Why Your Heat Pump Is Freezing Up

A heat pump freezing up is only a problem when the ice does not go away. Frost on the outdoor coil is part of winter operation, and every air-source unit has a defrost cycle to deal with it. When that cycle fails or the coil cannot breathe, the frost becomes a solid block and heating stops.

Is frost on a heat pump normal?

Yes, within limits. In heating mode the outdoor coil runs colder than the outside air, so moisture freezes on it. The unit then reverses into cooling mode for a short time to push heat to the coil, and the frost melts. Energy Vanguard notes that the outdoor fan switches off during defrost so the heat goes into the ice, not the yard.

That means a little steam and a brief pause in heating are ordinary. Damp mornings in the 30s make frost more likely than very cold, dry air, because dry air carries less moisture.

What makes a heat pump freeze up?

Lennox lists a handful of common causes, and they match what technicians see.

• A defrost control board, timer or sensor that no longer starts the cycle
• Low refrigerant from a leak, which makes the coil run colder than designed
• Restricted airflow from a dirty filter, debris, a collapsed duct or blocked vents
• Poor drainage, so meltwater pools and refreezes under the unit
• Freezing rain, sleet or wet snow that overwhelms the defrost system
• A failing reversing valve or outdoor fan motor

A dirty filter looks too small to matter, but it is on the list for a reason. Less warm air crossing the indoor coil means less heat for the system to move. If you need a refresher on filters, read our filter guide.

How does the defrost cycle decide when to run?

Lennox says defrost cycles should run about every 30 to 90 minutes while the unit is heating in frost-prone weather. The control board decides, using a timer, a coil sensor or both. If the sensor reads the coil as warm when it is actually iced over, the board never calls for defrost, and the frost keeps stacking up between cycles.

Two details help you judge what you are seeing:

• The outdoor fan stops during defrost, so the heat goes into the ice, according to Energy Vanguard.
• Indoor air can feel cooler for a few minutes, because the system briefly runs in cooling mode. Backup heat strips may switch on to soften that, according to the same source.

Cool air for a few minutes every hour or so is the design. Cool air that lasts, with ice that never clears, is the fault.

Outdoor coil or indoor coil: which one froze?

Everything above is about the outdoor coil in winter. A heat pump also cools in summer, and in that mode the indoor coil is the cold one. Ice on the refrigerant line at the air handler, water around it, or weak airflow from the vents in July points indoors. The usual culprits are a dirty filter, a blocked return or low refrigerant. The safe first step is the same: set the thermostat to Off, then run the fan alone so the coil thaws.

What can you safely do while it is iced over?

Switch the thermostat to Off so the compressor stops working against the ice. Then do the following.

1. Clear snow, leaves and debris, and keep about two feet of open space around the unit.
2. Check the indoor filter and replace it if it is dirty.
3. Do not chip the ice or use hot water. Tools damage fins, and hot water can crack parts.
4. Switch to emergency heat if the house is getting cold. It uses the backup heat, not the outdoor unit.

What should you write down before you call?

A technician can narrow the fault faster when you hand over specifics. Jot these down while the unit is still iced:

1. Where the ice sits: all over, only at the base, on one side, or on the lines.
2. What the fan is doing now, and whether it spins after the ice melts.
3. The weather for the last day: damp and in the 30s, freezing rain, or dry and cold.
4. How long it has been iced, and whether it thawed and came back.
5. Any noise: grinding, squealing or banging.

Lennox lists ice on only one side of the unit and an outdoor fan that will not spin after the thaw as reasons to call.

If the unit is clear and still freezes again soon after thawing, that is a repair call. Our heat pump repair page covers how a frozen coil, a weak defrost cycle and a low charge are told apart.

From the field: what cold-night calls teach

Cold-weather failures are the ones people remember. One Google reviewer, Gaquan C., wrote that Kenan Corporation came out in 10-degree weather and the family had heat again after a furnace call. Calls like that are why a heat pump with a frozen coil should be treated as a heating emergency once the house starts to cool.

General trade knowledge adds one more point. The frost is usually the symptom, not the fault. A technician who only chips ice off has not fixed a bad sensor or a slow leak, and the unit will be back to ice by the next damp morning.

Can the way the unit is installed make it freeze?

Yes, in two ways, and both are fixable. Meltwater has to leave the base of the cabinet, so a unit sitting in a low spot, on a settled pad or under a roof edge or downspout will pool and refreeze overnight. Energy Vanguard shows how enough ice at the base can block drainage altogether.

Raising a unit is rarely worth the cost if the freeze-ups come from a failed sensor instead of drainage.

Worked example: a damp 34-degree morning

Say you find a heat pump iced over at 7 am, with the house a few degrees below the thermostat setting.

1. Turn the thermostat to Off, then to emergency heat if the house is cooling. Backup heat uses more electricity than the heat pump, so treat it as a short-term fix.
2. Clear debris and check the filter. Lennox says an above-freezing thaw takes about two to four hours.
3. If ice remains at six to eight hours, or comes back quickly, stop waiting and book the repair.

The trade-off is cost against risk. A day on emergency heat adds to the bill, while a compressor running into a blocked coil can cost far more.

When is a frozen heat pump a repair, and when is it a replacement?

Repair first, almost every time. A defrost board, a sensor or a fan motor are common, bounded repairs. A leak is different, because a coil leak on a unit near the end of its range changes the math. ENERGY STAR suggests having a professional evaluate a heat pump that is more than 10 years old, and contractor sources commonly put the working life of these systems at 15 to 20 years with routine maintenance.

If you are weighing it, our heat pump installation page explains how a new unit is sized, and ENERGY STAR cold climate models are tested down to 5°F. Keep in mind that St. Louis logged 4,100 heating degree days in 2025 against a normal of 4,433, according to the National Weather Service. A mild winter does not make a faulty defrost cycle safe.

Shrewsbury and Webster Groves: the next step

For homes around Shrewsbury and Webster Groves, the pattern is familiar: mid-century houses, aging equipment and a first hard freeze that exposes the weak part. Turn the system off, clear the unit, and call.

We are open 24 hours. Same-day service depends on the schedule, not a promise. You get a clear, upfront quote before work starts, and we check what else is working after the repair.