Comfort science
Why rooms feel different temperatures (it's almost never the furnace)
If your bedroom is 4 °C colder than your office, the answer is probably ductwork, returns, or window radiation — not the furnace, the thermostat, or the seasons.
- Author
- Patryk K.
- Role
- Lead estimator
- Published
- September 18, 2025
- Reading time
- 7 min
We get this call constantly. "My furnace must be broken — the upstairs is 4 °C colder than the basement." The furnace is almost always fine. The ducts almost never are.
Three things that cause it
- Return-starved bedrooms (one register, no return, door closed at night)
- Trunk lines that lost balance after a basement renovation
- Single-pane or undersized double-pane windows in one specific room
How we diagnose it
Anemometer at every register, infrared scan of windows and headers, smoke pen at supply boundaries to see how air actually moves. The whole thing takes about 90 minutes. Cost is $189 and is credited against any work we do.
What usually fixes it
- 1Return-air pathway under the door (short answer: cut a 1" gap or add an over-door transfer)
- 2Damper rebalancing in the basement supply trunk (often free of charge during a maintenance visit)
- 3Add a supply or return drop where the room is starved
- 4Insulating film on a single problem window (third-party trade)
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