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I recently moved into a new house and during the inspection, the inspector noted that there was a fan in the attic space above the garage.

He also mentioned that I could probably just unplug it because they do not typically install them anymore now that houses have air conditioners because:

  • The fan moves the hot air out of the attic area
  • This pulls air from the main floor of the garage into the attic
  • Which pulls the air-conditioned air out of the house and into the garage.

Thus making the air conditioner have to run more often. He said that they instead are now just putting vents in those areas.

Is this true? Are there other considerations I should take into account? The attic area is pretty nicely finished and will be used for storage of things we will use about once a year (mostly Christmas decorations). I do not think anything that would be stored there is super temperature sensitive.

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    Wow, I think he missed the boat on this one. Unless the garage is open to the house the fan draws the hot air from the vents in the attic helping to cool the hot area, This keeps the house cooler so the AC runs less. This is one of my first upgrades as it reduces my AC bill by 10X the cost of the small vent fan. It should be temp activated so it is not running when it is cold and raining outside or it could create mold in the attic space. also if you garage has access doors to the roof over the living space open them this will further cool the entire house. – Ed Beal May 20 '16 at 17:54
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    When you say "garage attic", what do you mean? A garage without a ceiling between the attic and garage? Or is there a ceiling between the attic and garage? – wallyk May 20 '16 at 18:00
  • @wallyk There is a ceiling between the garage and attic area – Josh Pennington May 20 '16 at 18:07
  • @Ed Beal that is the theory of their operation, but in practice they tend to draw non-trivial amounts of conditioned air out of the home, particularly if the attic vent area is small. If the building envelope isn't well sealed (which most aren't), they can be counterproductive. Even when the fans do work, they don't work as well as insulation or radiant barriers. – Zhentar May 20 '16 at 18:15
  • I have done this in several homes and in the first it extended the time until I had to turn the AC by several weeks, They even make solar ones so other than the install they cost nothing to run. In my first place I did this My AC bill dropped 100+/ month. If the garage is so poorly sealed it is drawing from the living space it would not pass code in Oregon. R40 in the ceiling wont stop 140-150 degree heat if it is trapped it heats up and acts like a heater all night, with the fan drawing cool air in my first house was well over 20 deg F cooler in the summer. – Ed Beal May 20 '16 at 18:39
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Negative pressure in your garage is far more likely to be relieved through flimsy overhead door seals than from inside the adjacent home (presumably through the service door weather seals alone).

However, it's probably not doing you much economic good to have a fan running in the garage attic. What little energy you save conditioning your home (with its separate attic space) may be offset by the cost of the fan operation.

If your garage gets uncomfortably warm in the summer it may be worthwhile to run the fan. Otherwise, it's apparently not serving a purpose.

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The fan is meant to remove excessively hot air to make the air conditioning less expensive. The exhausted attic air should be replaced by air from outside, not from inside the house. The garage ceiling should be (mostly) sealed to prevent air movement between the attic and garage. Unless it is a structure built before about 1960.

Also, the attic fan should have a thermostat which turns it on only when it is more than about 120 °F/50 °C in the attic. Check for a control; if there isn't one, obtain a thermostatically controlled outlet and install it.

  • How does cooling a garage attic help with home cooling? Seems to me that the relationship is negligible. – isherwood May 20 '16 at 18:00
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    @isherwood: I just posted a question to the OP seeking clarification of the situation. In our case, the attic over the garage is part of the rest of the house so it makes great sense to keep it all cool during hot weather. – wallyk May 20 '16 at 18:02
  • Fair enough. Around here garages need full-height firewalls, so a shared attic is not common. – isherwood May 20 '16 at 18:03
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Definitely keep the garage attic fan (and install one in the house if appropriate).

The one change I would make is to upgrade the fan (or convert it) to run on solar power. It's only needed on hot days anyway, which often have more than enough sun to run the fan. A nice simple first solar project!

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    I could get behind a solar powered fan. – Josh Pennington May 20 '16 at 18:36
  • they make solar attic fans about double the cost but the install is way cheaper (no wiring to run). – Ed Beal May 20 '16 at 18:44

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