Heat pump won't defrost

Frost or ice builds on the outdoor coil and the defrost cycle does not clear it.

Heat Pump

normal

Diagnosed here

Heat Pump Not Heating
Troubleshooting
Working through weak or absent heat pump heating, including auxiliary heat and defrost behaviour.

Background

Heat Pump Defrost Cycle
Concept
Why an outdoor coil frosts in heating, how the control decides to defrost, and what normal defrost looks like.

Local Code Notice

HVAC installation, venting, combustion air, ventilation air, gas piping, clearances, termination locations, electrical requirements, permits, and equipment requirements vary by jurisdiction and equipment. Always verify the applicable local codes, adopted code edition, regulations, permits, and manufacturer installation instructions before installation or service.