A furnace short cycles when it turns on and off in a very short time, before reaching the set temperature. Trane says a normal furnace cycle lasts at least 7 minutes, and that frequent cycling on a very cold night can be normal because the house is losing heat quickly. Cycling day and night in mild weather is abnormal. Trane lists the common causes as a thermostat problem or bad thermostat placement, restricted airflow, a faulty limit switch, a cracked heat exchanger and an oversized furnace, and calls an oversized furnace the most common cause.
Carrier adds a dirty flame sensor. You can safely check the thermostat, the filter and the returns and registers, and look for a blocked exhaust or intake outside. Limit switches, flame sensors and heat exchangers are technician work.
What this covers
- Residential gas forced-air furnaces that start and stop repeatedly before the house reaches the set temperature
- Single-stage, two-stage and variable-speed furnaces
- Customer-accessible checks: cycle timing, thermostat, filter, registers and returns, and the outdoor vent terminations
What it does not cover
- ×A furnace that never starts, which is the no-heat page
- ×Heat pumps and boilers
- ×Testing or replacing the limit switch, flame sensor, pressure switch or heat exchanger
Try in this order
0 of 6 completedTime the cycles before you decide it is a problem
ReversibleWatch three complete heating cycles with a clock. Trane says a normal cycle is at least 7 minutes. On a very cold night more frequent cycling can be normal because the house loses heat faster. A modulating or variable-speed furnace may run longer at quieter, lower speeds. Cycling in mild weather, day and night, is the abnormal case.
Start with the thermostat
ReversibleTrane says a malfunctioning or badly placed thermostat can call for heat at random times: faulty wiring, dying batteries and calibration problems all count. A thermostat on a wall that gets direct sunlight can turn the heat off no matter what the rest of the house is. It should be central, away from vents and heat-producing appliances and out of direct sun. Replace batteries first.
Fix airflow: filter, returns, registers
ReversibleTrane says a clogged or dirty filter can make the furnace overheat and shut down before the cycle is done; change filters every 1 to 3 months depending on type. Make sure return air vents are not blocked by furniture or drapes. Carrier lists closed vents, kinked ductwork and a slow blower as other airflow causes of uneven heat and short cycles.
Look outside at the vent and intake
ReversibleTrane says that if the exhaust flue or vent pipe is clogged, with a bird's nest, a rodent or a wedged toy, the flue limiter shuts the furnace down to prevent carbon-monoxide poisoning. Look at the exterior terminations for snow, ice, leaves and nests from the ground without touching the pipe.
Read the status light after a shutdown
ReversibleWrite down the status light or displayed code after the furnace stops. On a Carrier furnace, codes 33 and 13 mean a limit, rollout, draft-safeguard or blocked-vent switch opened, and 32 or 43 point at the low-heat pressure switch. On a Lennox furnace, E250, E274 and E311 point at the limit circuit and restricted airflow. Do not treat a code as proof a part failed.
Recognize the causes only a technician can settle
ReversibleA faulty limit switch can think the furnace is overheating and shut it down early. A cracked heat exchanger can make it overheat, cool and try again. Trane says an oversized furnace is the most common cause and that it can start after home improvements that reduce the heating load. Ask the HVAC company to check sizing, the limit switch and the heat exchanger.
Evidence ledger
Sources behind this answer
Trane's own guide: a normal cycle is at least 7 minutes; causes are the thermostat, restricted airflow, a faulty limit switch, a cracked heat exchanger and an oversized furnace, the last being the most common.
Trane's own guide to what the limit switch does, the symptoms of a failing one, and not to bypass or reset it.
Carrier's own guide to the signs a furnace needs repair, common causes and quick checks, and the problems that need a professional.
Carrier's own guide for the 58CVA and 58CVX. Two-digit status codes flashed on an amber LED, each with its own section.
Lennox's own service literature: the E-code table for the 103130-XX ignition control, including soft lockouts.
Exact questions
Common questions about “Furnace starts and stops every few minutes”
How long should a normal furnace heating cycle be?+
Trane says at least 7 minutes. Shorter cycles that repeat in mild weather are abnormal. Cold weather can legitimately make cycles more frequent because the house loses heat faster.
Is furnace short cycling dangerous?+
Trane says short cycling is not itself dangerous but costs money through higher energy bills, extra wear and possible heat-exchanger damage, so fix the cause soon. A safety switch that keeps tripping, a carbon-monoxide alarm or a blocked vent is different: stop using the furnace and call for service.
Can a dirty filter cause short cycling?+
Yes. Both Trane and Carrier say restricted airflow can make the furnace overheat, and the safety limit shuts it down before the cycle finishes. It is the first thing to check because it is free and quick.
Why did my furnace start short cycling after we added insulation?+
Trane says a furnace sized correctly at installation can become oversized after home improvements that reduce the heating load. Ask an HVAC company to check whether it is now oversized.
Should I replace the limit switch myself?+
No. Trane says not to attempt to bypass or reset the limit switch and to call a professional. It is a safety device that prevents overheating.
Why does the furnace short cycle?+
Overheating from poor airflow, a bad flame sensor or a thermostat issue can cause frequent starts and stops.
Is short cycling harmful?+
Yes, it wears parts and wastes energy, so have it checked.