When the furnace quits on a cold morning, a few simple checks solve a surprising number of cases before you need a technician. Here is a safe, ordered list to work through, plus the warning signs that mean it is time to call.
Start with the simple checks
Confirm the thermostat is set to heat and the target temperature is above the room temperature, and replace its batteries if it is battery powered. Check that the furnace switch, which looks like an ordinary light switch nearby, is on, and that the breaker has not tripped. These three account for many no-heat calls.
Check the filter and the vents
A severely clogged air filter can choke airflow until the furnace overheats and shuts itself off on a safety limit. If the filter is dirty, replace it. Make sure supply and return vents are open and not blocked by furniture or rugs.
Confirm gas and the condensate drain
Make sure the gas is on, for example by checking that other gas appliances work. On a high-efficiency furnace, a full or blocked condensate drain can trigger a safety shutoff, so look for water around the unit. If you smell gas, stop, leave, and call your gas utility from outside.
Look at the error code
Many furnaces have a small light that blinks a diagnostic code. The pattern, read against the legend on the access panel, often points to the problem and is useful information to share with a technician.
When to call a professional
Stop and call if you smell gas, see a yellow or sooty burner flame, hear loud banging or grinding, the system short-cycles, or your checks do not restore heat. Anything involving gas, the heat exchanger, electrical components or refrigerant should be handled by a qualified technician.
Frequently asked questions
Why did my furnace suddenly stop working?
Common causes include a thermostat setting, a tripped breaker, an off furnace switch, a clogged filter, or a full condensate drain. Work through the simple checks first, then call a pro if heat does not return.
Is it safe to keep resetting my furnace?
Reset it once. If it shuts down again, repeated resets can be unsafe and usually mean a fault that needs diagnosis. Stop and call a technician.
I smell gas near my furnace. What should I do?
Do not operate switches or look for the source. Leave the home and call your gas utility or emergency line from outside. Treat any gas smell as urgent.
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