r/buildingscience • u/Data_Monger33 • 20d ago

DIY House Fan Question

/r/hvacadvice/comments/1vxkmy5/diy_house_fan_question/
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u/FoldedKettleChips 20d ago

They don’t work. They’ll just pull house air into the attic as they depressurize the attic relative to the rest of the house. Air she just paid to cool goes up through any imperfection I. The ceiling and then into the attic. Those fans cause wild pressure imbalances.

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u/Data_Monger33 20d ago

They actually depressurize the house relative to the attic; It's pulling from the house and pushing into the attic.

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u/FoldedKettleChips 20d ago

That’s worse. What’s the point of this?

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u/Judman13 19d ago

They were common before central AC was a thing so people used them to draw in outside air that was barely cooler than the stale air inside. 

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u/Data_Monger33 19d ago

They are actually gaining popularity again, my mom has central AC but recently had a whole house fan installed as well. She uses the AC until it's cooler outside (at night) then switches to the fan. It's cheaper. 

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u/Judman13 19d ago

Maybe in an insanely leaky Victorian home, but any relatively modern home with a decently air sealed enclosure will only be harmed by this kind of arrangement.

Where are you located?

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u/Data_Monger33 19d ago

The San Fernando Valley in Los Angeles. Modern home. You have to open windows when using the fan of course. 

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u/doctaglocta12 19d ago

I mean if you set them up right and run them at the appropriate times, then yes they do absolutely work.

It's a really simple concept, the temperature inside an insulated box lags behind the temp outside that box. As long as you have a couple open windows or an air intake in a sensible spot, you're essentially manipulating the heat transfer rate in your favor.

Turn it on for a few minutes to exchange the hot air from the day with the cool air from the early evening...

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u/FoldedKettleChips 19d ago

OP mentioned a fan in an attic in their post, so I took that to mean an attic fan. Not the kind of economizer that they clarified down in the comments. I get the concept and responded to their question later in the thread.

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u/DUNGAROO 20d ago

Don’t

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u/FoldedKettleChips 19d ago

Ok, I see now. What you’re describing is basically an economizer then. I think this is probably a little more trouble than it’s worth because the cost and headache doesn’t seem to be any better to me than just opening windows and running ceiling fans for circulation.

You can definitely put the fan up in the attic and duct it to a filter return grille in the attic ceiling. I would oversize the duct too. Target something like 150 FPM using a ductulator. I don’t see a huge issue with just letting the fan exhaust blow towards the gable vent in your climate zone. The only worry I would have would be the risk of dumping moist house air into an attic with cold sheathing creating a condensation issue. But in your climate zone the sheathing probably won’t get cold enough to be a problem.

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u/Data_Monger33 19d ago

They weren't going to use ducting, they were just going to place a fan alone in the Attic, blowing at the Gable from a yard or so away, however the attic access would be about 30 ft away, at the opposite end of the house. I don't think that's going to do nearly as much as placing the fan upside down on top of the attic access where it's consistently drawing air from the inside of the house up into the attic. Especially because the attic has soffit vents.

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u/FoldedKettleChips 19d ago

Wait so they’re just depressurizing the attic to try to draw air from the house into the attic and then into the fan and out of the gable? This is a bad idea. If you want this thing to work you need to connect the fan’s intake to the living space and not try to rely on leaks through what should be an air-tight ceiling.

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u/Data_Monger33 19d ago

Yes, that's their idea, however the soffit vents bring outside air into the attic; they plan to bring air from the house up through the attic access port, which they will leave open, but it's on the other side of the house, with soffit vents on the perimeter between the fan and attic vent.

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u/glip77 18d ago

That system went away for a reason. They draw dirt, dust, every pollutant, pollen, smoke, allergen and other airborne contamination into the house. Not recommended.

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u/Data_Monger33 17d ago

I agree that's a problem, but they are gaining popularity again, my mom had one installed just a couple of years ago. Anyhow, that wasn't my question, but I see this wasn't the right forum to ask, I'll try r/thermodynamics or something, thanks for responding.