Symptoms
- Mode selector won't move off Cool, Off, or Auto to Heat
- Selecting Heat doesn't start the furnace or heat pump
- System may run in the wrong mode when heat is called for
Likely causes
- CommonSetting simply not selected or saved correctlySome thermostats need the mode confirmed with a separate button or menu step.
- CommonProgram or schedule overriding manual mode changesA programmed schedule can revert the mode shortly after you change it manually.
- PossibleWiring issue at the thermostat (common on DIY installs)A miswired heat or common wire can prevent heat mode from functioning.
- PossibleStuck reversing valve on a heat pump systemThis valve switches the heat pump between heating and cooling and can stick with age.
- Less commonThermostat compatibility issue with the system typeA thermostat not matched to a heat pump or dual-fuel system can behave incorrectly.
What to do
- Select Heat mode again and confirm any on-screen prompt, then wait a minute to see if it engages.
- Check for an active schedule or program that might be overriding your manual selection, and disable it temporarily.
- Replace batteries and restart the thermostat as a general troubleshooting step.
- If you or someone else recently installed the thermostat, double-check the wiring against the base plate labels.
- For heat pump systems, if cool air blows even in heat mode, suspect the reversing valve rather than the thermostat itself.
- Call an HVAC technician if wiring looks correct and the mode still won't engage properly.
Tools and parts
- Small screwdriver
When to call a pro
Call an HVAC technician if heat mode won't engage after checking settings and wiring, especially on a heat pump system where the reversing valve may be at fault.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.