Heat Pump Not Heating

The reversing valve switches the heat pump between cooling and heating modes. If stuck in cooling, you'll get cold air on heat call. Sometimes stuck partway, you get lukewarm air.

Set to HEAT. After 5 minutes, feel the larger insulated copper line at the outdoor unit. Should be HOT in heating mode. If it's cold (the unit is still pumping heat OUT instead of IN), the valve is stuck.

Heat pumps need full refrigerant charge to move heat efficiently. A leak that lost only a small amount can significantly reduce heat output (more sensitive than cooling).

Symptoms: warm air feels weak, longer run times, ice forming and not defrosting normally, indoor air handler making hissing sounds.

Heat pumps build frost on outdoor coils in cold weather (normal). They run a defrost cycle every 30-90 minutes to clear it. If the defrost system fails, the coil ices over completely and heat absorption stops.

Inspect the outdoor unit. A small amount of frost is normal. A solid block of ice covering most of the coil is failure. Defrost cycle (which should run automatically) involves reversing into cooling mode briefly and the outdoor fan turning off.

When outdoor temps drop below the balance point (usually 30-40°F), heat pumps need supplemental aux heat (electric strips or gas furnace backup). If aux doesn't engage, you only get heat pump output — insufficient on cold days.

Look at thermostat for AUX or EMERGENCY HEAT indicator. Should illuminate when outdoor is very cold or indoor is far below setpoint. If never lit, aux heat isn't being called for.

Same failures as in AC mode — failed capacitor, contactor, fan motor, or compressor. In heating mode, system blows neutral or cool air through the indoor unit.

Walk outside. Outdoor unit should be running. If silent or only the fan runs (no compressor), use the same diagnostic flow as AC: failed capacitor, contactor, or compressor.

Heat pump thermostats have multiple modes (HEAT, AUX, EMERGENCY, AUTO). Wrong mode means wrong behavior.

Verify mode is HEAT, not EMERGENCY (which uses ONLY aux heat — inefficient but a clue). Verify setpoint above current room temp. Check for HOLD mode disabling normal scheduling.