Quote:
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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The truth depends on whether your furnace has an outdoor air intake or not. This branch ties into the return side of the ductwork, and mixes into the return before the furnace reheats the mixed air. Many houses do not have an outdoor air intake, and if yours doesn't this whole conversation is moot.
If your house does have an outdoor air intake, the proportion of outdoor to return air is very small, usually 10% or so. Often even less, depending on how crappy your duct installation is.
Hypothetically then you're bringing in outdoor air in every time you run the furnace fan, but you're also doing so every time you run your bathroom fan, or every time your water heater fires up (if you don't have a more modern direct-vented heater). The proportion is pretty small.
What you're really accomplishing running the furnace fan is mixing cold return air from your basement with warmer air in the upstairs, and redistributing the tempered mix of air back throughout the house. This will gradually warm the basement up and cool the upstairs down, but if your basement is losing heat quickly enough (i.e. if it's unfinished/uninsulated/poorly insulated) you may not notice much heat gain in the basement. In which case, so much the better for you.
Note that you do not want to block any outdoor air intakes. If you have older conventional gas-fired appliances you should have a combustion air intake that quite deliberately lets outdoor air into the house for your furnace/water heater to burn; DO NOT BLOCK A COMBUSTION AIR INTAKE, it could #### up the venting of your gas-fired appliances and result in flue products not being expelled properly out the flue.