Heat pump not heating

Start with the checks below. They are taken from manufacturer owner's manuals and are safe for a homeowner to do. If none of them explains the problem, call a technician.

First: is it actually a problem?

Some things that look like "not heating" are normal operation:

• Defrost. The outdoor fan stops, steam rises, and a ductless unit may stop blowing for several minutes. Mitsubishi's wall unit manual: "The operation stops for about 10 minutes in the heating operation. Outdoor unit is in defrost."^{[Source 5]} See Heat pump defrost cycles.
• Startup delay. Many units have built-in delays. Rheem's manual describes a protective timer that holds the unit off for "approximately five minutes."^{[Source 3]} Carrier's manual also notes built-in time delays are normal.^{[Source 2]}
• Cooler supply air than a furnace. NRCan notes heat pumps deliver air at "normally between 25 and 45°C" (77 to 113°F), compared with 55 to 60°C from a furnace.^{[Source 4]} The air can feel cool to the hand while the house is still being heated.
• Running a long time. NRCan describes variable-speed heat pumps as "designed to vary the capacity of the system to more closely match the heating/cooling demand of the house at a given moment," which means long, low-output run times.^{[Source 4]} Long run time alone does not indicate a fault.

Safe checks you can do yourself

Work through these in order. They match the checks in Lennox's and Carrier's owner's manuals.^{[Source 1][Source 2]}

1. Thermostat mode and setpoint. Lennox: "Verify room thermostat settings are correct."^{[Source 1]} The mode should be Heat (or Auto), and the setpoint above the current room temperature. If the thermostat runs on batteries, check them.
2. Breakers and disconnects. Lennox: "Verify that all electrical disconnect switches are ON. Check for any blown fuses or tripped circuit breakers."^{[Source 1]} There may be separate breakers for the outdoor and indoor units, plus a disconnect switch next to the outdoor unit. If a breaker has tripped, you can reset it once. If it trips again, stop and call a technician.
3. Filter. Lennox: "Verify air filter is clean."^{[Source 1]} A clogged filter restricts airflow. See Heat pump filters and maintenance.
4. Vents and registers. Carrier says "interior vents should be open and unobstructed."^{[Source 2]} NRCan: make sure registers are "not blocked by furniture or carpeting."^{[Source 4]}
5. Outdoor unit. Look for snow drifted against it, heavy ice that is not clearing, or debris blocking the coil. Clear snow from around the unit by hand or with a broom. Lennox: "never pour hot water on the unit... don't try to chip or break the ice."^{[Source 6]} See Ice on the outdoor unit.
6. Emergency heat. If the heat pump is clearly not running and the house is getting cold, setting the thermostat to Emergency Heat (on systems that have backup) will heat with the backup while you wait for service. Rheem describes this mode as being for "a suspected problem with the outdoor unit."^{[Source 3]} It costs more to run; switch back once the system is repaired. See Heat pump backup heat and lockout temperatures.

Clues to report to the technician

Write these down before you call. They shorten the diagnosis.

• Is the outdoor fan running? Is the compressor running (a steady hum)?
• Is the indoor blower running? Is the air warm, cool or room temperature?
• Does the thermostat show "aux," "emergency" or an error code? If a ductless unit flashes an indicator light, note the pattern.
• The outdoor temperature when it happens. Below the Thermal and economic balance points the heat pump alone may not keep up, which is a sizing or backup question rather than a fault.
• Whether it happened after a power outage, a storm or recent work on the system.

Leave these to a technician

• Anything with refrigerant. Adding, removing or checking refrigerant requires EPA certification. EPA requires that "technicians who maintain, service, repair, or dispose of equipment that could release refrigerants into the atmosphere must be certified."^{[Source 7]} Homeowners should not open refrigerant connections or add refrigerant. DOE notes that "too much or too little refrigerant" reduces heat pump performance, so the charge needs to be checked with proper instruments.^{[Source 8]}
• Opening service panels on the units. Outdoor and indoor units contain high-voltage electrical components.
• A breaker that trips again after one reset.
• Ice that will not clear, or a unit that is iced solid. See Ice on the outdoor unit.
• Repeated short cycling. See Heat pump short cycling.
• Burning smells, smoke, or loud grinding or banging. Turn the system off at the thermostat and call.

If it heats, but not enough

If the system runs and the air is warm but the house will not reach the setpoint on cold days, possible reasons include an undersized system for the house's heat loss, backup heat that is locked out or not wired, duct problems, or a heat pump running below its rated range. These are design or setup questions for the installer. Ask what the system's capacity is at the current outdoor temperature compared with the house's load.