Learn why a furnace may repeatedly start and stop, including airflow restrictions, high-limit trips, flame-sensing, thermostat and equipment-sizing problems.
Furnace short cycling can mean different things depending on where the cycle stops. A furnace may shut down before ignition, shortly after flame starts, or after operating long enough to overheat.
Repeated cycling increases wear and usually indicates a control, safety, airflow or system-design issue that should not be ignored.
Timing the cycle and observing whether the burners or blower stop first can help a technician narrow the problem.
Low airflow can cause furnace temperature to rise and open the high-temperature limit.
Incorrect speed, motor failure or dirty components can reduce heat removal from the furnace.
Burners can light and shut off quickly if flame is not proven.
Poor location, wiring or control issues can create abnormal calls.
Excessive heating capacity can create short run times and temperature swings.
Pressure-switch or other safety conditions may interrupt the heating sequence.
Several different HVAC faults can produce the same homeowner symptom. Proper diagnosis may require electrical readings, refrigerant measurements, airflow testing, static-pressure measurements, combustion testing or control-sequence checks.
Do not bypass safeties, open sealed refrigerant circuits, work on gas piping or perform live electrical troubleshooting unless you are qualified to do so.
Sequence failures can appear similar to short cycling.
Airflow restrictions are a common contributor.
Distribution issues can accompany airflow problems.
Understand heat exchangers, blowers and safety controls.
When requesting service, include the equipment type, approximate age, when the problem started, whether the system still operates and any unusual noise, water, odor or thermostat behavior you have noticed.
Understand normal system operation before narrowing a symptom.
Identify the components commonly mentioned during diagnosis and repair.