Troubleshooting guide

Why Is My Heat Pump Running but Not Heating Enough?

Operating mode, defrost, cold weather, airflow and backup heat limit heat pump output. Includes a balance-point example.

Mechanical, HVAC & Fluids · Last reviewed October 17, 2026

A heat pump moves heat from outdoors to indoors, and its capacity falls as the outdoor air gets colder, just when the building needs the most heat. Warm but not hot supply air is normal. The question is whether the system delivers the heat the building needs.

The equipment has high voltage, and the outdoor unit has fans and a refrigerant circuit. Refrigerant and electrical work belong with qualified technicians. Do not disable defrost or lockout controls.

Symptom	Likely cause	What to check
Thermostat shows cooling or off, unit runs	Wrong mode or reversing valve not shifting	Thermostat mode and settings, then valve operation (technician)
Air from the registers is lukewarm and the unit runs constantly in cold weather	The load is above the heat pump capacity at that outdoor temperature	Balance point (see below). Auxiliary or backup heat should be adding heat
Outdoor unit is covered in ice, heat falls off	Defrost not working, or a blocked airflow	Defrost sensor and control, coil airflow, snow or debris around the unit
Steam or short stops from the outdoor unit in cold weather	Normal defrost cycle	Check the defrost cycle length and that indoor heat continues from backup
Weak airflow indoors	Dirty filter, closed registers or a failing blower	Filter, registers, blower speed
Backup heat comes on at mild temperatures	Thermostat staging, outdoor sensor, or a heat pump fault	Backup lockout temperature and staging settings
Little heat at any temperature	Low refrigerant, failing compressor, or reversing valve fault	Technician diagnosis, with pressures and temperatures

A balance-point example

Suppose a house needs 25,000 Btu/h when it is 15 °F outside and 70 °F inside, so its heat loss is 25,000 ÷ 55 = 455 Btu/h per °F of temperature difference. A heat pump is rated 36,000 Btu/h at 47 °F and 22,000 Btu/h at 17 °F, which makes its capacity fall by 467 Btu/h for each degree of drop.

Capacity = 22,000 + 467 × (T − 17)     Load = 455 × (70 − T)

Setting the two equal gives T = 19.3 °F. Above that outdoor temperature the heat pump can carry the house. Below it, the house needs more heat than the heat pump delivers, and auxiliary heat has to make up the difference. At 5 °F the house needs 455 × 65 = about 29,500 Btu/h, while the heat pump gives only 22,000 − 467 × 12 = 16,400 Btu/h, a shortfall of about 13,100 Btu/h.

A heat pump that is working correctly can therefore run all day below the balance point and still leave the building cool, if the backup heat is off, too small, or locked out.

Check, in this order

Mode and thermostat settings, then airflow (filter, registers).
Outdoor unit: ice, debris, defrost operation, and clearances.
Supply air temperature compared with what the equipment should deliver at the current outdoor temperature.
Backup heat staging and lockout settings.

Write a useful problem statement

Equipment capacity, age and backup heat type. Outdoor and indoor temperatures.
Supply air temperature and how long the unit has been running.
Whether the outdoor unit is icing, and whether defrost happens.
Whether backup heat comes on, and at what outdoor temperature.

Related tools and labs

Calculators, labs and simulators are part of the All-Access Pass.

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Questions

Why is heat pump air cooler than furnace air?

A heat pump normally delivers supply air in the range of about 90 to 105 °F, compared with 120 °F or more for a furnace. It feels cooler on the skin but moves a similar amount of heat for longer.

Is defrost normal?

Yes. In cold, humid weather frost builds on the outdoor coil, and the unit reverses briefly to melt it. A short stop with steam from the unit is normal.

What does a cold-climate heat pump change?

It keeps more of its rated capacity at low temperatures, which moves the balance point lower.

Educational material. Examples use simplified, hypothetical inputs and are not a design, a construction document or professional engineering advice. Confirm requirements with the codes and standards adopted where the work is built and with a licensed engineer.

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