both died on the mushroom fans and we had one AC contractor suggest we ditch the mushrooms and add a thermostat to the whole house fan for the same effect during the day. It sounds logical, but he is the only place I've heard of such a thing.
 
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Old 01-14-05, 07:27 AM
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I don't like that idea. Whole house fans don't work unless you open the windows. You have to have supply air in order to exhaust air. They aren't made to cool attics; they're made to cool houses. That idea would be fine on days where you aren't running your A/C and are leaving your windows open. I think you have an excellent setup the way it is. Attic ventilator thermostats are easily replaced. If anything, you could install a timer switch on the whole house fan, if you don't have one already, so you can cut it on when you go to bed and have it cut off automatically after an hour instead of running all night (unless that's what you want).
 
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Old 01-14-05, 08:33 AM
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[QUOTE=mcjunk]I don't like that idea. Whole house fans don't work unless you open the windows... QUOTE] True -again only one guy told me that. I guess the theory is that with the windows closed it would draw throught the eave vents and cool the attic, but I can see where that would unduly load the fan motor.
 
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Old 01-14-05, 09:13 AM
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The whole house fan draws air from the house and blows it out the attic (and out the eaves and gable vents). In order to pull air out of the house, air has to get into the house somewhere. If you're house is sealed up good (no leaky windows & doors, etc.), then where will the air come from. You will create a window rattling negative pressure in the house, or the air will be somehow pulled around the fan from the attic and then blown back into the attic. Maybe the latter is what the a/c tech was counting on. I'm no HVAC mechanic nor engineer, and I've never owned a home with a whole house fan, so I may be wrong.
 
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Old 01-14-05, 09:55 AM
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Okay, here's why I think a thermostat is a bad idea for a whole house fan:
  • You must open the windows, so you don't want it coming on automatically when you're not home.
  • You only want it to come on when the outside temperature is lower than the inside temperature. Since a thermostat only measures the temperature at one place, it can't make that decision.
If you run a whole-house fan without the windows open, it will of course not be effective. Even worse, it will pull air from anyplace it can get it. If you have any gas-burning appliances (stove, water heater), one such place it will pull air is down the flue. Not only does this have the potential of introducing carbon monoxide into your house, it can blow out the pilot lights.

In my opinion, every whole-house fan should be installed with a timer. A simple mechanical "turn-past" timer works the best I think. I like it to shut off about an hour after I go to sleep. Then maybe I'll run it another hour in the morning when I wake up to pull in the cool morning air. If I do it just right, I get enough cool morning air in, then I shut up the house and the house won't get too hot for most of the day. Depending on your climate and heat tolerance, this may or may not allow you to get by without air conditioning. But it should certainly allow you to reduce your air conditioning use.

If you live in a humid climate, then dehumidification may be a more important function of your air conditioner than cooling. In this situation, the whole-house fan may not be so effective.
 
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Old 01-14-05, 10:12 AM
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We live in northern California, recent transplants from Minnesota where heating/cooling are totally different issues. Tho we do have central A/C, we'd like to reduce energy use if possible. If the attic ventilator fan is useful then we'd like to replace it--I notice these fans at Home Depot are relatively inexpensive but could not figure out from the box label instructions whether it has an on/off switch that overrides the thermostat and there was nobody to ask! Given our location is a whole house fan the way to go? Our humidity is only high in the early morning hours (fog too) but dissipates rapidly to very low humidity, like 40%--that's in general. We have had many days of 90's tho--sporadically, like in March, then in August & September.
 
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Old 01-14-05, 10:46 AM
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When you wire an attic ventilator fan, you mount a disconnect switch in line (within sight, generally right at the fan, for servicing) and then to the thermostat. The fan is prewired from the thermostat. The thermostat is not an on/off device, but is normally set for 95 degrees or so. There's really no reason to manually control the fans. They simply operate when your attic reaches a set temperature, which is when you will want them to come on.

As far as a whole house fan goes, you have to consider how often you might use it. I live in NC, and I if I had one I would probably use it 50 days a year (obviously guessing). Someone in Michigan might use it 150 days a year and not have central A/C. That's why they're more prevalent the farther north you go. Higher humidity in the south lessens their practicality. You have to balance the cost of the fan / installation with any energy savings you may realize. You may just want one to enjoy the cool breeze it provides inside the house.
 
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Old 01-14-05, 03:18 PM
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Why I wanted to know whether I can turn the fan on independent of the thermostat is because when it went on automatically, it made a huge racket--I just want to test it now to see what the problem is. It's in an attic space that is not easy to access. Our attic must get awfully hot if as you say it comes on at 95 degrees because in the summer, it often would go on around 10am and not turn off until 9pm. Is the thermostat hardwired for certain temperature range? The fan can be heard when it's on, somewhat annoying and definitely you are aware the fan is running.
 
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