Quote:
Originally Posted by timun
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.
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Yep it seemed to somewhat redistribute the heat but since eupstairs was so hot and still had the sun it just got hotter as expected. The basement temp went from a pleasant ~70F up to about 78 which is more than I wanted. Oh well. And the temps Are in F because my office heater fan constantly displays in F... by WRGMG is that although I can reset that to C, if the unit gets knocked off its foot (or moved) it totally resets to F and warming to 85F.