Why Is My Heat Pump Running But Not Heating?
If your heat pump runs but blows cool or lukewarm air in mild Frederick weather, a component has failed — this is not the same as a unit that simply struggles below freezing. The usual causes are a stuck reversing valve, low refrigerant from a leak, a defrost control fault, or a dirty outdoor coil that blocks heat transfer. A reversing valve stuck in cooling mode is the classic culprit: the system runs normally but pushes cold air. Low charge and a fouled coil both starve the system of the heat it should be moving indoors.
Why a heat pump runs but will not heat
| Cause | Symptom |
|---|---|
| Reversing valve stuck in cooling | Outdoor unit runs, indoor vents blow cold air |
| Low refrigerant (leak) | Weak, barely-warm airflow; long run times; possible hissing |
| Defrost control fault | Outdoor coil ices over and never clears |
| Dirty outdoor coil | Reduced heat output; unit runs constantly |
| Failed compressor valves | Fan spins but little to no temperature change |
Start with what you can safely check
Rule out the simple stuff before assuming a major fault. Confirm the thermostat is set to Heat, not Cool or Auto, and that the setpoint is above room temperature. Check that the outdoor unit is clear of leaves, snow, and debris, and that the air filter is clean — a clogged filter chokes airflow and mimics a heating failure. If those are fine and the vents still blow cool, the problem is inside the sealed system, where a technician's tools are required. Reliable Frederick heat pump repair begins with a refrigerant-pressure and reversing-valve check.
Do not open the electrical panel, add refrigerant, or probe wiring yourself. Refrigerant is handled under EPA rules, and a stuck reversing valve needs sealed-system service. These are diagnosis-and-repair jobs for a licensed tech, not DIY fixes.
- Set the thermostat to Heat and raise it a few degrees above the room.
- Clear snow, ice, and leaves from around the outdoor unit.
- Swap a dirty filter — restricted airflow can look like lost heat.
What the diagnosis usually finds
A reversing valve stuck in cooling is the signature fault: the compressor runs, refrigerant flows, but the system moves heat the wrong direction and blows cold. Low refrigerant from a slow leak is next most common, and simply topping it off without fixing the leak wastes money. A defrost board that misfires leaves the outdoor coil iced, which then blocks heat transfer entirely.
Ice on the outdoor unit can be a symptom here, though brief frosting is normal during a defrost cycle. If the coil ices and never clears, that points to a defrost fault worth diagnosing. A loud or grinding unit alongside weak heat suggests a failing part, so noise is a clue worth reporting.
- Persistent outdoor ice can signal a defrost problem rather than normal frosting.
- New rattles or screeches with weak heat? See why a heat pump gets loud.
- Tell the tech when it started and what the weather was — it narrows the cause fast.
Questions homeowners ask next
Why Is My Heat Pump Running But Not Heating?
In normal weather, a running heat pump that blows cool air has a component fault: a reversing valve stuck in cooling, low refrigerant from a leak, a defrost control failure, or a dirty outdoor coil. Each blocks the system from moving heat indoors, so a technician's diagnosis is the reliable fix.
Can I fix a stuck reversing valve myself?
No. The reversing valve sits inside the sealed refrigerant circuit, so replacing it requires recovering refrigerant, brazing, and recharging under EPA rules. This is licensed-technician work, not a DIY repair.
Is low refrigerant the same as needing a recharge?
Not exactly. A sealed system does not use up refrigerant, so low charge means a leak. Topping it off without finding and fixing the leak only buys a few weeks before the problem returns.