The heat is on, but no heat is coming out

Urgent first

If you smell gas or a carbon monoxide alarm sounds, get everyone out and call 911 or your gas provider from outside. If the night is forecast below freezing and someone in the house is very young, elderly or unwell, say so at the start of the call.

What to check before calling

• The thermostat is set to heat, above room temperature, and has fresh batteries.
• The breakers. An electric furnace often has its own large breaker, sometimes two. A tripped one may explain everything; reset it once only. See breaker keeps tripping.
• The filter. A clogged filter can trip a high-limit switch and shut the heat off.
• Propane: the tank gauge. A tank that ran empty may need a leak check before the supplier refills it and the furnace is relit.

What it usually is

Electric furnace or air handler. The blower runs but the air is cool: a heat strip element, a sequencer or relay that no longer brings strips on, or a limit switch. Nothing runs at all: a breaker, a transformer, or the control board.

Heat pump. Cool air in heat mode, or a thermostat that shows auxiliary heat all day: the reversing valve, the defrost control, low refrigerant, or the backup strips failing alongside. See heat pumps.

Gas or propane furnace. The inducer runs, a click, a glow, then nothing: the ignitor or the pressure switch. The burner lights and drops out after a few seconds: usually a dirty flame sensor. These first-cold-morning failures cluster in November, when the median first freeze arrives at the Smithville station on November 19.

What a technician checks

On electric heat: the current each strip draws, the sequencer and the limit. On a heat pump: the reversing valve, pressures, the defrost board and the strips. On gas or propane: the ignition sequence step by step, and combustion and carbon monoxide readings that should be written down.

What to say when you call

• Electric, heat pump, gas or propane
• What happens: nothing runs, blower runs cold, burner lights then stops
• Thermostat setting and the current reading in the house
• Whether any breaker tripped, and whether you smell anything
• Your ZIP code and where the indoor unit sits

Common questions

The blower runs but the air is cold on an electric furnace. Usually a strip or the part that switches strips on. It is a repair, not a replacement, on most units.

The heat worked last winter and has not run since March. That is the most common pattern of all. Running the heat briefly on a cool evening in October finds these faults early.