Heat Pump Not Heating
Working through weak or absent heat pump heating, including auxiliary heat and defrost behaviour.
Safety
Compressor: The compressor raises refrigerant pressure and temperature, so hot discharge gas leaves it in either mode.
Symptom
Symptom
The heat pump runs but the air feels cool and the space loses temperature.
Symptom
Or the outdoor unit is covered in ice and does not clear.
Symptom
Or the system only heats when auxiliary heat is energized.
Possible causes
- Reversing valve not shifting into heating, or leaking internally.
- Defrost control or coil sensor failure allowing the outdoor coil to ice over.
- Refrigerant undercharge, which shows up first in heating mode.
- Restricted indoor airflow.
- Outdoor temperature below the balance point, so the heat pump alone cannot keep up.
- Thermostat configured incorrectly for a heat pump, especially the reversing valve terminal.
- Auxiliary heat not energizing when required, or energizing all the time.
What to check, in order
Supply air temperature with auxiliary heat locked out
Heat pump supply air is normally warm but noticeably cooler than a furnace. Cool supply air with the compressor running suggests a refrigerant or valve problem
Reversing valve solenoid voltage in heating mode, against the wiring diagram
Manufacturers differ on whether the coil is energized in heating
Line temperatures at the valve body
An internally leaking valve blends hot and cool gas and shows unusually close temperatures
Superheat and subcooling using the manufacturer heating mode method
Heating mode charging differs from cooling mode and should follow the manufacturer procedure
Defrost operation using the forced defrost procedure
Confirms the valve shifts, the outdoor fan stops and auxiliary heat energizes
Indoor airflow and temperature rise
Low airflow raises head pressure and reduces heating capacity
Thermostat configuration and outdoor temperature lockouts
A misconfigured thermostat can prevent auxiliary heat or run it constantly
Outdoor temperature versus the balance point
Below the balance point, needing supplemental heat is normal operation, not a fault
Next step
If ice has built up, allow a full defrost or thaw before taking readings. Once the repair is complete, verify operation in both heating and cooling, confirm defrost initiates and terminates correctly, and check that auxiliary heat stages as designed.
Tools involved
- Digital manifold with temperature clamps
- Multimeter
- Clamp ammeter
- Manufacturer charging and defrost procedures
Symptoms
Practice & Mastery
- Heat Pump Not Heating Troubleshooting Scenarios2
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