A heat pump that freezes up has ice on a coil that should be cold but never frozen solid. In heating season it’s the outdoor unit that ices; a little frost there is normal, and the unit melts it off in a defrost cycle. In cooling season the indoor coil is the cold side, and ice there is never normal. When ice stays, the cause is usually a failed defrost control or outdoor sensor, low refrigerant from a leak, blocked airflow, or a faulty fan motor.
Winter: ice on the outdoor unit
To heat your home, the outdoor coil has to be colder than the air around it so it can absorb heat from that air. On a cool, damp morning the coil surface can sit below 32°F while the air is several degrees warmer, and the moisture in that air turns to frost on it. At intervals the control board checks the coil, usually through a temperature sensor clipped to it, and starts a defrost: the system briefly switches into cooling mode, the outdoor fan stops, and hot refrigerant warms the coil until the frost lets go. Steam and water running off the cabinet mean it’s working.
Trouble starts when that cycle doesn’t happen or doesn’t finish:
- Defrost cycle failure. A failed defrost board, timer or relay never starts the defrost, and frost thickens into ice.
- Outdoor sensor issues. A coil or outdoor-air sensor that reads wrong can make the board skip a defrost or end it too soon.
- Faulty fan motor. Apart from defrost, the outdoor fan should run whenever the unit heats. If it stops, no air crosses the coil and ice builds fast.
- Low refrigerant. A leak makes the coil run colder than designed, often with frost covering just a section of it.
- Blocked airflow. Leaves, weeds, a fence or shrubs too close, or water dripping onto the unit from a roof edge.
Summer: ice on the indoor coil
In cooling mode a heat pump works exactly like an air conditioner. The indoor coil runs cold to pull heat and moisture from the house air. Starve that coil of air, or let the charge run low through a leak, and its surface falls below freezing until ice spreads across the fins. You’ll notice weaker cooling, frost on the larger copper line outside, and later water around the indoor unit as it thaws. The usual culprits are a dirty filter, covered return grilles, a weak blower motor, a dirty coil or a leak.
Which kind of ice is it?
| What you see | Probable cause | Usual fix |
|---|---|---|
| Coil stays frosted all morning, never steams or clicks over | Defrost board, timer or sensor failure | Defrost controls tested; failed part replaced |
| Ice growing while the outdoor fan stands still | Failed fan motor or its capacitor | Motor or capacitor replacement |
| Frost on one section of the coil, weak heating or cooling | Leak, so the charge is low | Leak repair and recharge |
| Weak heat and ice that never clears | Reversing valve stuck partway | Valve diagnosis and repair |
| Summer ice inside the air handler or on the outdoor copper line | Low airflow or low charge | Filter, blower or coil cleaning; leak search |
| Ice packed around the base or top | Debris, poor drainage or roof runoff | Clearing the area; redirecting water |
| Grinding or scraping from the outdoor unit | Fan blade hitting ice | Unit off; technician frees and checks the fan |
Right now
- Don’t attack the ice. No chipping, scraping or hot water.
- In winter, if your system has backup heat, set the thermostat to Emergency or Aux Heat. The outdoor unit stops and the backup warms the house while the ice melts. Backup heat usually costs more to run, so use it only until the repair.
- In summer, set the system to Off and the fan to On. Room air blowing across the coil thaws it sooner; make sure the drain keeps up with the water.
- Pull the outdoor disconnect before you clear plants or debris from around the unit, and fix a gutter that drips on it.
- Swap the indoor filter in either season, and switch off a unit whose fan is scraping ice.
When to call
Call if ice remains two or three hours later without any defrost, if it returns within a day, if the outdoor fan isn’t turning, or if summer icing comes back even with a new filter. A technician starts by forcing a test defrost to see whether the board reacts, the coil sensor reads correctly and the valve shifts. Next come the fan motor and capacitor, indoor airflow and refrigerant pressures. Low refrigerant means a leak, which is located and sealed before the system is recharged. Our technicians are EPA-certified for refrigerant work.
Repair cost
Most heat pump repairs fall between $125 and $700, and larger jobs can run higher. Sensors and capacitors sit near the bottom of that range; a fan motor, defrost board or leak repair costs more. You get the price before any work starts. If a big leak or a failed reversing valve on an older unit makes repair hard to justify, compare it with a new heat pump; installation estimates are free. Ductless units ice up for the same reasons; see mini-split repair.
Local note
Heat pumps in San Jose work both seasons: long cooling runs on hot afternoons and heating on cool winter nights. Outdoor-coil frost shows up mostly on cold, damp mornings, and more often in the foothill neighborhoods than on the warmer valley floor. Long hard freezes are uncommon here, so an outdoor unit sitting in a block of ice is a repair call, not a cold-weather quirk. Ice in July always is.