Heat Pump Not Heating? 7 Causes & How to Fix It

When the heat pump runs but won't warm the house

Heat pumps heat by moving heat rather than burning fuel, so they behave differently from a furnace - and some "not heating" complaints are actually normal heat-pump behavior. Here are the seven real causes, and how to tell normal from broken.

Quick takeaways:

• Heat-pump air feels cooler than furnace air (90-100°F) - that's normal
• Brief cool air during a defrost cycle is normal too
• A stuck reversing valve leaves it in cooling mode - a real fault
• A thick ice block on the outdoor coil is not normal - call a pro
• In deep cold, confirm auxiliary/backup heat is working

1. Thermostat set wrong

Confirm it's on HEAT (not COOL or EMERGENCY HEAT unless intended), the target is above room temperature, and batteries are fresh. On heat pumps, an incorrectly configured thermostat can also fail to call for backup heat when needed. (See thermostat troubleshooting.)

2. Dirty air filter

As with any system, a clogged filter chokes airflow, weakens heat output, and can freeze the coil. First thing to check.

3. Stuck reversing valve

The reversing valve is what lets a heat pump switch between cooling and heating. If it sticks, the unit stays in cooling mode and blows cold air in winter. This is a signature heat-pump fault and a pro repair. (How the valve works: what is a heat pump.)

4. Frozen outdoor coil / defrost problem

Heat pumps normally frost up outside and run a defrost cycle to melt it. If the defrost control, sensor, or board fails, ice builds into a thick block that blocks heat transfer. A light frost that clears is fine; a coil encased in ice needs a technician.

5. Low refrigerant

A heat pump moves less heat when the charge is low, so it struggles to warm the house - and low charge always means a leak. Expect weak heat and possibly more icing. Refrigerant is a licensed-pro repair (recharge costs).

6. Auxiliary / backup heat not engaging

In cold weather, heat pumps rely on auxiliary heat (electric strips or a gas furnace in a dual-fuel setup) to supplement. If that backup fails or isn't wired to engage, the house can't keep up on the coldest days even though the heat pump runs. A pro checks the strips, sequencer, and controls.

7. Outdoor unit blocked or iced at the base

Snow drifts, ice, or debris packed around the outdoor unit restrict airflow and defrost. Keep the unit clear and elevated per the install; never chip ice off the coil with a tool - you'll puncture it.

"It's blowing cool air" - is it actually broken?

Two normal things fool people: heat-pump supply air is naturally cooler to the touch than a furnace's (it's warmer than the room, just below skin temperature), and the system briefly blows cool during defrost. If the air is genuinely cold, the room temperature is dropping, or the outdoor coil is a solid ice block, then it's one of the faults above.

Safe to DIY vs. call a pro

Safe yourself: thermostat mode and batteries, replacing the filter, clearing snow/debris around (not off) the outdoor unit, letting a normal defrost finish.

Call a pro: stuck reversing valve, defrost-control faults, low refrigerant, failed auxiliary heat, or a coil that keeps freezing solid.

Bottom line

Heat-pump "not heating" is often a thermostat or filter issue - or normal cool-feeling air and defrost cycles mistaken for a fault. Genuine failures (reversing valve, refrigerant, defrost control, backup heat) need a technician, especially before a cold snap.

Heat pump not keeping up? Connect with a licensed local pro for same-day heating service - no-heat priority, upfront pricing, no overtime fees.