Opposite operation can come from heat-pump reversing-valve configuration, mislabeled wiring or equipment fault. Turn the system off; repeated testing can heat or damage the home. Check the thermostat's equipment type and the O/B wire setting against the installer's record. Compare the wire labels with the photograph or the installation record.
Differential
Likely causes of “Smart thermostat runs heat when cooling is selected”, and how to tell them apart
- 1
The reversing-valve setting on a heat pump is wrong
What separates it The system heats when set to cool and cools when set to heat, and the equipment is a heat pump.
How to confirm Check the thermostat's equipment type and the O/B wire setting against the installer's record.
- 2
The O/B wire is connected or labeled incorrectly
What separates it The problem began after a thermostat swap and the old wire labels were photographed or not.
How to confirm Compare the wire labels with the photograph or the installation record.
- 3
A stuck or failed reversing valve
What separates it The thermostat settings are correct but the outdoor unit still runs in the opposite sense.
How to confirm A technician should test the valve and the outdoor unit.
- 4
A conventional system has heat and cool wires swapped
What separates it A simple heat and cool system runs the wrong mode from the first installation.
How to confirm Have a qualified person check the wiring against the thermostat labels.
Before you touch a setting
Record this first — changing settings destroys the evidence
- The equipment type, the wire labels and a photograph of the wiring at the thermostat.
- Whether the outdoor unit and the indoor air behave opposite to the setting.
- Whether the problem began after a thermostat replacement.
Try in this order
0 of 4 completedCapture the exact symptom
ReversibleRecord thermostat equipment type, wire labels/photo and whether outdoor unit and indoor air behave opposite the selected mode.
Run the smallest isolation test
ReversibleUse the thermostat’s supported equipment configuration check without moving wires; compare installer documentation.
Correct only the proven cause
ReversibleHave a qualified HVAC technician correct O/B configuration or wiring if the original installation record is uncertain.
Verify before changing anything else
ReversibleRun a brief supervised cool call and verify supply temperature moves in the correct direction.
Stop boundary
When this stops being a self-service repair
- !Turn the system off; repeated testing can overheat the house or damage the equipment. A qualified HVAC technician should correct the O/B configuration or the wiring.
Evidence ledger
Sources behind this answer
Exact questions
Common questions
Can I fix the O/B setting myself?+
The thermostat's equipment setting can sometimes be changed in the menu, but wiring changes and valve diagnosis belong to a qualified HVAC technician.
Why does the thermostat heat in cool mode?+
The wiring may be reversed, the heat pump's reversing valve setting may be wrong or the thermostat may be set for the wrong system type.
Should I check the wiring?+
Turn off power first. If the wiring was recently changed, an electrician or HVAC technician should confirm it.
Can a wrong O/B setting cause this?+
On a heat pump, yes. The reversing valve setting must match the unit, so check the manufacturer's setting.
Should I turn the system off at the breaker before checking?+
Yes, switch off the power before touching thermostat wires.
When should I call an HVAC technician?+
If the wiring is correct and the system still runs the wrong mode, a technician should check the reversing valve or control board.