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Heat Pump Not Heating? Settings, Defrost and Fixes to Try

Published 2026-10-19 · Last reviewed 2026-10-19 · Checked against manufacturer sources

Quick answer

A heat pump that seems to stop heating is often just in its defrost cycle, which Trane says usually lasts 3 to 5 minutes and can send cool air or steam out of the outdoor unit. If it lasts longer, check that the thermostat or remote is set to Heat, not Cool or Auto, then look at the filters and clear snow or debris from the outdoor unit.

Safety first: Switch the system off at the thermostat and at the outdoor disconnect before clearing debris from around the outdoor unit, and never chip ice off the coil or fan. Refrigerant, reversing valve and control board work is for a licensed HVAC technician.

Rule out the defrost cycle first

In heating mode the outdoor coil runs colder than the outside air. Trane puts the difference at about 10 to 20°F (6 to 11°C), so frost can build on the outdoor coil even above freezing. Trane says defrost can start in the low 40s°F and is most common in the 20s and 30s°F (about -7 to 4°C). To clear it, the system briefly reverses into cooling mode and sends heat to the outdoor coil instead of the house.

That cycle is easy to mistake for a breakdown. The outdoor fan stops, you hear a whoosh as the reversing valve shifts, the compressor changes pitch, and a cloud of steam may rise off the unit. Inside, homes without backup heat may get cooler air from the vents for a few minutes. Trane calls all of this normal and says its cycles usually run 3 to 5 minutes before heating resumes.

Ductless systems show it on the display. Samsung shows dF and pauses the indoor fan so you don't get a blast of cold air. GREE shows H1. Wait 10 to 15 minutes before you decide anything is wrong.

Check the mode, set point and schedule

Confirm the thermostat or remote is on Heat and the set point is a few degrees above the room temperature. It sounds basic, but a thermostat left on Cool from the fall, a smart thermostat schedule that's running a setback, or a mini-split remote set to Dry or Fan accounts for a lot of no-heat calls.

NYSERDA's heat pump guidance recommends picking Heat in winter instead of Auto, because Auto can bring on a backup furnace or boiler when it doesn't need to. On multi-zone mini-splits, all the indoor units on one outdoor unit have to agree. If one room is set to Cool while another calls for Heat, the system shows a mode conflict, such as E7 on GREE or CH07 on LG, and one of them won't run.

NYSERDA also advises holding a steady setting and keeping the heat pump running in very cold weather, even below zero. If a room feels short on heat, its tip for ductless units is to raise the set point, try a higher fan setting if the heat isn't reaching far enough, and point the vanes down toward the area you want warm.

Clean or replace the filters

A clogged filter cuts heat output the same way it hurts cooling. On a central system, pull the filter at the air handler or return grille and replace it if it's gray and you can't see light through it. Mini-split wall units have mesh filters behind the front panel. NYSERDA says to vacuum or rinse them when the filter light comes on or they look dirty, and at least on the schedule in your manual, which may be monthly in a dusty house.

Heavily restricted airflow in heating mode can also trip high-temperature protection. Daikin A5 and Mitsubishi Electric P6 cover the indoor coil overheating while heating, and GREE ties its E8 code to low indoor airflow. Clean filters are the first thing to rule out before you call.

Clear snow, ice and debris from the outdoor unit

The outdoor unit needs to pull air freely through its coil. NYSERDA lists the things to keep away from it: leaves, debris, snow, ice and water dripping off the roof. Brush snow away from the top and sides, keep a path clear around it after a storm, and trim back shrubs that touch the cabinet. Your installation manual gives the exact clearances for your model.

Water from a roof edge or a broken gutter dripping onto the unit freezes into a solid shell that defrost can't keep up with. Fix the gutter or add a small cover over the unit, which NYSERDA suggests can cut down on cleaning. The Department of Energy also notes that strong winds can cause defrost problems, and that a fence or shrub upwind of the coil can help.

A coil that stays iced solid after the debris is gone means defrost isn't doing its job. Don't chip at the ice or the fins; switch to emergency heat if you have it and book a service call. Snow, ice or leaves against the fan blade can trigger an outdoor fan code, such as CH67 on LG, E458 on Samsung, U8 on Mitsubishi Electric, L3 on GREE or E7 on Daikin.

Understand auxiliary and emergency heat

Most central heat pumps have a backup heat source, usually electric resistance strips in the air handler. Carrier separates the two ways it runs. Auxiliary heat (AUX) comes on automatically to help the heat pump when it's very cold outside, when you raise the set point more than 2 or 3 degrees at once, or during a defrost cycle. Emergency heat (EM Heat) is a manual setting that shuts the heat pump off and runs only the backup.

Carrier's advice is to use emergency heat only when the heat pump has actually broken down, and to get an HVAC pro out instead of relying on it. Electric backup heat costs far more to run. Carrier says AUX running when it's above about 40°F (4°C) outside can be a sign of a dirty filter, a refrigerant leak, a faulty sensor or a set point raised too quickly.

Many ductless mini-splits have no electric backup of their own. If the outdoor unit stops in extreme cold, it may be protecting itself. Midea shows PC 0L when it's too cold outside to run, and Samsung uses E440 when it's too warm outside to heat. Check the low-temperature limit in your manual.

Check power to both units

If the outdoor unit isn't running at all, look at the breaker panel. A central heat pump usually has its own breaker for the outdoor unit, and most installations also have a disconnect box on the wall next to it. Reset a tripped breaker once. If it trips a second time, leave it off and book a service call. Indoor and outdoor units that lose contact show a communication code on mini-splits, and those also need a technician once power is confirmed.

Faults that need an HVAC technician

Some no-heat problems sit inside the sealed system or the controls. A heat pump that blows cool air in Heat mode, never switches into defrost, or stays stuck in defrost may have a reversing valve or defrost control problem. Fujitsu's code 99 points to a reversing valve that isn't changing over. Low refrigerant cuts heating output as well as cooling, and American Standard notes that a defrost cycle that keeps getting longer can mean a defrost control board is failing.

None of these are homeowner repairs. Refrigerant work requires an EPA 608 certified technician, and the outdoor boards run on line voltage. Write down any code, what the outdoor unit was doing and the outdoor temperature, and give that to the tech.

When to call a professional

Call an HVAC technician if heat doesn't return within about 15 minutes of a defrost cycle with the correct mode set, if the outdoor unit stays iced over, or if a breaker trips more than once. Use emergency heat to stay warm until they arrive.

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Error codes for this problem

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