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Originally Posted by RichieRich
OK not AC question... but kinda related. My wife has now turned on the home furnace recirculation fan. I think it's bringing in the hot hot outside air plus circulating inside and warming up the basement and soon the rest of the house. She thinks there's no outside air being added and helping to cool the house. Where does the truth lie?
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Unless you have a damper you can switch on the fresh air intake (and I don't know if that is even legal) the fan is bringing in outside air 100% for sure. Follow the ducts in the basement. I can't remember my classes, but it isn't 100% fresh air, something like 35% fresh air rest re-circulated air?
It won't really warm the basement up too much, unless your ducting is leaking. Beyond the downstairs vents (which share the flow with all vents in the house, they just get the advantage of more flow since closer to the source), it can only really heat up downstairs via convection to the metal duct, then conduction through the metal ducting and then convection to the downstairs... that seems like a very minimal impact given the small delta T.
But it is also circulating your air around the house, potentially mixing the air and helping.
My guess is at peak hot hours you may want it off as it the air circulation vs hot outdoor air isn't balancing in your favor. If your house indoor temp is close to outdoor temp then less worry.
But, turning on bathroom fans may help, they indirectly suck air from outside via the ducts (likely so little it doesn't matter), but will also pull cold air upwards from the basement. How much, who knows.