Why Is My Heat Pump Freezing Up?

A heat pump freezes up when the outdoor coil gets colder than it can shed, usually because of poor airflow, a dirty coil, a failed defrost control or low refrigerant. A thin layer of frost on a cold morning is normal and the unit clears it on its own. A thick block of ice that stays on the coil is a sign of a problem, and continuing to run the system can damage the compressor.

Frost you can ignore and ice you cannot

In heating mode, the outdoor coil absorbs heat from cold air, so it runs colder than the air around it and moisture can freeze on it. To handle this, the heat pump briefly switches to a defrost cycle, sending warm refrigerant to the outdoor coil. During that time you may see steam rise from the unit and hear the fan stop, and the system may blow cooler air indoors for a few minutes. That is all normal.

The concern is a unit that stays coated for hours, has ice on the fan blades or on the lines, or shows ice climbing the whole cabinet. That means the defrost cycle is not doing its job, or something is starving the coil of heat or air.

Common causes

Restricted airflow is the most common cause. A clogged air filter or a dirty indoor coil cuts the air across the system, and a dirty outdoor coil, leaves, or a pile of snow against the cabinet does the same outside. Keep at least a couple of feet of clear space around the unit and brush loose debris off the coil area gently.

The defrost control or a temperature sensor can fail, so the system never triggers a defrost. A stuck reversing valve or a fan motor that is not turning will also cause ice. Low refrigerant, which points to a leak, makes the coil run colder than it should. Refrigerant work is a licensed technician's job, and a leak has to be found and repaired properly rather than simply topped up.

Winter freezing fog and wet snow in the Tri-Cities can add to the load, and a unit sitting where roof runoff drips or drifts onto it will ice up more easily.

What you can safely do

• Switch the thermostat to emergency or auxiliary heat, or turn the heat pump off, so the compressor is not running against ice.
• Check and replace the indoor air filter if it is dirty.
• Clear leaves, snow and debris from around the outdoor unit by hand.
• Let the ice melt naturally, or run the fan only if your technician has told you it is safe.

Do not chip at the ice with a tool. The coil fins and refrigerant lines are fragile, and a small puncture becomes an expensive repair. Skip the hot water too, which can refreeze overnight and make things worse.

When to call

If ice returns after it has melted and the filter and surrounding area are clear, the cause is inside the system and needs a technician. Repeated freezing points to a control fault, a refrigerant problem or a failing part, and each day of running that way adds wear. Regular maintenance, including coil cleaning and defrost control checks, is the best way to prevent it.

Prevention is mostly routine. Change the filter on time, keep the outdoor unit clear through leaf fall and snow, and have the system looked over before heating season so a weak defrost control is caught in October rather than in a January freeze.

Air-Tech is a family-owned company in Pasco, and you can reach the team at (509) 392-2021, including emergency service around the clock if the heat pump quits in the middle of a cold night.