Why Is My Heat Pump Running But Not Heating?

Common Symptoms

• Vents blow air but feel cool or room temperature in heating mode
• Heat pump runs constantly but the house never reaches set temperature
• System seems to be cooling the house while set to heat
• Outdoor unit is covered in frost or thick ice that won't clear
• Emergency heat light is on or aux heat is running constantly

Most Likely Causes

1. 1

   Stuck Reversing Valve

   The reversing valve is what allows a heat pump to switch between heating and cooling modes. If it sticks in the cooling position, the system blows cold air even when set to heat — you may notice the house actually getting colder. This typically requires a technician to replace the valve ($300–$600 with labor).

2. 2

   Low Refrigerant Charge

   Heat pumps need a precise refrigerant charge to transfer heat efficiently. Low refrigerant (usually from a slow leak) significantly reduces heating capacity — the system runs but can't keep up, especially below 40°F outdoor temperature. A technician with gauges can confirm this quickly.

3. 3

   Dirty Air Filter Restricting Airflow

   A clogged air filter reduces airflow across the indoor coil, making it nearly impossible for the heat pump to transfer warmth into your home. This is the first thing to check — it takes 5 minutes and costs under $20 to fix.

4. 4

   Normal Defrost Cycle

   Heat pumps periodically enter a defrost cycle to melt frost that builds up on the outdoor coil in cold weather. During defrost (usually 5–15 minutes), the system temporarily blows slightly cool air — this is completely normal. If defrost seems to last longer than 20 minutes, the defrost board may have failed.

5. 5

   Auxiliary Heat Breaker Tripped

   Many heat pumps have auxiliary electric heat strips that kick in when the heat pump alone can't maintain temperature. If the aux heat breaker is tripped, or if there's a temperature lockout setting, your home may never reach the set temperature during cold snaps.

Quick DIY Checks

1. 1Check and replace the air filter if dirty. Verify thermostat settings: mode should be HEAT (not EM HEAT unless in a true emergency), fan set to AUTO, and temperature set above the current room reading.
2. 2Go outside and observe the outdoor unit while in heating mode. Some frost on the coil is normal — the unit should clear it periodically. A solid block of ice that doesn't clear after 30 minutes indicates a defrost system problem.
3. 3Time the defrost cycle. If the system seems to be blowing cool air, watch for 10–15 minutes. If it stays in 'cool mode' longer than 20 minutes while the thermostat shows HEAT, the reversing valve or defrost board may have failed.

1. 4Locate the auxiliary heat breaker at your electrical panel (often labeled 'AH', 'Air Handler', or 'Heat Strips'). Reset it if tripped. Aux heat is critical for maintaining temperature when outdoor temps drop below 35°F.
2. 5As a diagnostic test, switch the thermostat to EM HEAT (emergency heat). This bypasses the heat pump and runs only the electric heat strips. If the house warms up, the heat pump itself has a refrigerant or reversing valve issue — call an HVAC tech.

Repair vs Replace

Recommended Tools & Parts

• Heat Pump Air Filter (4-Pack)

  Pleated MERV 8 or MERV 11 filters for heat pump air handlers. Replace every 1–3 months. Check filter size on your existing filter frame before ordering.

  $18–$35

  Buy on Amazon →

• Dual Run Capacitor for Heat Pump

  Replacement capacitor for heat pump condensers. Fixes sluggish fans and hard-starting compressors. Match MFD and voltage rating to your unit's data plate.

  $15–$40

  Buy on Amazon →

• Heat Pump Thermostat

  Thermostat with dedicated heat pump O/B reversing valve wire support and aux/emergency heat management — not compatible with a standard furnace thermostat.

  $30–$120

  Buy on Amazon →

Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.

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