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Heating & Air Conditioning

AC and Attic Fan

9 years ago

We bought a fixer upper and the air conditioner is five years old. Some of the windows in the house have not yet been serviced so I cannot open the windows for a breeze on warm days. For now it is AC on or a hot house.

The attic has a huge fan vented outside. My question is should I run this fan when the weather is extremely hot to get rid of any warm air that rises, or should I leave it off?

Thanks in advance!

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Comments (9)

mike_home
9 years ago
I think the best way to use a whole house fan is to wait until the evening when the temperature begin to drop. Make sure you have enough windows open so you won't cause a negative pressure situation.
Acadiafun
Original Author
9 years ago
Thanks Mike. Can you explain to me what a negative pressure situation means in regards to the house? I will google it, but your input will be appreciated.
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Houses develop mold from having too high interior relative humidity. In muggy areas (East Coast and South), that comes from leaky houses/HVAC ducts and/or oversized AC systems that don't run long enough to dehumidify. People make that situation worse by keeping the fan running, that causes interior air to reabsorb humidity that was removed by the AC coil. Some houses need hard wired/plumbed dehumidifiers in addition to AC systems to provide comfort on hot yucky days. The kind of fans that blow air out from the attic are looked at unfavorably these days, because they cause a negative pressure area that draws air in to the attic from the house, through the ceiling and wall openings. The air drawn out of the house is replaced by outside air into the leaky/porous house, making the interior more humid. Passive atttic ventilation, with gable vents or ridge and soffit vents, is what's recommended. Honestly, a lot of the old time approaches have been discredited. The problem sources are much better understood today. The oversized AC problem is a big one - HVAC contractors always oversell size (more $$$ in the equipment sale) and the consequence is too humid interiors.
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mike_home
9 years ago
Negative pressure will occur if the air flow being pulled out of the house by the fan is greater than the air being supplied by the open windows. This means the air pressure inside the house is less then the air pressure outside. If you have appliances using combustion (hot water heater, fireplace, etc.) the fumes which would normally be rising up through the chimney will reverse direction and flow into the house. This could lead to carbon monoxide poisoning.

Whole house fans were popular before the days of central AC. I become concerned when someone wants to switch between the two. It is very easy to switch on the fan and forget to open the windows. You have to be careful.
over40
9 years ago
Years ago I had a house that had a walk up attic, and a big exhaust fan. We would leave the door open to the attic, being sure to block it open, so it doesn't slam shut!
We would open a window at the other end of the house, and got a great breeze thru the house.
Acadiafun
Original Author
9 years ago
Thanks over40 and thank you very much Mike for that detailed explanation. Who knew that improper use of the attic fan could be dangerous. So glad I asked this question and for your educational advice which I will be sure to take.
User
9 years ago
Best use of whole house fans is with a few bottom windows slightly open, AFTER the outdoor temperatures are below house temp.

You suck cooler air into the house and vent the warmer air out the top

Just be METICULOUS about making sure you have the right things on and open and closed.
energy_rater_la
9 years ago
what location?

in my hot humid climate whole house fans pull
in lots of humidity, overloading the house & making
hvac system work harder to reduce Relative Humidity.
(RH).

I remember as a child the wonders of the whole
house fan & the wisteria vines outside the
windows. smelled heavenly!
Acadiafun
Original Author
9 years ago
Lazygardens and Energy I have one more question and an answer and appreciate you answering my post. First is the answer to my location- it is Northeast Ohio where the temps can get very hot and humid but not for longer than a few months in summer. I run a dehumidifier in the basement if that makes any difference.

My question for Lazy- is how many windows in how many rooms to keep open? The ground level has 6 rooms. I can open two windows in one room, two windows in another room and that is it until I get the windows serviced. I can open two out of three attic windows. The attic is finished and is four rooms.

Thanks again!!!!
countryboymo
9 years ago
With a 15 seer or higher system that has variable speed motors or at least dc motors and sized correctly it might be hard to beat leaving the air on vs having a whole house fan run all night.

I would have a fan but sealing and insulating them decent when not in use involves a insulated cover with weatherstrip that. I try to keep every btu trapped in the house in the winter that I can. Right now the weather pattern is mild in the day and drops in the 60s at night with low humidity which is rare but perfect for a fan.

It is purely a preference. I have no allergies so I love the windows open when possible but my wife not so much.

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