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Heat Pump Not Heating

Step 1: thermostat

Confirm it is set to Heat (not Cool, Off or Fan only), the set point is above room temperature, and batteries are good. Smart thermostats may have schedules or eco modes overriding the set point. If you see "Aux" or "Em heat", note it; that tells the technician whether backup is running.

Step 2: power

Check the breakers for the outdoor unit, air handler and any heat strips. A tripped breaker can be reset once; if it trips again, stop and call an electrician or technician. Check the outdoor disconnect switch is on. Some air handlers have a service switch that looks like a light switch.

Step 3: airflow

A clogged filter can cut airflow and trigger safety limits. Replace or wash it. Make sure supply and return registers are open and not blocked by furniture. For mini-splits, clean the head filters.

Step 4: outdoor unit

In winter, look for snow drifts, ice blocks or roof drip coating the unit. Gently clear loose snow around it; do not chip ice from the coil. If the coil is fully iced and defrost does not clear it after an hour, switch to emergency heat and call for service. See defrost mode.

Step 5: is it really not heating?

Heat pumps deliver warm, not hot, air, often 90 to 105°F at the register. On a very cold day, the unit may run constantly and still struggle if it is near its capacity limit. If the house holds temperature, the system may be working as designed. Compare with cold-climate heat pumps and your unit's performance table.

Common causes technicians find

• Low refrigerant from a leak.
• Failed capacitor, contactor or fan motor.
• Stuck reversing valve.
• Faulty defrost sensor or control board.
• Wiring issues or thermostat compatibility problems.
• Blocked condensate drains triggering safety switches.

These need a licensed technician. Refrigerant work requires certification, and electrical components hold dangerous voltage even after a breaker trips.

What to tell the technician

Note the outdoor temperature, thermostat display messages, any error codes or flashing lights on the outdoor unit or head, sounds, smells and when it started. Photos of the outdoor unit help. This speeds diagnosis.

Preventing problems

Annual professional maintenance, regular filter changes and keeping the outdoor unit clear prevent many no-heat calls. See heat pump lifespan and thermostat settings.

Worked example

Figures below are illustrative examples (2026-10-29), not quotes or guarantees.

On a 10°F morning the house is at 64°F with the thermostat at 70°F. The heat pump's table shows 26,000 BTU/h at 10°F, and the load calculation shows a 30,000 BTU/h loss at that temperature. The gap of 4,000 BTU/h means the house will slowly drift down unless backup heat runs. If backup strips are locked out above 15°F by mistake, adjusting the setting may solve it.

Checks you can safely do

Confirm the thermostat is in heat mode and set above room temperature, replace a dirty filter, make sure supply and return registers are open, and check that the outdoor unit is not buried in snow or ice. Look at the breaker panel for a tripped breaker, but do not reset it repeatedly; a breaker that trips again signals a fault for a technician.

Normal behaviour that looks like a problem

Steam rising from the outdoor unit in winter usually means it is in defrost. Lukewarm air from registers is normal for heat pumps, which deliver lower air temperatures than furnaces over longer run times. A brief whoosh or pause when defrost starts and ends is expected. If the house holds temperature, the system is likely working.

Signs that need a technician

Ice that covers the whole coil and does not clear, a fan that does not spin, a humming outdoor unit with no airflow, short cycling, error codes on the thermostat or indoor unit, and unusually high bills all call for service. Refrigerant leaks and electrical faults must be diagnosed by licensed professionals; do not open sealed panels.

Information to give the service company

Note the model numbers, when the problem started, the outdoor temperature, any error codes and whether backup heat is running. This helps the technician arrive with the right parts. If the house is getting cold, use emergency heat as a temporary measure and protect pipes in unheated spaces.

Preventing repeat calls

Ask the technician to explain the root cause and what would prevent it, such as a better drain path, adjusted defrost settings or a new filter schedule. Keep the service report with your maintenance records.

Thermostat issues

Dead thermostat batteries, a wrong mode setting after a power outage, or wiring configured for a furnace can all stop heat. Replace batteries, check the mode and look for messages on the screen. If the thermostat was recently replaced, wrong reversing-valve settings are a common cause of cold air in heat mode.

Airflow problems

Closed or blocked registers, collapsed flexible ducts and a dirty indoor coil restrict airflow and can cause the system to shut off on high pressure. Walk the house and make sure furniture and rugs are not covering vents.

Outdoor fan and coil

If the outdoor fan is stopped while the compressor hums, switch the system off and call for service; running it that way can damage the compressor.

What to tell the technician

Before the service call, note outdoor temperature, whether the outdoor unit fan is spinning, whether ice covers the coil, which thermostat mode you used, and how long the problem has lasted. That short list saves diagnostic time and keeps the visit focused on the real fault rather than a long interview at the door.

FAQ

Is it safe to run emergency heat?

Yes, but it costs more; use it until service arrives.

Why does it blow cold air?

Possibly defrost, low refrigerant or a reversing valve problem.

Can I add refrigerant?

No; call a licensed technician.

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