Why the Heat Is Not Working
Start with the thermostat mode, the filter, and whether anything is trying to run. Gas odor or a carbon monoxide alarm means leave. The rest is a diagnosis, not a new furnace by default.
No heat has a short list of harmless causes and a short list of reasons to leave the house. Do the harmless ones. Do not improvise on the rest.
Leave first if
- You smell gas. Do not hunt for a switch. Leave and call the gas utility from outside.
- A carbon monoxide alarm is sounding, not merely chirping once a minute. Get everyone out and call emergency services from outside.
- The furnace cabinet is hot to the touch, discolored, or you heard a bang from the burner area. Leave it off.
Checks that are reasonable
- Thermostat mode is Heat, fan is Auto, setpoint is above the room temperature. Hold is on if a schedule keeps setting it back.
- The screen is not blank. A blank thermostat is a power problem, covered separately.
- The filter is not collapsed. If you change it, use the same size and put the airflow arrow toward the blower.
- The switch on the furnace that looks like a light switch is on. That switch exists so a technician can kill power. It also gets bumped off.
- The breaker for the furnace is on. If it is tripped, reset it once. If it trips again, leave it off.
- For a heat pump, Emergency Heat should not be the thing you discover later. If the outdoor unit is a block of ice that never clears, that is a defrost or airflow problem, not a thermostat quirk.
A furnace that clicks, spins the inducer, and never lights is past this list. So is a heat pump that runs and the air stays cold for more than the first few minutes. Those are heating repair or heat pump repair. We find out why it stopped before we talk about a new system. A room-by-room version of the same idea is in what to check before you call.
Common questions
The blower runs but the air is cold.
The fan can run without the burners or the heat pump actually heating. Mode, a tripped limit, or a heat pump stuck in defrost or aux are different faults. It needs a diagnosis.
Should I reset the furnace again?
One reset is enough to see if it was a one-time lockout. Repeated resets on a furnace that will not stay lit hide the fault and can flood the heat exchanger with unburned gas attempts.
Could it just be the filter?
Yes, if the filter is packed and the furnace is overheating and shutting down. If the filter is clean, the filter is not the story.
