Should upstairs a/c run to help cool downstairs?
by Marsha
(Plano TX)
Our 2 story home has 3 a/c units. The upstairs area is rarely used but is open enough that the air flows to the downstairs. Should we keep the upstairs unit at a lower temp so it will help cool the downstairs? Or is it more energy efficient to close off as much of the upstairs as possible and turn that thermostat higher?
Answer from Green Energy Efficient Homes
It's true that warm air rises and cold air falls, so you might think it would make sense to run an air conditioner upstairs to cool the downstairs.
The trouble is that heat also passes through exterior walls and ceilings, even with insulation in the walls and ceilings. (Insulation doesn't stop heat transfer - it just slows it down.) So unless your upstairs walls and attic are perfectly insulated - which of course they cannot be - that cold air in your upstairs will start absorbing heat from the outside.
That's why it's always better to close off an unused space and reduce its heating or cooling load, than to leave it open and heat or cool it along with the rest of your house. In essence your upper floor acts as a buffer between heat coming from outside walls and the attic, and the part of the house you actually use.
At the very least, closing the HVAC registers in unused rooms (on any floor), and turning off room air conditioners or heaters in such rooms, will save you energy. Closing off the entire floor will help even more. The only issue is with heating in cold climates: if you close off a room or floor where there is plumbing, and the temperature drops below freezing, there is a risk of pipes freezing, and then bursting. But that may not be a concern for you that often in Plano TX!
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