A furnace that starts, runs a few minutes and shuts off is short cycling. Sometimes it's minor. Sometimes a safety switch is telling you something is wrong.
Common causes
Dirty flame sensor
The furnace lights, can't confirm the flame, and shuts the gas off. A cleaning often fixes it.
Clogged filter
The furnace overheats and the high-limit switch shuts it down.
Blocked vent or intake
Leaves, nests or ice can block high-efficiency vent pipes.
Oversized furnace
It heats the house too fast and shuts off before it runs efficiently.
Thermostat problems
Bad placement near a heat source, or wiring issues.
Cracked heat exchanger
A serious safety problem that can let combustion gases into the air stream.
Don't keep pressing the reset button. Repeated lockouts mean a safety control is tripping. If you smell gas or your CO alarm sounds, get outside and call 911 or your gas utility.
See furnace repair.
Written by the Ace Mechanical team. We've been fixing heating, cooling, refrigeration and plumbing across Hampton Roads for more than 30 years. Questions about your own system? Call (757) 495-7782.
