Heat Pump Not Heating? Causes and Fixes

Work through these in order. The first four are free, take minutes, and solve the majority of cases. The rest narrow things down enough that you can tell a technician something useful.

Normal behaviour vs a real fault

The first three rows account for most "my heat pump is broken" calls. Supply air around 90–100°F feels cool against skin at 98.6°F — that is the machine working correctly, not failing.

First: is it actually not heating?

Heat pump air is 90–105°F at the vent. Furnace air is 120–140°F.

Body temperature is about 98°F. So heat pump air can feel cool against your hand while genuinely heating the house. This is the single most common false alarm, especially in the first winter after switching from a furnace.

The test: is the house temperature rising toward the setpoint? If yes, the system is working — the air just feels different. See heat pump vs furnace.

1. Check the thermostat

• Mode set to HEAT, not COOL or OFF. Obvious, and it happens.
• Setpoint above room temperature. Also obvious, also happens.
• Fan set to AUTO, not ON. Fan ON runs the blower continuously, circulating unheated air between cycles and making the system feel like it's blowing cold.
• Batteries, on battery-powered thermostats.
• After a power outage: settings may have reset, and many systems impose a compressor restart delay of several minutes.

2. Check the air filter

The highest-value free check. A clogged filter restricts airflow, which reduces heat output, triggers backup heat, and in bad cases causes the system to shut down on a safety limit.

Hold it up to a light. If light doesn't pass through, replace it.

Filters need changing more often than most people think — every 1 to 3 months depending on type, pets, and use. See HVAC air filters and MERV explained.

3. Check the breakers

A heat pump typically has two circuits: one for the outdoor unit and one for the indoor air handler. Both need to be on.

A tripped breaker must be pushed fully to OFF before returning to ON — moving it straight from the tripped position doesn't reset it. See how to reset a tripped breaker.

If a breaker trips again immediately, stop. That's a fault, not a nuisance — see why does my breaker keep tripping.

Also check the disconnect at the outdoor unit — a switch or pull-out in a small box on the wall nearby. These get switched off during yard work and forgotten.

4. Look at the outdoor unit

• Snow, leaves, or drifts blocking airflow. Clear them gently.
• Ice on the coil. Light frost is normal and clears during a defrost cycle. Solid ice that persists is a fault — see heat pump defrost cycle.
• Is the fan spinning? If the compressor runs but the fan doesn't, that's a failed fan motor or capacitor.
• Is anything running at all? Complete silence with the thermostat calling for heat points at power, the control board, or the contactor.
• Vegetation and clearance. Manufacturers specify clearances; hedges grown in around the unit choke it.

5. Is it stuck in defrost?

A defrost cycle lasts a few minutes: outdoor fan stops, steam pours off the unit, indoor air may cool briefly. Normal in cold damp weather.

A unit stuck in defrost — steaming continuously, never returning to heating — suggests a failed defrost control, sensor, or reversing valve. Technician.

6. Is auxiliary heat doing all the work?

If the thermostat shows AUX constantly and the outdoor unit isn't running, the heat pump portion has failed and backup is carrying the house. You'll stay warm and your bill will roughly triple.

This is worth catching early precisely because it doesn't feel like a failure. See heat pump backup heat explained.

7. Suspect refrigerant charge — technician only

Low charge reduces capacity, causes excessive frosting, and makes the system run continuously without reaching setpoint.

Refrigerant doesn't get consumed. Low charge means a leak, and topping it up without finding the leak is throwing money at a symptom. Venting refrigerant is also illegal — recovery requires EPA certification.

Signs: ice on the refrigerant lines, long run times with poor output, a hissing sound, and a system that worked fine last winter and doesn't now.

See refrigerant leak signs.

8. Reversing valve — technician only

The reversing valve is what makes a heat pump reversible. Stuck in the cooling position, it will genuinely blow cold air with the thermostat calling for heat.

Symptom: the system runs, air is genuinely cold at the vents, and switching to cooling mode produces warm air — the exact opposite of what's requested.

What to tell a technician

Have this ready. It shortens the visit:

• Is the outdoor unit running? The fan?
• Is the compressor running?
• What's the air temperature at a supply vent?
• Does the thermostat show AUX or EM?
• Any ice, and does it clear?
• When did it last work normally, and what changed?
• Filter last replaced when?

Should you switch to emergency heat?

Don't chip ice off the coil. The fins and tubing puncture easily.

Don't leave it on emergency heat longer than necessary — it's two to three times the running cost.

Don't cover the outdoor unit. It's built for weather and needs airflow.

Don't add refrigerant yourself. It requires certification, and a system low on refrigerant has a leak that needs finding.

Standards and code reference

The standards behind this guide, for looking up in the edition your jurisdiction has adopted:

• EPA Section 608 — required certification for any refrigerant work
• AHRI 210/240 — rated capacity is measured at 47°F, which is why cold-weather output differs

Code editions and local amendments vary. Confirm the adopted edition with your AHJ, and treat manufacturer instructions as governing wherever they are more restrictive.

Where to go next

• Heat pump maintenance
• Heat pump defrost cycle
• Refrigerant leak signs
• Run the numbers: electricity cost calculator

More in our heating and cooling guides.