Why Is My Heat Pump Freezing Up?

Normal compared with a problem

• Normal: a layer of frost that appears and clears within a few hours
• Normal: steam rising from the unit right after a defrost cycle
• Normal: a whooshing sound as the reversing valve shifts
• Problem: ice covering the entire coil, the fan blade, or the top of the unit
• Problem: ice that never clears across a full day
• Problem: the outdoor unit is silent while backup heat runs constantly

What causes defrost failures

• Failed defrost sensor or thermostat, so the control never learns the coil is frozen
• Defrost control board failure
• Reversing valve that will not shift into the cooling position needed for defrost
• Low refrigerant charge, which drops coil temperature and increases frost accumulation
• Restricted airflow across the outdoor coil from debris or a failing fan motor
• Water dripping onto the unit from a gutter or roof line and refreezing

What you can safely do

Turn the system to emergency or auxiliary heat so the house stays warm without asking the heat pump to run while iced. Do not chip ice off the coil, since the fins and refrigerant tubing damage easily.

If you can safely reach it, pouring warm water over the coil can clear ice. Never use a sharp tool or a torch.