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I've been researching fans and the ones I've found all seem to vent to the attic. Why is that? Why can't they vent directly to the outside? It seems more efficient that way. Otherwise you have to have anothe fan to vent the air in the attic.

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    If the attic is vented properly it shouldn't need it's own fan. The pressure from the whole house fan should be enough to move it out of the attic. – Steve Jun 30 '16 at 15:15
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    Cooler house air vented into the attic pushes hot attic air out the attic vents, what's not to like about that? Two ventings for the price of one. – Fiasco Labs Jun 30 '16 at 15:58
  • What if you don't want to kick up dust in your attic? We may have some asbestos fibers in the attic. – milesmeow Jun 30 '16 at 18:08
  • In that case, I think you answered your own question. :) You could lay down housewrap over the insulation, but that doesn't sound fun. – isherwood Jun 30 '16 at 18:57
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    If you've got asbestos, then you need to remove it anyway. It's drifting into your house proper all the time. – Carl Witthoft Jun 30 '16 at 19:10
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As was said in the comments on your question, you get additional efficiency by clearing hot air in the attic in the process. Your house air is maybe 75-80* F, while your attic can be 100* or more. Ventilating the home and the attic results in greater overall cooling by reducing heat that would conduct back through any attic insulation.

I assume here that whole-house ventilation is used exclusively for cooling, and that we're not discussing heat recovery ventilators (HRVs), which are typically only used for bath and kitchen ventilation and are installed according to strict protocols, which wouldn't allow attic discharge.

  • What about Andy's comment about condensation? – milesmeow Jun 30 '16 at 18:06
  • When does a person vent a home during cold weather? Aren't you paying for that heat? The question doesn't involve bath fans, which is a different issue. – isherwood Jun 30 '16 at 18:26
  • Good point @isherwood. – milesmeow Jun 30 '16 at 18:27
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    I do. We're not talking HRVs here, which are mostly tied to bath fans. In my experience, HRVs are not used as whole-house ventilators. – isherwood Jun 30 '16 at 18:50
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    Here, HRVs are used as whole house ventilators. Not just for kitchens and baths. That's the point. You cannot safely transfer warm climate practices and experience to cool climes. And until you've lived in a cool clime, I guess that wouldn't be evident. So Miles, if you live in a warm clime, you may be OK doing as isherwood says. If you're in a cool clime, especially if you get freezing weather, your house will suffer by venting into the attic. There's extensive explanations here: nrcan.gc.ca/energy/efficiency/housing/home-improvements/… – AndyW Jul 1 '16 at 1:49
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There are three reasons why whole house fans generally vent into attics. First, whole house fans are designed to pull air from the highest point in the living space of a house, which is generally where the warmest air collects. This removed air is replaced by cool air entering windows in the first floor, flushing the warm air out of the house. The highest point is generally the ceiling of the top floor, so the obvious route for the air is at least into the attic. However, whole house fans are designed to handle large amounts of air, so it's better to vent the fan into the attic rather than directly out an enormous vent in the roof or gables.

Second, the attic is often the low-rent district of the house, which makes it a good location for the fan mechanism itself.

The third reason was raised in another answer: cooling the attic will reduce the heat load on the main house. However, a properly insulated house (at least in non-tropical climes) will have enough insulation between the top floor ceiling and the attic itself that it won't matter much what the attic temperature is; the heat won't have much of an effect on the floors below. Still, there is some benefit to having a cooler attic.

Add all this together, and you find the majority of whole-house fans will vent into the attic, with the attic in turn vented to the outside by any of a wide variety of (generally passive) methods.

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Depending on your climate, venting a bathroom or other house fan to the attic is a very bad idea. If you live in a cold climate (outside temp lower than room temp), the colder attic will cause condensation of the warm moist home air or even freezing of the water vapour in the attic, potentially leading to mould or rot of the wood, or ice in the attic insulation. These are all undesirable.

Vent any house fan directly to the outside. This is usually done with a flex duct run from the fan outlet, through the wall or ceiling to a vent cover on an outside wall (not through the roof, unless you install a fan roof vent specifically designed for this purpose).

The attic should be properly vented on it's own with soffit vents on the bottoms and ridge or roof vents near the top, and not rely on house fans to motivate air flow in the attic.

  • Yes, the attic should have soffit vents, ridge vents, etc. No, it's not a problem running a whole house fan so long as it's not mounted in a bathroom. – Carl Witthoft Jun 30 '16 at 19:11
  • I guarantee you it's a problem in subzero weather. So if you never see snow, then maybe. Otherwise, don't try this at home. – AndyW Jul 1 '16 at 1:16
  • What sort of idiot runs a whole house fan in the winter? Nobody I know of in New England (and we've got plenty of idiots here) – Carl Witthoft Jul 1 '16 at 11:32
  • Do you still cook and breathe in the winter? If you read this: energy.gov/energysaver/whole-house-ventilation and this: en.wikipedia.org/wiki/Heat_recovery_ventilation carefully, you might learn something. Maybe even enough to not call those who practice what you don't understand "idiots". – AndyW Jul 1 '16 at 16:58
  • Well, Andy, apparently you don't know that, left all alone, any house that's not superinsulated completes an air exchange all by itself every 20 mins to 2 hours. And further don't seem to understand the difference between a kitchen exhaust vent and a whole house fan. – Carl Witthoft Jul 1 '16 at 17:55

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