Heat Pump Frozen? Normal Defrost vs. a Real Ice Problem, and What to Do
On a cold, damp morning it is common to see frost on a heat pump, or steam rising from it. Most of the time that is the defrost cycle doing its job. A unit buried in solid ice that does not clear is different: it cannot heat the house well, and running it that way can damage it.
Is some frost normal?
Yes. In heating mode, the outdoor coil of a heat pump runs colder than the outside air so it can pull heat out of it. When the air is near or below freezing and damp, moisture freezes on the coil, the same way frost forms on a car windshield. A thin, even layer of white frost is normal.
The heat pump is built to clear it. Every so often it runs a short defrost cycle, and the frost melts off.
What defrost mode looks like
Defrost mode can look and sound strange the first time you notice it. During a normal defrost:
- The outdoor fan stops, but the unit keeps running.
- You may hear a whoosh or a click when the system switches over, and again when it switches back.
- Steam rises from the unit. That is melting frost, not smoke.
- Water drips from the coil and may refreeze on the ground beside the unit.
- The air from your vents may feel cooler for a few minutes. Many systems turn on backup (AUX) heat to cover this.
A defrost usually lasts a few minutes. How often it runs depends on the weather. In cold, wet conditions it can run every half hour to hour and a half. Afterward the coil should look mostly clear, and the fan starts again.
When ice is a problem
Call it a problem when you see any of these:
- The coil, the top or the whole cabinet is covered in thick, solid ice, not a light frost.
- The ice does not clear after an hour or two of normal running.
- The outdoor fan is not turning, or the fan blades are frozen in place.
- Ice is building up on the top of the unit or on the fan grille.
- The house is not staying warm, or the thermostat shows AUX or EM heat running all the time.
- You hear grinding or scraping from the outdoor fan.
Why a heat pump ices up
1. The defrost system is not working. A failed defrost control board, a bad defrost sensor or a stuck reversing valve means the unit never clears its frost, so the ice keeps building. This is one of the most common causes, and it needs a technician.
2. Low airflow. A dirty air filter, closed vents or a dirty indoor coil cut the airflow and throw off how the system runs. Leaves, grass or a cover around the outdoor unit block airflow on the outside. See how your air filter affects your heater.
3. The outdoor fan motor or capacitor has failed. If the fan does not spin, the coil cannot pull in outdoor air and it freezes fast.
4. Low refrigerant. A refrigerant leak makes the coil run colder than it should. Refrigerant is not used up like fuel, so low refrigerant means there is a leak to find and fix.
5. Water dripping onto the unit. A leaking or overflowing gutter above the unit drips water that freezes on top of it. Defrost mode is not built to melt a block of ice from roof runoff.
6. Freezing rain or an ice storm. Freezing rain can coat a unit faster than defrost can clear it. Once the weather breaks, the unit should clear itself. If it does not, call.
What to do if your heat pump is frozen
- Watch it for a little while. If it is a light frost, give it time to run a defrost cycle.
- Change the air filter and make sure supply vents and return grilles are open.
- Clear the outdoor unit. Brush off snow and pull away leaves or anything leaning on it. Keep about two feet of open space around it.
- Look up. If a gutter or roof edge is dripping on the unit, that needs fixing.
- If it is a solid block of ice, turn the system off at the thermostat. You can rinse the outdoor coil with lukewarm water from a garden hose to melt the ice. Let it drain before you turn it back on.
- Keep the house warm. If your thermostat has an EM or emergency heat setting, use it until the unit is repaired. It runs the backup heat only and keeps the outdoor unit from running iced up. See EM heat vs. heat.
- If it freezes again, the defrost system, the fan or the refrigerant charge needs a technician.
What not to do
- Do not chip the ice with a screwdriver, ice pick, hammer or shovel. The coil fins and refrigerant tubes are thin, and a puncture means a refrigerant leak.
- Do not pour boiling water on the coil. Sudden heat can damage parts.
- Do not cover the unit with a tarp while it is running. It needs airflow.
- Do not keep resetting the breaker if the unit trips it or will not run. That points to an electrical or motor problem.
- Do not let it run for days covered in ice. That is hard on the compressor, the most expensive part of the system.
AUX heat and EM heat during a freeze
Most San Antonio heat pumps have electric backup heat strips. AUX on the thermostat means the backup heat is helping the heat pump, which is normal during defrost and on the coldest mornings. EM heat means you turned the heat pump off and are running the backup heat by itself. Backup heat costs more to run, so use EM heat as a stopgap, not as a fix. Read EM heat vs. heat on your thermostat and the 20-degree rule for heat pumps.
Getting ready for a San Antonio freeze
San Antonio only gets a handful of freezing nights most winters, so heat pumps work well here. The problem nights are the ones with freezing rain or a hard freeze, when the system is working its hardest. Before a cold snap:
- Put in a clean air filter.
- Clear leaves, grass and clutter from around the outdoor unit.
- Clean out the gutters above the unit.
- Check that the thermostat switches to heat and that backup heat comes on.
- Have the system checked in the fall. A tune-up includes testing the defrost controls, the outdoor fan and the refrigerant charge. See our maintenance plan.
When to call
- Solid ice that does not clear after an hour or two, or comes back after you melt it.
- The outdoor fan is not spinning while the unit runs.
- The unit never seems to go into defrost.
- AUX or EM heat runs all the time and your electric bill jumps.
- The house will not stay warm. See heater blowing cold air and heater not turning on.
- You see smoke (not steam) or smell burning. Turn the system off. See heater smells like burning.
Cowboy's answers 24/7 with our own dispatch on Blanco Road. See heat pump repair in San Antonio, heating repair, or call (210) 495-7771.
Frequently asked questions
Is it normal for a heat pump to freeze up in winter?
A light, even layer of frost is normal when it is cold and damp, and the defrost cycle clears it. Thick, solid ice on the coil or the top of the unit that does not clear after an hour or two is not normal and needs a technician.
How long does a heat pump defrost cycle last?
Usually a few minutes. How often it runs depends on the weather; in cold, wet conditions it can run every half hour to hour and a half. During defrost the outdoor fan stops and you may see steam.
Why is steam coming from my heat pump?
Steam during defrost is melting frost and is normal. Smoke with a burning smell is not. If you see smoke or smell burning, turn the system off and call for service.
Can I pour hot water on a frozen heat pump?
Turn the system off first, then rinse the outdoor coil with lukewarm water from a garden hose. Do not use boiling water, and never chip the ice with tools, because the coil fins and tubes are easy to puncture.
Should I turn off my heat pump if it is frozen?
If it is covered in solid ice, yes. Turn it off at the thermostat, or switch to EM heat if you have it so the backup heat keeps the house warm, and have a technician check the defrost controls, outdoor fan and refrigerant charge.
Related service: heat pump repair in San Antonio · heating repair · all heating and cooling services. Cowboy's AC answers 24/7 at (210) 495-7771.
