hvac troubleshooting
Indoor fan runs but no heating or cooling
Common symptoms: fan blows no heat; blower runs no cool; air handler runs no conditioning; no heat or cool fan only
Stop and call a pro if:
- burning odor
- smoke
- sparks
Step-by-step diagnostic flow
Step 1
Any burning smell, smoke, or sparks?
Step 2
Is the thermostat fan setting on 'On' or 'Auto'?
'Fan: On' runs the blower continuously regardless of heat/cool demand. If the system isn't currently calling for heating/cooling, the fan will still run but blow ambient air.
Step 3
Is the thermostat mode set to Heat (with setpoint above room temp) or Cool (with setpoint below room temp)?
Step 4
When the system is supposedly running, is the outdoor unit (compressor/condenser/heat pump outside) running?
Listen and look at the outdoor unit. Both the compressor (large can shape) and the fan on top should be running for cooling or heat-pump heating. For a gas furnace, no outdoor unit is involved.
Possible outcomes
Stop and seek professional help immediately
high confidenceActive hazard.
Fan is set to continuous — switch to Auto
high confidence'On' (or 'Circulate') runs the blower constantly regardless of heating/cooling demand. With no current call, the blower will just push around indoor air.
- Switch the thermostat fan setting from 'On' to 'Auto'
- If the blower keeps running after switching to Auto with no heat/cool call, switch to the 'Blower runs constantly' workflow
Thermostat setpoint isn't calling for heat or cool
high confidenceIf 'Heat' setpoint is below room temp, or 'Cool' setpoint is above room temp, the system won't run the heat or AC.
- In Heat mode, raise setpoint above current room temp by 5 °F
- In Cool mode, lower setpoint below current room temp by 5 °F
- Wait 5 minutes for the call to engage (most thermostats have a built-in delay)
Indoor air-handler issue — call a pro
medium confidenceOutdoor unit is doing its job but the indoor side isn't transferring heat/cool. Possible causes: stuck reversing valve (HP), TXV stuck closed, indoor coil iced over, or a duct-side problem.
- Measure supply-vs-return air temperature with a probe thermometer if available
- Photograph the indoor coil if visible without removing panels
- Do not open sealed refrigerant components
- Mode
- Supply and return temps
- Equipment make/model
- Any error codes
Outdoor unit has no power — check the disconnect and breaker
high confidenceIf the outdoor unit doesn't run at all on a clear call, it likely lost power.
- Check the dedicated breaker in the electrical panel for the outdoor unit
- Check the disconnect box on the wall next to the outdoor unit — the pull-out fuse can be installed backwards or be blown
- If the breaker keeps tripping after a reset, do not keep resetting — call a pro
- Switch to the 'Outdoor unit won't start' workflow for deeper diagnosis
- Breaker label and amperage
- Whether disconnect fuse is blown
- Equipment make/model
Humming but not spinning — likely capacitor or contactor
medium confidenceHumming means a motor (fan or compressor) is trying to start but can't get past its starting torque. Run capacitors fail in this exact pattern.
- Turn the system off at the disconnect — running a humming compressor damages it
- Don't open the outdoor unit — run/start capacitors store lethal energy
- Call a pro and describe the symptom — capacitor replacement is a common, quick fix
- While waiting, you can run the indoor fan-only mode for air movement if it's safe
- Sound description
- Equipment make/model
- Approximate unit age
Gas furnace blower runs but burner won't fire — separate workflow
medium confidenceBlower running with no burner ignition is the 'Furnace no heat' diagnostic.
- Switch to the 'Furnace no heat' workflow
- Check for any error LED codes flashing through the furnace inspection window
- If you smell gas at any point, leave and call the utility
- LED error code count
- Furnace make/model
- Last service date
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