A heat pump blowing cool air in heating mode is normal in two situations — during a defrost cycle (a few minutes at a time, when the outdoor unit stops to melt frost) and because heat pump supply air is simply cooler than furnace air (85–100°F, which can feel lukewarm) — and a problem in the others: the thermostat is in the wrong mode, the outdoor unit isn’t running, refrigerant is low, or the reversing valve is stuck in cooling. Here’s how to tell them apart in a couple of minutes.
Normal: defrost
In heating mode the outdoor coil gets colder than the outdoor air and frost builds on it, especially between about 25°F and 45°F with damp air, which describes most North Carolina winter days. Periodically the system reverses to melt the frost: the outdoor fan stops, the unit may steam, and for two to ten minutes the indoor air is cool unless the auxiliary heat strips cover it (they usually do, briefly). If cool air comes and goes in short episodes and the house holds temperature, that’s defrost doing its job.
Normal: heat pump air isn’t furnace air
A gas furnace delivers 120–140°F air; a heat pump delivers roughly 85–100°F, cooler on the coldest days. Below skin temperature, moving air feels cool even when it’s heating the house. If the room is at setpoint and the vent air is warmer than the room, nothing is wrong. Heat pump vs furnace in NC covers what to expect.
Not normal: check these
Thermostat mode. Set to Heat, not Cool or Fan. Fan-only mode blows unheated air continuously; a surprising number of “broken heat pump” calls are this.
Outdoor unit. Is it running when the thermostat calls for heat? If the indoor blower runs and the outdoor unit doesn’t, you’re getting strip heat only (which is warm) or nothing (cool air). Check the breaker and disconnect. If it’s running but iced solid, defrost has failed.
Filter and vents. Restricted airflow drops output.
Aux heat indicator. If the thermostat shows aux on constantly and the air is still cool, the strips may have failed or the heat pump is badly underperforming. Aux heat stuck on.
Likely causes when it’s not defrost
| Cause | Sign | Who fixes it |
|---|---|---|
| Thermostat mode / settings | Fan or Cool selected; schedule surprise | You |
| Outdoor unit not running (breaker, disconnect, capacitor, contactor) | Indoor blower runs, outdoor silent | You (power) / technician |
| Defrost failure (board, sensor) | Outdoor unit iced solid; cool air for long stretches | Technician |
| Low refrigerant | Air lukewarm all the time; system runs constantly | Technician (leak repair) |
| Reversing valve stuck | Blows cold in Heat, or won’t switch between modes | Technician |
| Failed heat strips (aux) | Cool air only during defrost and cold snaps | Technician |
| Compressor problem | Outdoor unit hums or runs but moves no heat | Technician; often a replace decision |
When to call
Call if the outdoor unit isn’t running or is a block of ice, if the air is cool for more than 15–20 minutes at a stretch outside of defrost, if the house is drifting below setpoint, or if the thermostat is calling for aux constantly. Until the technician arrives, emergency heat will keep the house warm (expensively) and protects the compressor if the outdoor unit is iced. Ask for refrigerant readings, defrost operation and reversing valve check. Repair costs are in the repair cost guide; if the system also runs constantly, start there. Every contractor on the North Carolina registry shows the license status we found.