Quick answer
A heat pump is short cycling when the compressor runs less than about 10 minutes, shuts off and restarts, over and over. The common causes are an oversized single-stage unit, a clogged filter or indoor coil tripping the high-pressure switch, low refrigerant tripping the low-pressure switch, and a thermostat set up for the wrong equipment. Change the filter first; if it continues, book a diagnosis.
Your heat pump is short cycling if the outdoor unit starts, runs for a few minutes, stops, and starts again before the house reaches the set temperature or moments after it does. Time it with a phone. A single-stage heat pump in mild weather runs 10 to 20 minutes at a stretch, and in a cold snap it runs almost without stopping. Three back-to-back runs shorter than 10 minutes is the line between normal and a fault.
Inverter heat pumps ramp down, and that is not short cycling
A variable-speed heat pump is supposed to run for hours at low output. An inverter compressor slows down as the house nears the setpoint, and at low speed the outdoor unit is quiet enough to seem off. Changes in pitch are the compressor ramping, not cycling.
True on/off cycling on an inverter unit means the smallest output it can hold is still more than the house needs, which points to sizing, or that a safety control is shutting it down. Use the same test: stand beside the outdoor unit and time how long the compressor stays on.
Check this yourself first
- Filter: pull it and hold it to a light. If you cannot see through it, replace it. In heating mode the indoor coil is where the heat pump gives up its heat, and starved airflow drives refrigerant pressure up until the high-pressure switch opens.
- Registers and returns: open every supply register and move furniture off the return grilles. Closing off rooms does the same thing a dirty filter does.
- Outdoor coil: clear leaves, cottonwood fluff and snow from all sides.
- Thermostat location: a thermostat above a supply register, in afternoon sun or beside a television reaches setpoint early and ends the cycle.
- Thermostat setup: in the installer menu, confirm the equipment type is heat pump. Honeywell-style thermostats set compressor run time in cycles per hour, and a value meant for a gas or electric furnace starts a compressor far too often.
The causes a technician looks for, most likely first
- Oversized single-stage unit: the house reaches setpoint in a few minutes, so the unit shuts off before it settles into efficient running. It is most obvious in spring and fall, and in summer it leaves the house cool but clammy because short runs remove little humidity.
- High-pressure trips in heating: a plugged filter, a matted indoor coil or a failing blower motor. The unit restarts once the pressure bleeds down, then trips again.
- Low refrigerant charge: a leak lets suction pressure fall until the low-pressure switch opens. Look for frost on only part of the outdoor coil in winter, or ice on the indoor coil in summer; the signs of a refrigerant leak lists the rest.
- Faulty defrost sensor: a coil sensor that reads colder than the coil really is tells the board to defrost on schedule whether or not there is frost. The unit heats briefly, flips into defrost on a bare coil and flips back, which looks like short cycling from indoors.
- Weak run capacitor or an overheating compressor: the compressor starts, draws high current, and its internal overload shuts it off until it cools.
Most thermostats and outdoor boards enforce a delay of about five minutes before the compressor may restart. If yours restarts sooner than that, the delay has been disabled or the board is failing, and that alone is worth a call.
Why short cycling should not wait a season
Starting is the hardest thing a compressor does. Each start pulls several times the running current and happens before oil and refrigerant flow have settled, so a unit that starts ten times an hour wears faster than one that starts twice. Short runs also never reach steady efficiency, which shows up on the hydro bill. It is not a same-night emergency unless the house cannot hold temperature.
- Call the same day if the breaker trips when the unit tries to start.
- Call the same day if the outdoor unit hums or buzzes and shuts off after a few seconds.
- Call the same day if ice covers the outdoor coil, or the indoor temperature keeps falling while the thermostat calls for heat.
What fixing a short-cycling heat pump costs
The price depends on which control is ending the cycle. These are the published bands for the usual repairs:
- Diagnostic visit: $90–$150, applied to the repair if you proceed.
- Run capacitor: $150–$350.
- Refrigerant leak repair and recharge: $300–$800+, because finding and sealing the leak is most of the work.
- Defrost or control board: $400–$800.
- Thermostat settings and airflow corrections: handled during the visit.
An oversized unit has no repair. Longer minimum run times at the thermostat help, and at replacement time a properly sized variable-speed unit ends the problem; what size heat pump you need explains the load calculation. General price ranges are on our pricing page.
ZK Mechanical times the cycles, reads the pressures and checks the thermostat setup before recommending any part. Details of a visit are under heat pump repair; to book one in Burlington, Oakville, Milton or Hamilton, phone (647) 801-1252 or use the contact form.