Every winter, heat pump owners hold a hand over the vent, feel cool air, and conclude the system is broken. Most of the time it isn't. A heat pump heats differently from a furnace, and the difference is big enough to fool your hand. This guide separates the three normal explanations from the faults that actually need a technician.
A gas furnace blasts air at 120–140°F. A heat pump in heating mode delivers air at roughly 85–95°F — warmer than the room, but cooler than your 98.6°F skin. Moving air below skin temperature feels like a draft, so a hand over the vent reports "cold" while the house is quietly gaining heat. The thermometer tells the truth: hold it in the supply stream for a full minute. Supply air 15–25°F warmer than the room means the heat pump is doing its job, however cool it feels. This is also the core trade-off in the heat pump vs furnace comparison — gentler, longer heating instead of short hot blasts.
In cold, damp weather, frost builds on the outdoor coil — it's pulling heat from outdoor air, so the coil runs colder than the air around it. Every 30 to 90 minutes, the system briefly reverses itself into cooling mode to melt that frost. Indoors, that means a few things happen at once:
A normal defrost ends on its own and warm air returns. Defrost becomes a problem when it runs far longer than 10–15 minutes, repeats every few minutes, or never quite clears the ice — patterns that point to a failed defrost sensor, board, or low refrigerant charge, all of which are technician diagnoses.
When outdoor temperatures drop into the 20s and below, a heat pump alone can't keep up, and the thermostat brings on auxiliary heat — electric strips or a backup furnace. During defrost, aux heat is what keeps the vent air from feeling arctic. If the air feels cold specifically during cold snaps and defrosts, the aux heat may not be coming on at all. Our explainer on what aux heat means on your thermostat covers the indicator lights and the bill impact. A tripped breaker for the air handler is a homeowner-level check; failed heat strips or a dead blower are not.
Measure, don't guess. Thermometer in the supply vent for a full minute while the system runs. Supply air 15–25°F above room temperature is normal heat pump output. Supply air at or below room temperature is a real problem — continue down the list.
Wait out one defrost. If the outdoor unit is steaming and its fan has stopped, you're mid-defrost. Give it 10 minutes. Warm air returning on its own closes the case.
Read the thermostat. Confirm it's set to HEAT, not COOL or AUTO with a cooling call left over from fall — a mis-set thermostat in changeover season is embarrassingly common. Note whether AUX shows when it should. If the display says heat is on but nothing at all runs, that's a different fault tree — see our guide for when the thermostat says heat on but there's no heat.
Look at the outdoor unit. A light, even frost is normal in cold weather. A unit entombed in thick ice that defrost never clears is not. Check that leaves, snow, or a cover left over from summer aren't choking airflow around the coil, and clear what's loose with a gloved hand — no tools against the coil.
Check the air filter. A clogged filter strangles airflow across the indoor coil, which can drop supply temperatures and, in heating mode, stress the whole cycle. If the filter is gray and matted, replace it and re-measure the vent temperature after 20 minutes of running.
| What you observe | Verdict |
|---|---|
| Vent air feels cool but measures 15–25°F above room temp | Normal heat pump output |
| Cold blast for 5–10 minutes, steam at the outdoor unit, then warm air | Normal defrost cycle |
| AUX light on during a cold snap, house holds temperature | Normal backup heat |
| Supply air at or below room temperature for an hour or more | Fault — call a tech |
| Outdoor unit encased in ice that never melts off | Defrost failure — call a tech |
| House temperature falling despite the system running nonstop | Fault or undersized backup heat — call a tech |
| System stuck blowing cold with EM HEAT off and no defrost signs | Possible reversing valve or charge problem — diagnosis only, call a tech |
Three real faults account for most genuinely-cold heat pumps. Low refrigerant charge cuts heating capacity until the air barely warms; charge is added only after the leak is found, with gauges and a license. A stuck reversing valve can leave the system in cooling mode while the thermostat calls for heat — the one case where a heat pump truly air-conditions in January. A failed defrost control lets ice strangle the outdoor coil until almost no heat gets in. A technician distinguishes these in one visit with temperature and pressure readings. Your job is the observation notes: supply temperature, defrost behavior, ice pattern, and what the thermostat showed. That list shortens the visit and the invoice.
Feel is a bad instrument; a thermometer is a good one. Cool-feeling air 15–25°F above room temperature, a 10-minute defrost with steam, and an AUX light in a cold snap are a healthy system being misunderstood. Air at room temperature or below for an hour, ice that never clears, and a house losing ground are the call-a-tech pattern.
Usually because heat pump supply air is only 85–95°F, which feels cool against your skin even while it heats the house. The other normal cause is the defrost cycle, which briefly reverses the system to melt frost off the outdoor coil. Measure the vent air: 15–25°F above room temperature means it's heating.
Typically 2 to 10 minutes, every 30 to 90 minutes in frosty weather. The outdoor fan stops, the compressor runs, steam may rise off the unit, and the vents blow cool air while backup heat covers the gap. A defrost that runs past 15 minutes or repeats every few minutes suggests a sensor or board problem.
Only as a stopgap while waiting for a repair. Emergency heat runs the backup strips alone, which heats fine but costs roughly two to three times as much per hour as the heat pump. If the air measures normal with a thermometer, leave EM HEAT off — there's no fault to bypass.
Around 85–95°F at the supply vent for most heat pumps in typical winter weather, or 15–25°F above the room temperature. Colder outdoor weather pulls the number toward the low end. Air that measures at or below room temperature during a heating call is the reliable sign of a fault.
It depends on the diagnosis: a defrost sensor or board repair commonly runs $250 to $600, a reversing valve $800 to $1,500, and a refrigerant leak repair plus recharge $500 to $1,200 or more. The service call itself is typically $100 to $200, and several of the "faults" homeowners report turn out to be normal defrost or cool-feeling air — which costs nothing.