Heat pump not cooling

In cooling mode a heat pump works exactly like a central AC, so most of the causes are the same. The system needs three things to cool: correct instructions from the thermostat, enough air moving across both coils, and the right amount of refrigerant. When one of those is missing, the air from the registers turns lukewarm or the system runs all day without reaching the setpoint.

One cause belongs to heat pumps alone. The reversing valve decides whether the system heats or cools. If it sticks, the unit can blow warm air while the thermostat asks for cooling.

1. Thermostat set wrong. Heat mode, fan set to On, a schedule override or dead batteries.
2. Dirty filter or blocked vents. Low airflow cuts cooling and can freeze the indoor coil.
3. Tripped breaker or drain safety switch. The indoor fan runs but the outdoor unit does not.
4. Dirty outdoor coil. The unit cannot shed heat, so it runs hot and cools poorly.
5. Failed capacitor or contactor. The outdoor unit hums, clicks or stays silent.
6. Low refrigerant from a leak. Long run times, weak cooling and sometimes ice on the lines. The page on an AC refrigerant leak explains the signs and the repair, and it applies to heat pumps too.
7. Reversing valve stuck in heat. Warm air in cooling mode with everything else running.
8. Compressor failure. The least common and most costly cause.

If the system heats when you ask for cooling, rule out the thermostat first. A thermostat that was replaced or reconfigured can be set for the wrong reversing valve type, which swaps heating and cooling. Check the installer settings in the thermostat manual, or ask whoever installed it.

If the thermostat is right, the fault is in the valve circuit: the wire to the valve, the solenoid coil that shifts it, or the valve body. A solenoid coil is a modest repair. Replacing the valve itself means recovering the refrigerant, cutting the valve out, brazing in a new one and recharging the system, so it is one of the larger heat pump repairs. You cannot tell which of these it is without electrical and pressure tests.

The tech will confirm the thermostat setup, measure voltage at the outdoor unit and at the reversing valve, test the capacitor and contactor, clean or inspect the coils, measure the temperature drop across the indoor coil and read refrigerant pressures. If the charge is low, a proper repair includes finding and fixing the leak before adding refrigerant. Ask for the findings and a written quote before work starts.

Settings, filters, capacitors and dirty coils are routine fixes. Think about replacement when the compressor has failed, when a coil is leaking, or when a reversing valve replacement is quoted on an older system that has already needed major work. Compare the repair quote with a replacement quote before deciding. The heat pump overview links to the cost and replacement pages.