A heat pump that is running but not producing heat is one of the most common HVAC service calls. Unlike a furnace that generates heat directly, a heat pump moves heat from the outside air into your home. When that transfer stops working, you get lukewarm or cold air from your vents.
Most causes fall into one of three categories: a control or setting problem, a refrigerant issue, or a mechanical failure in the reversing valve or outdoor unit.
Jump to Fix
- Thermostat Setpoint and Mode Check
- Emergency Heat Mode Misunderstanding
- Reversing Valve Failure
- Heat Pump in Defrost Cycle
- Low Refrigerant Charge
- Dirty or Blocked Outdoor Unit
- Frozen Outdoor Coils
1. Thermostat Setpoint and Mode Check
This is the easiest fix and the one most often missed. A heat pump thermostat needs to be set to HEAT mode, and the setpoint must be above the current indoor temperature. Many programmable thermostats use a schedule that can override manual settings, especially after a power outage or battery change.
What to check:
- The thermostat display reads HEAT, not COOL, OFF, or AUTO
- The setpoint is at least 3 degrees above room temperature
- The schedule is not holding a lower temperature than expected
- The thermostat has fresh batteries (for battery-powered units)
The fix: Confirm the mode and raise the temperature 5 degrees. If the heat pump still does not respond within 5 minutes, move to the next steps.
2. Emergency Heat Mode Misunderstanding
Most heat pump thermostats have an EMERGENCY HEAT or AUX HEAT setting. This bypasses the heat pump and runs the electric resistance (strip) heaters or a gas backup furnace. Emergency heat is designed for extreme cold when the heat pump cannot keep up.
Many homeowners accidentally switch to emergency heat mode and then wonder why the system blows cool air. In emergency heat mode, the heat pump compressor stops running. The backup heat alone may not be enough to heat the whole house, producing a sensation of weak or cool air from some vents.
The fix: Check that the thermostat is set to HEAT, not EM HEAT or AUX. Emergency heat should flash or show a separate indicator. If it is active, switch back to standard HEAT mode.
3. Reversing Valve Failure
The reversing valve is the component that switches the heat pump between heating and cooling mode. In heating mode, the valve directs refrigerant flow so the indoor coil releases heat and the outdoor coil absorbs heat. If the valve gets stuck or the coil that controls it fails, the heat pump stays in cooling mode regardless of the thermostat setting.
How to diagnose a stuck reversing valve:
- The outdoor unit blows cold air (it should blow cold in heating mode, but if the indoor air is also cold, the valve is stuck in cooling)
- Listen for a metallic click when the heat pump starts a heating cycle. If there is no click, the valve solenoid may not be getting power
- If the valve is stuck mid-travel, you may hear a hissing sound from the refrigerant line
Can you fix it? Sometimes tapping the reversing valve body gently with the handle of a screwdriver frees a stuck valve. This works about 20% of the time. If it does not work, the valve needs replacement, which requires a technician to recover the refrigerant, braze in a new valve, and recharge the system.
4. Heat Pump in Defrost Cycle
Heat pumps routinely switch to cooling mode for a few minutes to defrost the outdoor coil. When this happens, the indoor fan keeps running but the air coming from the vents is cool or lukewarm. The electric resistance heat usually activates to compensate, but if the backup heat is not working, you feel cold air.
Normal behavior: A defrost cycle lasts 5-15 minutes. The outdoor fan stops, the coil warms up, and water or steam may drip from the outdoor unit. After the defrost completes, the system returns to heating mode and warm air resumes.
The problem: If the defrost control board is faulty, the system may stay in defrost mode indefinitely or cycle into defrost too frequently. If the outdoor coil is freezing repeatedly, that points to a refrigerant or airflow problem.
The fix: If the outdoor unit is steaming and the indoor air is cool, wait 15 minutes before investigating further. If the system does not come out of defrost, the defrost control board, defrost thermostat, or outdoor fan motor may be faulty.
Defrost control board on Amazon
5. Low Refrigerant Charge
Heat pumps rely on a precise charge of refrigerant to absorb and release heat. A leak reduces the charge, which reduces the system’s ability to transfer heat. Low refrigerant is the most common mechanical cause of a heat pump not heating.
Signs of low refrigerant:
- Frost or ice on the outdoor unit, especially on the copper refrigerant lines
- A hissing sound from the outdoor unit (refrigerant leaking)
- The system runs continuously but never reaches the setpoint
- Higher than normal electric bills from extended run times
The fix: Low refrigerant is not a DIY fix. Adding refrigerant without fixing the leak is a temporary measure. A technician must locate and repair the leak, then recharge to the manufacturer’s specified amount.
6. Dirty or Blocked Outdoor Unit
The outdoor coil releases heat to the outside air during heating mode. If the coil is covered in dirt, grass clippings, leaves, or snow, heat transfer drops and the heat pump runs longer without producing enough warmth.
What to check:
- The coil fins are free of debris and not crushed or bent
- There is at least 24 inches of clearance around the unit
- The area under the unit is not overgrown with grass or weeds
- Snow and ice are not blocking the unit
The fix: Turn off power to the unit. Use a garden hose to spray the coil from the inside out to push dirt through the fins. Do not use a pressure washer (it bends the fins). Trim vegetation back to 24 inches minimum.
7. Frozen Outdoor Coils
If the outdoor coil is heavily iced over, heat transfer essentially stops. The heat pump runs but produces minimal warmth. Ice on the outdoor coil in heating mode is a sign of a defrost system failure, low refrigerant, or an airflow restriction.
The fix: Turn off the heat pump and switch to emergency heat (if available) to keep the house warm while the ice melts. Do not try to chip the ice off the coil. Damaged coil fins cause permanent efficiency loss. Once the ice has melted, run the system and watch for early frost formation. If it returns within hours, call a technician.
Parts You May Need
When to Call a Technician
Call a professional for these situations:
- Low refrigerant charge (requires EPA-certified handling)
- Reversing valve replacement
- Defrost control board fault that persists after troubleshooting
- Compressor failure signs (loud buzzing, tripped breaker)
- Electrical issues in the outdoor unit
Most heat pump refrigerant repairs cost $200-$600 depending on the leak location and the amount of R-410A or R-32 needed. A full reversing valve replacement runs $400-$900.