Why does my furnace keep turning on and off?
A furnace that fires, runs a few minutes and quits, over and over, is usually overheating. Not enough air moves across the heat exchanger, a safety limit trips, and the burners shut off until things cool. Carrier's troubleshooting guide starts with the thermostat, the filter and the room vents, and so should you. If those check out, the fault is inside the cabinet.
Likely causes
A clogged air filter Safe to check
A choked filter starves the blower of air, so heat builds up in the cabinet and the high-limit switch shuts the burners off. Carrier recommends checking the filter monthly. This is the cheapest fix there is.
Closed or blocked vents and returns Safe to check
Furniture over a return grille, or many closed supply registers, has the same effect as a dirty filter. Walk the house and open things up.
Thermostat placement or settings Safe to check
A thermostat next to a supply register, a sunny window or a lamp reads warm and stops the call for heat early. A dying battery can also cause erratic cycling.
A dirty flame sensor or failing limit switch Pro job
The flame sensor proves the burners lit. When it is coated, the control shuts the gas off seconds after ignition and tries again. Carrier lists a dirty flame sensor and a faulty limit switch among causes of shutdowns. Both sit next to the burners and are a job for a licensed HVAC technician.
An oversized furnace Pro job
A unit with far more capacity than the house needs heats the air near the thermostat fast and quits. There is no repair for this, only a properly sized replacement when the time comes.
Safe things to check first
- Pull the filter and hold it to a light. If you cannot see through it, replace it with the same size.
- Open supply registers and clear anything blocking return grilles.
- Replace thermostat batteries and confirm it is on Heat with the setpoint above room temperature.
- Note whether the thermostat sits near a heat source, and tell the technician if so.
- Count how long each cycle lasts. A technician will want to know.
When to stop and call
Short cycling that survives the basic checks is a job for a qualified HVAC professional, as Carrier advises. Call sooner if you smell burning, hear a loud bang on ignition, or a carbon monoxide alarm sounds. A CO alarm means get outside to fresh air, then call 911. If you smell gas, leave the house without touching switches and call your gas utility from outside.
Repair or replace?
Start with its age. Type the serial from the label:
Replacing it: a new gas furnace (80,000 BTU, 95%+ efficiency), installed
National figures before permits and code extras. Prices for your ZIP code.
Most short-cycling repairs (sensor cleaning, a limit switch, thermostat relocation) are small. The exception is a unit that is badly oversized or has a cracked heat exchanger. DOE's analysis puts the average gas furnace at about 21.4 years, and inspector charts list 15 to 25. Date your furnace with the age decoder above. Past 18 to 20 years, weigh the repair against the replacement range above, and ask for a load calculation so the next one is sized correctly.
Common questions
Is short cycling dangerous?
The limit switch is doing its job, so the furnace is protecting itself. But repeated overheating stresses the heat exchanger, so do not ignore it.
How long should a normal heating cycle last?
It varies with weather and equipment, but cycles of only a couple of minutes, many times an hour, are a sign something is wrong.
Can a new filter fix it right away?
If the filter was the cause, yes. Give it a few hours of normal operation and watch the cycle length.