Heat Pump Not Heating? Fix It Fast (Step-by-Step)

Quick-Diagnosis Table

Cause 1: Incorrect Thermostat Settings

A misconfigured thermostat is the single most overlooked reason for a heat pump not heating. Before anything else, confirm the thermostat is set correctly and is actually communicating with the unit.

1. Set the thermostat to HEAT mode — not “Cool,” “Fan Only,” or “Auto” without a heat setpoint.
2. Raise the setpoint at least 3–5°F above the current room temperature so a heating cycle is triggered.
3. Set the fan to AUTO, not “On” (running the fan alone circulates unheated air).
4. If the display is blank or dim, replace the batteries and check the wiring connections at the thermostat base.
5. Restore factory defaults if settings appear corrupted — consult your thermostat manual.

Cause 2: Tripped Circuit Breaker or Blown Fuse

Heat pumps have two separate circuits — one for the air handler (indoor) and one for the outdoor compressor — and either can trip independently, causing partial or total heating failure.

1. Go to your main electrical panel and locate the breaker(s) labeled for the heat pump or HVAC. There are often two: one 15–20 A breaker for the air handler and one 30–50 A double-pole breaker for the outdoor unit.
2. If a breaker is in the middle “tripped” position, switch it fully OFF and then back ON.
3. Check the disconnect box mounted near the outdoor unit for a cartridge fuse. Pull it out and test continuity with a multimeter, or swap it for a known-good fuse of the same amperage.
4. If the breaker trips again immediately or repeatedly, stop — a repeated trip signals a wiring or compressor fault requiring a licensed electrician or HVAC technician.

Cause 3: Dirty or Blocked Air Filter

A severely clogged filter restricts airflow so much that the heat pump cannot transfer heat effectively, and the system may overheat and shut down on a safety limit switch.

1. Locate the filter — typically in the return-air grille or inside the air handler cabinet.
2. Slide it out and hold it up to a light source. If you cannot see light through it, it needs replacement.
3. Note the printed size on the filter frame (e.g., 16×25×1) and install a new filter of the same size and a MERV rating between 8 and 11 for most residential systems.
4. While the filter is out, inspect the return duct and air handler coil for visible dust buildup and vacuum any accessible surfaces.
5. Set a recurring reminder to check filters monthly and replace them every 60–90 days, or more often if you have pets.

Cause 4: Frozen or Blocked Outdoor Unit

Some frost on the outdoor coil is normal in cold weather, but a unit encased in thick ice cannot absorb heat from the outside air, stopping the heating process entirely.

1. Visually inspect the outdoor unit. A thin layer of frost is normal; solid ice covering the coil fins or top is not.
2. Check that the unit has at least 18–24 inches of clearance on all sides and is not blocked by leaves, debris, or a snow drift.
3. Clear away any debris with a soft brush. Never chip ice off the coil with a sharp object — you will damage the fins.
4. Switch the thermostat to Fan Only mode for 1–2 hours to let the unit thaw passively, or pour lukewarm (not boiling) water over the coil to speed the process.
5. Once thawed, switch back to HEAT and monitor the unit. Heat pumps have a built-in defrost cycle that runs every 30–90 minutes; if icing recurs quickly, the defrost control board or defrost thermostat may be faulty.

Cause 5: Faulty Reversing Valve

The reversing valve is the component that literally switches the heat pump between heating and cooling modes; when it sticks or fails, the system may stay in cooling mode even when heat is called for.

1. With the system in HEAT mode, go outside and listen. The outdoor fan should run. If the air coming from your indoor vents feels cooler than room temperature, the reversing valve is a prime suspect.
2. Check for a humming or buzzing sound at the outdoor unit — a buzzing solenoid on the reversing valve can sometimes be heard clearly.
3. Locate and inspect the reversing valve solenoid wire connector at the outdoor unit (power off first). Look for corroded or loose pins and reseat the connector.
4. If reseating the connector does not help, the valve itself or its solenoid coil requires replacement by a licensed HVAC technician — this involves recovering refrigerant and brazing copper tubing.

Cause 6: Low Refrigerant (Refrigerant Leak)

Refrigerant is the lifeblood of a heat pump — without the correct charge, the system cannot transfer heat efficiently and may produce little or no warmth even when running continuously.

1. Watch for signs: the system runs almost constantly without reaching setpoint, ice forms on the outdoor unit or indoor coil, or you hear a hissing or bubbling sound near the refrigerant lines.
2. Feel the larger (suction) line running to the outdoor unit with the system running. It should feel cool and may have light condensation; if it is warm or dry, refrigerant charge may be low.
3. Do NOT attempt to add refrigerant yourself. Handling refrigerants (R-410A, R-22, etc.) requires EPA Section 608 certification and specialized equipment.
4. Contact a licensed HVAC technician to perform a leak search, repair any leaks, and recharge the system to the manufacturer’s specified pressure.

Repair vs. Replace Cost Guide

For general guidance on heat pump operation and energy efficiency, see the U.S. Department of Energy’s Heat Pump Systems resource.

Frequently asked questions