HomeAnswersWhy does my furnace keep turning on and off?
Why does my furnace keep turning on and off?
A furnace that starts, runs for a minute or two, and quits before the house warms up is short cycling, and it is almost always the furnace protecting itself: restricted airflow, a tripping high-limit switch, a flame that will not stay proven, or a thermostat that keeps cutting the call short. It heats worse while it runs, and it wears parts at several times the normal rate.
Airflow and the limit switch
The most common pattern is a dirty filter, closed registers, or a blocked return letting heat stack up inside the cabinet. The high-limit switch reads an overheat, kills the burners, and the blower cools the furnace down until the limit resets and the cycle starts over. A limit that trips on its own can also fail mechanically, cycling a furnace that has perfectly clean air. Either way the fix starts with airflow and a meter on the limit, not a new furnace.
Flame dropping out
If the burners light and die within seconds, several times in a row, the flame sensor is not proving flame. A coated sensor makes the control shut the gas almost immediately, and the furnace retries on a lockout timer. That looks like turning on and off at the thermostat. Cleaning or replacing the sensor is one of the smallest repairs on a furnace repair visit.
Thermostat and duct causes
A thermostat in the path of a supply register, in direct sun, or on a cold outside wall satisfies too early and calls again within minutes. An oversized furnace does the same thing by design, blasting short blasts of heat. A cracked heat exchanger can also trip the limit through the same overheat pattern, which is why a short-cycling furnace deserves a professional look before a second winter of it.
Typical short-cycle repairs run from a filter and sensor cleaning at the low end to a limit switch or thermostat in the mid hundreds for the metro. Short cycling never fixes itself, and every cycle drags the igniter closer to failure. Send the pattern, three or four words, through the request form and a company can usually tell you over the phone whether it sounds like airflow or flame.