Heat Pump Not Heating: What Is Normal and What Is Not

By Ray Bennett · Updated 2 October 2026 · 7 min read · Checked against the sources at the end

Heat Pump Not Heating: What Is Normal and What Is Not

Short answer

Carrier puts the air from a working heat pump at around 85 to 100 degrees, which is at or below body temperature, so it feels cold when nothing is wrong. Measure it with a thermometer in the vent before anything else. Then check the thermostat is not sitting on Em Heat, and that the outdoor unit is running and clear of snow and leaves.

A heat pump not heating is often a heat pump that is heating perfectly well. The air coming out is below body temperature, so it feels cold even when the system is doing its job.

So the first thing to do is not a repair. It is to measure what is actually coming out of the vent.

Before you start

What to carry: a cheap thermometer, a torch and a new filter. Ten minutes.

A heat pump is not a furnace. A gas furnace burns fuel and sends out air at 120 to 140 degrees, which feels hot. A heat pump moves heat from outside instead and sends out around 85 to 100, Carrier says, with electric strips as backup for the cold days. Both warm the house to the same temperature. They just feel completely different at the vent. How cold each kind can work: how cold is too cold for a heat pump.

Which one do you have? If a box outside runs in winter and the indoor unit has no gas pipe and no flue, that is a heat pump. If it has a gas pipe and a flue, you have a furnace, and a different page: furnace blowing cold air.

Measure the air before you judge it

Body temperature sits right inside the range a working heat pump produces, which is why the air feels cold to the hand and is not.

Check 1Free, five minutes

Put a thermometer in the nearest supply vent

Let it run for ten minutes first so you are not reading the first cold air out of the ducts.

Then hold the thermometer in the vent and read it, and take a second reading at the return grille. A working heat pump should read clearly warmer at the supply than at the return, and Carrier puts heat pump supply air at around 85 to 100.

Anything in that range means the machine is fine and the complaint is comfort rather than fault. Under about 85, with the backup heat not running, is a real problem and the rest of this page applies.

Three jobs on a heat pump that are not do it yourself. Never chip ice off the outdoor coil. The fins and the tubing behind them are thin enough to go through with a screwdriver, and a punctured coil is a refrigerant leak and usually a new unit. Cool water from a hose, with the power off, is the only safe way to help it along.

Never top up refrigerant. A heat pump does not use refrigerant up. If it is low, it is leaking, and putting more in without finding the leak is money poured away.

The outdoor fan that hums but does not turn is usually a capacitor, and a capacitor holds a charge after the power is off. That one is a technician unless you already know how to discharge one safely.

Then look at the thermostat

Check 2

Make sure it is not sitting on Em Heat

Emergency heat locks the compressor out completely and runs only the electric strips. The house still gets warm and the bill triples, and the outdoor unit never starts, which is exactly what somebody looking for a fault will notice.

It is easy to leave on after a cold snap. Put it back to Heat and the outdoor unit should start within a few minutes.

Aux heat showing on the display is a different thing and is normal. That is the strips helping the compressor rather than replacing it.

Check 3

Check the thermostat knows it is driving a heat pump

In the installer settings there is a system type. Set to furnace rather than heat pump, it will never run the changeover valve properly.

The other half of the same fault is the O and B terminal. That wire tells the outdoor unit which way round to run, and swapping it gives you cold air when you ask for heat and warm air when you ask for cooling, with nothing actually broken. If the thermostat was replaced recently, that is the first place to look: what each letter and colour does.

Is the outdoor unit running, and can it breathe?

Check 4

Go outside and watch it

With the thermostat calling for heat, the outdoor fan should be turning and the unit should be humming.

Clear at least two feet all round it and keep the top clear. Leaves packed against the coil, a drift of snow, or a cover somebody left on for winter all stop it working. So does a unit standing in a puddle that has frozen solid, which is why they are meant to sit up on a stand in snow country.

If nothing happens at all, there are two breakers to check, one for the indoor air handler and one for the outdoor unit, plus a pull out disconnect box on the wall beside it.

Check 5

Pull the filter out and hold it to a light

No light through it means the indoor unit cannot move air, and a heat pump has much less heat to give away than a furnace, so it feels the restriction sooner.

Open the vents in unused rooms and pull the furniture off the return grille while you are there.

Steam, a stopped fan and cold air for ten minutes

Frost on the coil, a few minutes in reverse to melt it, backup heat covering the house, then back to normal. All four steps are the machine working.

This is the one that frightens people into calling somebody out. In cold damp weather the outdoor coil frosts over, so the unit runs backwards for a few minutes to melt it off. The outdoor fan stops, steam pours off the unit, and the indoor backup heat comes on so the house does not feel it.

Five to fifteen minutes, as often as once an hour in the worst weather, and then everything goes back to normal.

What is not normal is a unit that ices up completely and never clears, or one that defrosts every fifteen minutes all day. That points at the defrost sensor or the board, and it needs somebody with gauges.

When it genuinely cannot keep up

Every heat pump has an outdoor temperature below which it can no longer pull enough heat out of the air on its own. On an older unit that is somewhere around 25 to 30 degrees, Carrier says, and below it the electric strips do more and more of the work.

That is not a fault, it is the design, and it is why the bill climbs in a cold snap. A modern cold climate unit holds on much further down.

What is worth suspecting is a heat pump that used to cope and now does not. Long runs, weak air and a coil that ices oddly point at low refrigerant, which means a leak somewhere, and that is a technician with a leak detector rather than a part off a shelf.

Questions people ask

The air really does feel cold. Am I imagining it?

No, and you are also not broken. Ninety five degree air is below your body temperature, so it feels cool on your hand while it is still warming the room. The thermometer is the only honest judge.

Should I just leave it on Emergency Heat all winter?

Only if the compressor has actually failed and you are waiting for a repair. Electric strips cost roughly two to three times as much to run as the heat pump does.

My outdoor unit is covered in ice. Do I turn it off?

If it is not clearing on its own, yes, switch it off and let it thaw, or help it with cool water from a hose. Never anything sharp and never boiling water.

It blows warm for a minute and then cold.

If the outdoor fan stopped at the same moment, that is a defrost cycle. If the outdoor unit kept running, that is a changeover valve or an O and B wiring problem instead.

The fan runs all the time even when nothing is heating.

Check the fan setting on the thermostat. On ON it runs day and night and pushes room temperature air through the vents, which feels exactly like a failure.

How long should a heat pump last?

About 10 to 15 years, Carrier says, and around 15, Trane says. Past that, a compressor or a reversing valve is usually the point where a new unit makes more sense than a repair.

Sources: Carrier "Heat Pump Not Heating? Troubleshooting Cold Air Issues" (85 to 100 degree supply air, 120 to 140 from a furnace, separate breakers, older units below 25 to 30 degrees), Trane "Common Reasons for Your Heat Pump Not Heating" (body temperature) and "What Is the Heat Pump Defrost Cycle?", Lennox "Understanding the Heat Pump Defrost Cycle" and "Heat Pump Blowing Cold Air?" (defrost timing, disconnect box, two feet clear), Honeywell Home RTH6360 quick installation guide (Em Heat, system type, O or B), Google Nest Help "Emergency Heat for heat pump systems", Carrier and Trane heat pump vs furnace pages (life) and the DOE Energy Saver Guide 2017