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I have a whole house fan installed in my house from the previous owners. I love the fan, as it lets me cool down the house quickly in the evening -- or after I fill the house with smoke from forgetting about food on the stove! :)

However, as I try to air seal my house it is immediately apparent that the whole house fan is by far the leakiest thing in the house. Under a blower door test, the fan louvers actually open up and most blower doors can't achieve the pressure they normally operate at in order to perform a measurement.

I would like to install some sort of air tight louver or door on top of the fan in the attic that will close tightly until the fan switch is turned on, at which point the louver or door will open. The closest idea I've had for that so far has been a plywood door that closes over the fan against weather stripping and is opened and closed by a linear actuator, and then attaching insulation to the top of the door, but it really seems like overkill to build and maintain a system that complex.

I'm also aware of one commercial air sealed and insulated whole house fan, but it is $1,500 and pushes 2500 CFM less than my current fan. $1,500 buys a whole bunch of plywood and linear actuators if I do end up having to go that route.

I've also looked at motorized dampers, such as this one from Grainger, but due to the small width of each blade I wouldn't be able to put any reasonable amount of insulation on them. Also, they're rated for 3500 CFM for some reason, and the fan I have easily does at least 5500 CFM.

Does anyone have any better ideas and suggestions for how to accomplish this? Note that I am not interested in solutions that involve manually removing a lid, seal, or cover from the fan; this needs to be automatic.

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Is it really such a big deal to have it be a manual cover? I would just make it part of your winter prep routine: remove window screens, shut off outdoor faucets, remove leaves from gutters, etc. –  Henry Jackson Aug 21 at 20:53
    
@HenryJackson It would be a huge deal. That would leave it open all summer long, sending my daytime AC literally out the roof. –  David Pfeffer Aug 22 at 17:17

4 Answers 4

Check around the farm supply places - they carry big dampers. I used to have one about 36" square (the ex still has it). Try places like here and here.

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Great thought but I haven't seen any that carry dampers. The sites you linked for example don't carry any. –  David Pfeffer Aug 22 at 17:18

Brainstorming here, but a heavy enough door with a rubber gasket, that can rest on a gasket on the fan might do the trick. the levers to lift the door away will have to be strong enough. The only other idea is a set of electromagnets that could "pull" the door shut...wish I could offer a better solution.

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You could make an insulated panel that is spring-loaded to the open position. Keep it closed with door holder magnets like this .You could wire the fan and magnets to a double pole switch that cuts magnet power at the same time it turns on power to the fan. Might cost a bit and take some research... but it would be cool (pun intended)!

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Build a box that goes over the whole fan (on the attic side) in the winter-time. weatherstrip the bottom and weight as needed. This is also my preferred approach to the attic stairs that don't seal worth a darn. Duct-board (foil-stiff_fiberglass-foil) is probably the best material if you can find it - use aluminum foil duct tape for the joints. Nobody seems to stock it around here; I heard of it in books and on TV.

Otherwise XPS insulation sheets are more widely distributed and can be stuck together well with canned polyurethane foam. They have a somewhat higher fire risk potential, which is the main reason I'd prefer the duct-board if I could find any to buy. Put a note on the fan switch reminding you to remove the box before turning the fan on.

Ah - right, you don't want to do that - fine, put it on rails (garage door parts?) and have a motor pull it up out of the way.

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Other than the OP's "automatic" requirement, this is the right answer. You can even buy insulated covers pre-made at big box stores. –  Henry Jackson Aug 21 at 20:46
    
Doing it only in the winter leaves it wide open in the summer. My house does 12 air changes per hour at 50 Pa with the fan unsealed, and with a piece of plywood held against it (which isn't even 100% air tight) my house drops to 3 air changes per hour at 50 Pa. That's an enormous difference in summertime cooling costs. –  David Pfeffer Aug 22 at 17:20
    
A door blower test is useful for identifying the location of problems, but in this case it's probably exaggerating the rate of air loss—normally your house is not pressurized and the louvres stay shut (though leaky). I don't have a perfect solution for you but I think a cover in winter is a good start for two reasons: 1) in NJ your climate is still heating dominated (looks like approximately 4:1 ratio of heating degree days to cooling degree days), 2) AC-conditioned air is cool and will not tend to rise to the fan louvres, unlike heated air. Any air that does escape in summer is probably warm. –  Henry Jackson Aug 22 at 18:39

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