Ice on Your Heat Pump: Normal Frost or a Problem?

Normal frost or a real freeze-up?

Sources: Trane defrost cycle guide, Lennox.

Why a heat pump frosts up even above freezing

Trane explains that in winter the outdoor coil is typically 10 to 20 °F colder than the outdoor air. It collects condensation the way a glass of ice water does, and when the coil is cold enough, that water freezes. So a heat pump can go into defrost when it is in the low 40s outside, though it is more common in the 20s and 30s (Trane).

NYSERDA puts it simply: when the outdoor coil is below 32 °F, condensation on it freezes into a layer of frost, and the heat pump switches into a defrost cycle (NYSERDA).

What a defrost cycle looks and sounds like

During defrost, the reversing valve sends hot refrigerant to the outdoor coil, which briefly puts the heat pump into cooling mode. You may notice:

• A cloud of steam rising from the outdoor unit.
• A swooshing sound as the refrigerant reverses.
• The outdoor fan stopping.
• The compressor getting louder or quieter.
• AUX heat on the thermostat while backup heat keeps the house warm.
• Cool air from the vents, only if the system has no backup heat.

All of these come from Trane's guide, which calls them normal (Trane).

How long and how often varies by system:

Trane notes that many heat pumps use timed defrost instead, running after a set number of hours and for a set length whether ice is there or not (Trane). Your owner's manual will say which type you have.

What causes a heat pump to freeze up

Lennox lists these causes of heavy ice (Lennox):

Trane adds that ice which is not going away, or is growing, likely means low refrigerant, a faulty defrost control or a faulty sensor (Trane).

What to do if your heat pump is iced over

Steps from Lennox and Trane:

1. Wait first. Trane suggests waiting a few hours to see if the unit defrosts itself.
2. If ice is heavy and the house is not heating, set the thermostat to Off, or to Emergency Heat if you have it. Lennox says Emergency Heat lets the outdoor unit thaw while keeping the house warm.
3. Clear at least two feet around the unit of snow, leaves and debris. Do not pile snow against the sides.
4. Check the indoor air filter and replace it if it is dirty.
5. Make sure supply vents inside are open and not blocked by furniture.
6. Check that gutters and downspouts do not drain onto the unit.
7. Let it thaw naturally. Lennox says this typically takes 2 to 4 hours above freezing. If the ice has not melted in 6 to 8 hours, or it refreezes quickly, call a technician.

A dirty filter is one of the few freeze-up causes you can fix yourself; our guide on how to tell if your furnace filter needs changing covers the checks. If the thermostat shows a message such as "Waiting for equipment" when you switch back to heat, that is usually a built-in compressor delay.

What not to do

• Do not pour hot water on the unit. Lennox warns the sudden temperature change can crack components.
• Do not chip or pry at the ice. Tools can bend the coil fins and fan blades, and a small dent reduces efficiency.
• Do not wrap or cover the sides. NYSERDA says the outdoor unit needs airflow and should not be covered in winter; protect it from roof snow with placement under an eave or a snow shield (NYSERDA).

When to call for service

Lennox's list of reasons to call a dealer includes (Lennox):

• The unit refreezes quickly after thawing.
• Refrigerant lines are covered in ice, inside or outside.
• The outdoor fan does not spin after the ice melts.
• Grinding, squealing or banging noises.
• Ice forms only on one side or in unusual patterns.
• The defrost cycle never starts after several hours of running.

Trane adds: if the unit seems to be constantly switching into defrost, or defrost cycles keep getting longer while ice spreads, call for service (Trane).