Heat Pump Not Heating or Blowing Cold Air: Causes

Safety first. Do not open the outdoor unit or air handler, and do not chip ice off the coil. If you have a dual fuel system and smell gas, leave the house and call the gas utility from outside.

Check these first

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Set the thermostat to Heat, raise the setpoint a few degrees above room temperature and replace the batteries if it has them.
Make sure the fan is set to Auto. On the On setting, the fan blows unheated air between cycles.
Check both breakers: the outdoor unit and the indoor air handler. Reset a tripped breaker once only.
Replace a dirty filter and open any closed or blocked supply and return vents.
Look at the outdoor unit. Clear snow, leaves and debris, and note whether it is covered in solid ice.
Watch one cycle. If the outdoor fan stops and steam rises for a short time, the unit is defrosting, which is normal.
Write down any error code shown on the thermostat.

Still not working after these? That is the point to bring in a licensed technician.

Request a call back Repair or replace?

What it usually means

Start by separating normal behavior from a fault. Heat pump air is milder than furnace air, so it can feel cool on your hand while still heating the house. During a defrost cycle the system briefly runs in reverse to melt frost off the outdoor coil, and the registers may blow cool air for a short time. Neither of those is a problem.

A fault looks different: the room temperature keeps dropping, the outdoor unit is silent or iced solid, or the thermostat shows auxiliary heat nonstop. The page on auxiliary and emergency heat explains what that indicator tells you.

Likely causes, most common first

Thermostat settings or wiring. Wrong mode, fan set to On, dead batteries or a thermostat set up for the wrong system type.
Restricted airflow. A clogged filter or blocked vents cut heat delivery and can shut the system down on a safety limit.
No power to one half of the system. A tripped breaker can leave the indoor fan running with the outdoor unit off, so the air is never heated.
Outdoor unit iced over. A failed defrost control, sensor or outdoor fan lets ice build until the coil cannot absorb heat. See heat pump frozen or iced over.
Failed capacitor or contactor. The outdoor unit hums or does nothing.
Low refrigerant from a leak. The system runs constantly and heats poorly.
Reversing valve stuck in cooling. The system cools when the thermostat calls for heat.
Backup heat failure. The heat pump works but the electric strips do not, so the house falls behind on the coldest days.
Compressor failure. The least common and most expensive cause.

You can confirm the first three yourself. The rest need meters, gauges and a licensed technician. You cannot diagnose a refrigerant or valve problem by looking.

When the heat pump runs but cannot keep up

In very cold weather a heat pump loses capacity, and the backup heat is meant to fill the gap. A system that held temperature last winter and falls behind in the same weather now has a fault somewhere: airflow, refrigerant charge, defrost or backup heat. A system that has never kept up in cold snaps may be undersized, or the house may be leaking heat through ducts or poor insulation. A technician can tell these apart with measurements. Guessing from the thermostat alone will not.

What a technician will do

Expect the tech to check thermostat setup and wiring, measure voltage and current at the outdoor unit, test the capacitor and contactor, measure the air temperature rise across the indoor coil, read refrigerant pressures, force a defrost cycle and test each stage of backup heat. You should get a clear statement of what failed and a written quote before any repair. The heat pump overview links to the repair and cost pages.

When it points to replacement

Most no-heat calls end in a repair. Replacement enters the picture when the compressor has failed, when the outdoor coil leaks, when the system has needed several major repairs, or when it has never been able to heat the house properly. Ask for the repair quote and a replacement quote side by side, and ask whether any part is still under the manufacturer's warranty.

Common questions

Why is my heat pump blowing cold air in heat mode?

The usual reasons are a defrost cycle, the fan set to On, a tripped outdoor breaker, or an iced outdoor unit. If the air stays cold for a long stretch and the room temperature drops, the outdoor unit is probably not running or the reversing valve is stuck. Switch to emergency heat and call a technician.

My heat pump is running but not keeping up. Is that normal?

On the coldest days of the year, long run times and some auxiliary heat are normal. Falling several degrees behind the setpoint in ordinary winter weather is a sign of trouble. Check the filter and the outdoor unit, then have the refrigerant charge, defrost system and backup heat tested.

Should I switch to emergency heat?

Use emergency heat when the heat pump itself has failed or the outdoor unit is iced solid, and you need heat until a technician arrives. It runs only the backup heat source, which usually costs more to operate, so switch back once the heat pump is repaired.

Can I add refrigerant myself to fix weak heating?

No. Handling refrigerant legally requires EPA Section 608 certification, and low refrigerant means there is a leak. Adding more without finding the leak wastes money and the problem returns. A licensed technician has to locate the leak, repair it and charge the system by measurement.

Next step

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Heat pump not heating (or blowing cold air)