Symptom
Furnace short cycling
Short cycling is a furnace that starts, runs briefly and shuts off, over and over. Common causes are airflow restriction tripping the limit, a flame sensor that cannot prove flame, a pressure switch that opens under load, an oversized furnace, or a thermostat location problem.
Short answer
Short cycling means the furnace is shutting itself down before the thermostat is satisfied. Check and replace a loaded filter and make sure supply and return grilles are open; if the cycling continues, read the status code and call. A furnace that has always short-cycled since installation may simply be oversized for the house, which is a sizing problem rather than a fault.
Have a code on the display?
Causes that show up as codes
Limit trips: restricted airflow from a dirty filter, closed registers, a dirty blower wheel or undersized ducts lets the heat exchanger overheat, so the limit opens and the burner stops. The blower keeps running until the furnace cools, then it relights. On two-stage furnaces this often appears on high fire only.
Flame loss: a fouled flame sensor proves flame at ignition then loses the signal after a few seconds. The control shuts the gas valve and retries, which looks like a short cycle with a flame-loss code.
Pressure switch opening under load: a partially blocked vent, a marginal inducer or a trap that fills during the cycle lets the pressure switch drop out mid-run. Carrier's 59TP6C, for example, logs this as its pressure-switch codes.
Causes that do not show a code
Oversizing: a furnace with far more output than the house needs satisfies the thermostat in a few minutes and shuts off normally. Rooms far from the thermostat never warm up and the equipment wears faster. Two-stage and modulating furnaces reduce this, but the real fix is correct sizing from a heat-loss calculation.
Thermostat placement and settings: a thermostat in direct sun, above a supply register or with an aggressive cycle-rate setting produces short cycles the furnace is not responsible for.
Documented codes that point here
- Carrier 59SC6A status code 31: secondary pressure switch open or lockout
- Carrier 59SC6A status code 32.1: open main pressure switch (LPS)
- Carrier 59SC6A status code 33.1: a switch has opened in the main limit circuit
- Carrier 59SC6A status code 34: ignition fault or flame loss
- Carrier 59SC6A status code 13: limit lockout or ignition lockout
- Carrier 59SC6A status code 14: flame-loss lockouts during a heat request
- Carrier 59TP6C status code 31: high-heat pressure switch (HPS) open or slow to close
- Carrier 59TP6C status code 13: limit lockout or ignition lockout
- Carrier 59TP6C status code 14: flame-loss lockouts during a heat request
- Carrier 59TP6C status code 15.1: lockout - no blower rpms detected
- Carrier 59TP6C status code 32.2: open main pressure switch (LPS)
- Carrier 59TP6C status code 33: limit circuit, high-heat or ignition lockout
- Carrier 59TP6C status code 34: flame lost after ignition
- Carrier 59TP6C status code 41: blower RPM not detected
- Carrier 59TP7A status code 31: high-heat pressure switch (HPS) open or slow to close
- Carrier 59TP7A status code 13: limit lockout or ignition lockout
- Carrier 59TP7A status code 14: flame-loss lockouts during a heat request
- Carrier 59TP7A status code 15.1: lockout - no blower rpms detected
- Carrier 59TP7A status code 32.2: open main pressure switch (LPS)
- Carrier 59TP7A status code 33: limit circuit, high-heat or ignition lockout
- Carrier 59TP7A status code 34: flame lost after ignition
- Carrier 59TP7A status code 41: blower RPM not detected
- Carrier 59SP6B status code 31: secondary pressure switch open or lockout
- Carrier 59SP6B status code 32.1: open main pressure switch (LPS)
- Carrier 59SP6B status code 33.1: a switch has opened in the main limit circuit
- Carrier 59SP6B status code 34: ignition fault or flame loss
- Carrier 59SP6B status code 13: limit lockout or ignition lockout
- Carrier 59SP6B status code 14: flame-loss lockouts during a heat request
- Carrier 59SP6B status code 15.1: lockout - no blower rpms detected
- Carrier 59SP6B status code 41: blower RPM not detected
- Carrier 59TN7A status code 31: high-heat pressure switch (HPS) open or slow to close
- Carrier 59TN7A status code 13: limit lockout or ignition lockout
- Carrier 59TN7A status code 14: flame-loss lockouts during a heat request
- Carrier 59TN7A status code 32.2: open main pressure switch (LPS)
- Carrier 59TN7A status code 15: blower RPM lockouts
- Carrier 59TN7A status code 41: blower RPM not detected
- Carrier 59TN7A status code 33.1: a switch has opened in the main limit circuit
- Carrier 59TN7A status code 34: ignition fault or flame lost after ignition
Related
Questions people ask
Is short cycling bad for the furnace?
Yes. Every start stresses the ignitor, gas valve and motors, and running mostly in the start-up phase wastes fuel. It is worth fixing rather than living with.
Can a new thermostat stop short cycling?
Only when the thermostat is the cause. If the furnace is tripping a limit or losing flame, the code on the control will say so and a thermostat will not help.
Still no heat?
Call BC Wide with the brand, model number and anything on the display. Or
Request service online with this page attached.