Heat Pump Not Heating? Causes and How to Fix It

Last updated: July 2026 · Reviewed against Carrier, Goodman and Mitsubishi service documentation

A heat pump not heating usually comes down to a thermostat setting, a dirty filter or coil, a refrigerant problem, or the unit being stuck in defrost. Because a heat pump both heats and cools, some “no heat” complaints are really settings issues — and those you can fix in minutes. Here is how to diagnose it, from the free checks to the ones that need a technician.

Start With the Thermostat

Make sure the thermostat is set to HEAT (not AUTO or EM HEAT) and the target is several degrees above room temperature. A key quirk of heat pumps: the air from the vents feels cooler than a furnace — often around 85–95°F versus a furnace’s 120°F+. That is normal for a heat pump and does not mean it is broken. If it feels lukewarm but the house is slowly warming, the system is likely working as designed.

Check the Air Filter and Airflow

A clogged filter chokes airflow and cripples heating output, and can trip safety shutdowns. Replace a dirty filter and make sure return and supply vents are open. On the outdoor unit, clear away leaves, snow, and debris — a heat pump pulls heat from outdoor air even in winter, so a blocked outdoor coil kills its heating ability.

Is It Stuck in Defrost?

In cold, damp weather, frost forms on the outdoor coil, and the heat pump periodically runs a defrost cycle — temporarily blowing cool air indoors and steaming outdoors. This is normal and lasts a few minutes. But if the outdoor unit is encased in ice or defrost runs constantly, the defrost control or sensor may have failed. See our guide on a heat pump frozen on the outside.

Outdoor Unit Not Running

Go outside and check whether the outdoor unit is running when heat is calling. If the fan and compressor are dead, check the dedicated breaker for the outdoor unit and the disconnect box near it. A tripped breaker can be a one-off or a sign of an electrical fault — if it re-trips, call a pro.

Refrigerant and Reversing Valve (Pro Territory)

If the basics check out but the heat pump still will not heat, the likely culprits need a technician: low refrigerant (a leak — heat pumps do not “use up” refrigerant, so low charge means a leak), or a stuck reversing valve (the component that switches the system between heating and cooling). Both require diagnosis and are not DIY.

When to Call a Pro

Call an HVAC technician if the breaker re-trips, the unit ices over completely, you suspect a refrigerant leak, or the reversing valve is stuck. If the system is not cooling in summer either, the problem is more likely the whole unit — see heat pump not cooling. Diagnostic visits run $90–$180.

FAQ

Why is my heat pump blowing cool air in heat mode?

Heat pump air is naturally cooler than furnace air (85–95°F), so it can feel cool even when working. Truly cold air points to a defrost cycle in progress, a stuck reversing valve, or low refrigerant. If the house is not warming at all, have it checked.

What is EM heat and should I use it?

EM (emergency) heat runs backup electric strips only, bypassing the heat pump. Use it temporarily if the heat pump fails, but it is expensive to run — do not leave it on as your normal setting. If you need EM heat constantly, the heat pump needs service.

Why is my heat pump not heating below freezing?

Heat pumps lose efficiency as it gets colder and rely more on backup heat in extreme cold. Frequent defrost cycles are normal. If it cannot keep up at all, check the filter and outdoor coil first, then have the refrigerant charge checked.