Heat Pump Frozen or Iced Over Outside

Some frost on a heat pump's outdoor coil during cold weather is completely normal — the unit is designed to occasionally reverse itself and melt the frost off through a built-in defrost cycle. But a heat pump that's fully iced over for hours, encased in a solid block of ice, or dripping ice down onto walkways below is failing the defrost sequence. Here's how to tell which you're dealing with.

Most Likely Causes

1. Normal frost between defrost cycles (not a problem)

In outdoor temperatures between 25-45°F with high humidity, your heat pump will accumulate a light layer of frost on the outdoor coil. Every 30-90 minutes (depending on the defrost strategy), the unit briefly reverses to melt it. During that reverse cycle, you'll see steam rising off the outdoor unit and hear the reversing valve 'clunk' — both normal. If the frost is clearing itself within an hour, everything is working.

2. Failed defrost board or defrost thermostat

If the outdoor unit stays iced over for hours and never runs a visible defrost cycle, the defrost board (the control that decides when to reverse) or the defrost thermostat (the sensor that measures coil temperature) has failed. The heat pump keeps trying to pull heat from the frozen coil, can't, and gradually gets worse. Fix: replace the defrost board or thermostat.

3. Failed reversing valve

The reversing valve physically switches refrigerant flow between cooling and heating (and briefly to defrost mode). When it fails or gets stuck, the defrost cycle doesn't actually reverse the flow, so ice keeps accumulating. Symptoms: outdoor unit iced solid, indoor air feels cool even with the system running. Reversing valve replacement runs $680-$1,100.

4. Low refrigerant

Low refrigerant charge causes the outdoor coil to run colder than designed, which means more frost forms and defrost can't keep up. Symptoms: gradual loss of heating capacity over weeks, ice on refrigerant lines (not just the coil), higher electric bills. Requires leak detection and repair, then a proper charge.

5. Blocked airflow (leaves, debris, snow drifts)

If leaves, mulch, tall grass, or snow are blocking the outdoor coil, the unit can't get enough air across the coil to defrost efficiently. Clear a 2-foot perimeter around the outdoor unit and never bury it in snow drifts. Never chip ice off the coil — you'll damage the fins.

6. Fan motor failed on outdoor unit

If the outdoor fan isn't running (or running slowly), airflow across the coil drops to near zero and heavy ice builds. Look at the fan through the top grille when the unit is calling for heat — the fan should spin freely and steadily. If it's stopped or wobbling, the motor or capacitor needs replacement.

Try These Steps Before You Call

• Watch the outdoor unit for 60-90 minutes to see if a defrost cycle runs. You'll hear a clunk from the reversing valve and see steam rising off the coil as ice melts.
• Clear any leaves, mulch, snow drifts, or debris within 2 feet of the outdoor unit.
• Do NOT chip or scrape ice off the coil — you'll bend the fins and cause permanent damage. If the unit is fully encased in ice, switch the thermostat to 'Emergency Heat' (this uses only the backup heat strips), then call us.
• Look at the outdoor fan through the top grille when the system is running — it should be spinning steadily. Not spinning = motor or capacitor problem.
• If ice is dripping onto walkways below, redirect drainage or add gravel to prevent slip hazards until we can service.
• In Emergency Heat mode, your bill will run higher (all electric-strip heat) — but it keeps you warm and doesn't stress the frozen heat pump.

Other Common Heating Problems

See also: AC problems