Should your HVAC site explain a freezing heat pump so AI assistants cite you?

Frost at 6 a.m. is not the emergency it looks like

You walk out on a January morning and the outdoor unit is white. The coil is frosted over, the fan is still, and for a moment it looks like the system has frozen solid. Most of the time it has not. A heat pump pulls heat out of cold air, which means its outdoor coil runs colder than the air around it, and below about 40 degrees Fahrenheit the moisture in the air freezes onto it. Frost is a normal part of the job, and the system has a built-in routine for dealing with it.

The routine is the defrost cycle, and it runs every 30 to 90 minutes while the system is heating in cold weather. The reversing valve flips the system into cooling mode for a short time, the outdoor fan stops, and hot refrigerant melts the frost off the coil. You will see steam rising off the unit, which is the frost turning to vapor, and the vents inside will blow cool air for 30 to 60 seconds while the cycle runs. All of that is normal. The defrost cycle lasts 5 to 15 minutes, and then the system flips back to heating.

The defrost cycle, minute by minute

A defrost cycle has a telltale sequence, and knowing it keeps you from calling a technician for a working system. First the outdoor fan stops, which is the most noticeable change. Then you may hear the reversing valve switch with a soft whoosh. Steam rises from the coil as the frost melts, and water drips from the base of the unit. Inside, the auxiliary heat or emergency heat kicks in to cover the gap, which is why the vents feel cool for a short stretch before the warm air returns.

The cycle ends when the coil sensor reads warm enough, or when the timer hits its limit, usually 5 to 15 minutes. The fan starts again, the steam stops, and the coil looks clear. If you are watching the clock, a cycle that finishes inside that window and leaves a clean coil is a healthy system. The frequency matters too: on a damp night near freezing, defrost runs often. On a dry, very cold night, it runs less, because there is less moisture in the air to freeze.

The four signs the freeze is a fault, not weather

Ice that survives a defrost cycle is the first sign of trouble. A healthy cycle clears the coil completely. If the coil is still coated after the fan restarts, something is wrong with the defrost control, the reversing valve, or the refrigerant charge, and the system will keep freezing thicker with every cycle. A useful rule of thumb is four hours: if the unit has been iced over for four hours without clearing, call a technician.

The other three signs are a defrost cycle that runs 20 to 30 minutes or longer, the outdoor fan still running during defrost, and a unit that is encased in ice rather than frosted. A long defrost means the coil is not getting hot enough, which points at low refrigerant or a stuck reversing valve. A fan that keeps running during defrost fights the heating of the coil. And a solid block of ice, rather than a white coating, usually means a drainage problem: melt water is pooling and refreezing at the base instead of draining away.

What not to do with a frozen coil

The worst thing you can do to a frozen heat pump is chip the ice off with a tool. The coil is made of thin aluminum fins, and a screwdriver or a shovel bends them in seconds. Bent fins block airflow, which makes the freezing worse, and a punctured coil leaks refrigerant, which turns a service call into a repair bill. If the ice has to come off and the defrost cycle is not doing it, lukewarm water poured over the coil is the only safe method, and even that is a stopgap while you wait for a technician.

Never pour hot or boiling water on the coil. The sudden temperature change can crack the fins and the copper tubing. Never turn on emergency heat and leave it on as a permanent fix, either. Emergency heat is electric resistance heat, and it costs two to three times as much per unit of heat as the heat pump. It is a bridge to a repair, not a mode of operation. The useful thing a homeowner can do is keep the area around the unit clear: leaves, snow, and debris against the coil block airflow and make freezing more likely, and a unit sitting in a puddle will freeze from the base up.

Related reading

If this topic is useful, these earlier guides go deeper on connected questions: Should your HVAC site explain a frozen AC coil so AI assistants cite you?, Should your HVAC site explain a blank thermostat screen so AI assistants cite you?, and Should your HVAC site explain HVAC noises so AI assistants cite you?.

Frequently asked questions

Is frost on my heat pump normal in winter?

Yes. Below about 40 degrees Fahrenheit, moisture in the air freezes onto the outdoor coil, because the coil runs colder than the surrounding air. The defrost cycle melts it off every 30 to 90 minutes. Frost is a problem only when it survives a defrost cycle or builds into a solid block of ice.

How long should a heat pump defrost cycle last?

A normal defrost cycle lasts 5 to 15 minutes. The outdoor fan stops, steam rises from the coil, and the vents inside blow cool air for 30 to 60 seconds while auxiliary heat covers the gap. A cycle that runs 20 to 30 minutes or longer means the coil is not getting hot enough, and that is worth a service call.

Why does cold air come out of my vents during defrost?

During defrost the system briefly runs in cooling mode to send hot refrigerant to the outdoor coil, so the indoor coil goes cold for a few minutes. Auxiliary heat is supposed to cover that gap, but it takes a moment to catch up, which is why the vents feel cool for 30 to 60 seconds. If the cool air lasts the whole cycle, the auxiliary heat may not be working.

Should I turn on emergency heat when my heat pump freezes?

Only as a short bridge to a repair. Emergency heat is electric resistance heat, and it costs two to three times as much per unit of heat as the heat pump. If the unit is iced over and not clearing, switch to emergency heat to keep the house warm, then call a technician the same day.