Furnace Not Heating? Start Here

A furnace that won't heat is one of the more stressful home repair issues, mostly because of when it tends to happen — the first genuinely cold night of the year, or the coldest week of winter. Before assuming the worst, there are a few things worth checking yourself.

Start with the thermostat

It sounds obvious, but it's the most common culprit. Confirm the thermostat is set to "heat," not "cool" or "off," and that the set temperature is actually higher than the current room temperature. If it's a battery-powered thermostat, weak batteries can also cause it to stop communicating with the furnace properly.

Check the power and the breaker

Furnaces run on electricity even when they burn gas — the igniter, blower motor, and control board all need power. Check your breaker panel for a tripped breaker, and confirm the furnace's power switch (usually a light-switch-looking toggle near the unit, sometimes mistaken for a light switch) is in the "on" position.

Check the air filter

A severely clogged filter can restrict airflow enough that the furnace overheats and shuts itself off as a safety measure. If it's been a while since the filter was changed, this is worth ruling out — it's a five-minute fix if it's the cause.

Check the pilot light or ignition system

Older furnaces with a standing pilot light are simple to check — if the pilot's out, most have instructions printed on the unit for relighting it safely. Newer furnaces use electronic ignition, which is less something a homeowner can troubleshoot directly, but a furnace that clicks repeatedly without lighting is a sign the ignition system needs a professional look.

Signs it's time to call a pro

• The furnace won't turn on at all, even after checking power and the thermostat
• You smell gas — leave the house and call your gas utility immediately, then a technician
• The furnace turns on but shuts off shortly after (short cycling)
• You hear banging, screeching, or grinding noises
• The unit is blowing cold or lukewarm air instead of hot
• You see soot around the furnace, or the pilot light burns yellow instead of blue — both can indicate a carbon monoxide risk

A safety note

Gas furnaces involve combustion, and problems can occasionally involve carbon monoxide risk. If you have a CO detector sounding an alarm, or you smell gas, that's not a "wait and see" situation — leave the house, get everyone out, and call for help from outside. For anything short of that, a technician can usually diagnose the issue quickly once the simple checks above are ruled out.

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