Heat Pump Not Heating: Causes and What to Check
When a heat pump is supposed to be heating and the air coming out of the vents feels cool or only lukewarm, the house never quite catches up, or the outdoor unit is buried in ice, something is wrong. In a North Georgia winter that can mean cold mornings, high electric bills from emergency heat running flat-out, and a system that wears itself out. A few causes are easy to check yourself. Others need a technician with gauges and the right tools.
What “Not Heating” Looks Like
Heat pumps move heat from outside air into the house. Even on a cold day the supply air is often warm, not furnace-hot. That difference confuses a lot of people. Still, these patterns are real problems:
- Cool or barely warm air from the vents while the thermostat is calling for heat.
- The outdoor unit iced solid for hours, or ice that never clears after a defrost cycle.
- The system runs and runs without reaching the set point on a normal winter day.
- Emergency or auxiliary heat stays on almost constantly, or never comes on when it should.
Normal defrost is different from a frozen unit that will not thaw. Knowing which you are seeing helps a technician a lot.
Common Causes
1. Thermostat mode, emergency heat and settings
Heat pumps need the thermostat in Heat (or Auto), not Cool. Many thermostats also have an Emergency Heat or Aux Heat setting. Emergency heat turns on the electric strip heaters and usually shuts off the outdoor heat pump. That keeps the house warm, but it is expensive and it is meant for short stretches or a failed outdoor unit, not everyday use. A thermostat configured for a furnace instead of a heat pump, or with the wrong number of stages, can also leave the outdoor unit or the strips sitting idle when they should be working.
2. A dirty filter or restricted airflow
A clogged filter, closed supply registers or blocked return grilles starve the indoor coil of air. Low airflow cuts heating capacity, can trip safety limits and can contribute to icing. This is the same kind of restriction that shows up as high static pressure when measured. It is also one of the easiest homeowner fixes.
3. Outdoor unit iced over vs normal defrost
In cool, damp weather a thin layer of frost on the outdoor coil is normal. The heat pump periodically runs a defrost cycle: the outdoor fan may stop, you may see steam or melting ice, and the indoor unit may switch to auxiliary heat for a few minutes so the house does not go cold. That is expected.
What is not normal is a unit packed in thick ice for hours, ice bridging the fins so air cannot pass, or defrost that never finishes. That points to a defrost sensor or board problem, low refrigerant, airflow issues or a failing outdoor fan. Do not chip ice off the coil with tools; you can puncture the tubing.
4. A dirty outdoor coil or debris
Leaves, grass clippings, cottonwood fluff and mud pack the outdoor coil. When air cannot move through the fins, heating capacity drops and the coil ices more easily. Clearing loose debris from around and on the cabinet (without bending fins or spraying high-pressure water into the electronics) is a safe first step. A heavily fouled coil usually needs a professional cleaning.
5. Low refrigerant or a leak
Heat pumps need the correct refrigerant charge to move heat. A slow leak leaves the system short of charge, so heating fades as outdoor temperatures drop and the outdoor coil may ice heavily. Refrigerant work is for a licensed technician only: finding the leak, repairing it and weighing in the right charge. Topping off without fixing a leak is a temporary patch.
6. Reversing valve, contactor, capacitor or outdoor board issues
At a high level, several outdoor components can leave a heat pump stuck in cooling mode, unable to start or unable to defrost:
- Reversing valve stuck or not shifting, so the system still cools when it should heat.
- Contactor or capacitor failing, so the compressor or outdoor fan will not start or hums and trips.
- Outdoor control board or sensors failing, so defrost never runs or stages do not engage.
These need diagnosis with the covers off and the right meters. They are not DIY repairs.
7. Auxiliary or strip heat not engaging when needed
Below a certain outdoor temperature, or when the heat pump alone cannot keep up, electric strip (auxiliary) heat should come on to help. If the strips are burned out, miswired, locked out in the thermostat or never staged correctly, the house falls behind on cold mornings even though the outdoor unit is running. A technician checks strip amp draw, sequencers or heat strips and the thermostat’s auxiliary settings.
8. Undersized or failing unit approaching replacement
An undersized heat pump, a mismatched indoor coil, leaky attic ducts or a unit near the end of its life may heat fine in mild weather and fail on the coldest days. If repairs keep stacking up, or the system was never right for the house, replacement can be the better investment. See our heat pump replacement details when you are weighing that decision.
What You Can Safely Check
- Thermostat: Heat mode (not Cool), fan on Auto, set point above room temperature. Note whether Emergency Heat is on. Do not leave Emergency Heat on as a long-term fix if you can avoid it.
- Filter: replace if dirty; confirm size and airflow arrow direction.
- Registers and returns: open and clear of furniture, rugs or boxes.
- Outdoor unit: clear of leaves, snow drifts and stored items. Make sure the fan can spin freely when the unit is running.
- Breakers: outdoor disconnect and indoor air-handler breakers in the On position. If a breaker has tripped, flip it fully Off and back On once; if it trips again, stop and call.
- Ice vs steam: light frost with steam during a short defrost cycle is often normal. A solid block of ice that does not clear is not. Note which you see and how long it lasts.
What Not to Do
- Don’t chip or scrape ice off the coil with a screwdriver, hammer or stick. You can pierce a refrigerant tube.
- Don’t keep forcing Emergency Heat day after day without a diagnosis if the heat pump itself might be repairable. Strips cost more to run and they do not fix the outdoor unit.
- Don’t open sealed panels, add refrigerant or jump out safety controls.
- Don’t spray high-pressure water into the outdoor unit’s electrical compartment while “cleaning” it.
When to Call
Call if a clean filter and clear outdoor unit do not restore heat, the outdoor unit is iced solid, Emergency Heat is the only thing keeping you warm, breakers keep tripping, or the house will not reach the set point on a typical winter day. Short stretches of cold weather are when these problems show up hardest, so it is worth getting ahead of a deep freeze.
When Comfort Medics diagnoses a heat pump that is not heating, we check the measurements that matter for your equipment: airflow and static pressure, refrigerant charge and temperatures, defrost controls and sensors, auxiliary/strip heat operation, thermostat configuration and commissioning references from the manufacturer. Those are the same kinds of checks described on our heat pump replacement and Installation Standard pages.
If Replacement Makes Sense
If the outdoor unit is failing, the system is badly mismatched or repair costs keep climbing, a properly sized replacement may be the better path. Comfort Medics installs residential heat pump systems across North Georgia. New systems include a 10-year manufacturer parts warranty on qualifying equipment (registration may be required), a 10-year labor warranty and our lifetime craftsmanship guarantee, with financing available through FTL Finance and other lenders, for repairs and new systems (subject to credit approval).
Read more on heat pump replacement, or start with a free quote.
Related Guides
- Furnace and AC short cycling: causes and fixes
- Why is my upstairs hotter than downstairs?
- New AC not cooling: signs of a poor install
Get Your Heat Pump Checked
Comfort Medics Heating & Cooling repairs heat pumps across North Georgia, with 24/7 emergency service. We’re family-owned, licensed and insured (GA. REG. CR2011437). See our heating services.
Call (470) 360-8887 Request Service
