Carrier Heat Pump Not Heating: Causes, Fixes & Costs
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Carrier heat pump not heating: what it means
“Not heating” is a symptom, not a Carrier fault code. It means the heat pump runs, or tries to run, in heat mode but the house doesn’t warm up, or the system doesn’t start at all on a heat call.
There are two very different situations, and they have different causes:
- The outdoor unit runs but the air is cool. This points to defrost, reversing-valve, refrigerant or airflow problems.
- The outdoor unit doesn’t run at all. This points to power, thermostat, control-board or compressor/capacitor problems.
How the system reports faults depends on the model:
- Infinity / Greenspeed systems usually log a specific fault (for example, a pressure, temperature or communication fault) on the Infinity or CĂ´r wall control. Read and note it before calling.
- Non-communicating Comfort / Performance units may show a flash code on the defrost board inside the outdoor unit. Reading that board is technician work.
Lockout behavior also varies by board. Many Carrier boards hold a lockout after repeated high- or low-pressure trips until power is cycled. Some faults clear automatically after a delay. Check your model’s service literature.
If your backup heat is running nonstop, that is a related but different problem. See “Carrier Heat Pump Aux Heat On Constantly.”
Carrier heat pump not heating: common causes, ranked by probability
- Thermostat setting or setup. The thermostat may be in the wrong mode, set below room temperature, or have dead batteries. A newly installed non-Carrier thermostat may be configured for the wrong reversing-valve logic. Carrier energizes the reversing valve in cooling, so it uses O, not B.
- Normal defrost cycle or cold-weather limits. The unit pauses heating during defrost. Below roughly 25–35°F, a standard heat pump’s output drops, and the system relies on aux heat. This is expected, not a failure.
- Restricted airflow. A clogged filter, closed registers, or a blocked outdoor coil (snow, ice, leaves, a too-close shrub) cuts heat output and can trip pressure limits.
- Tripped breaker or outdoor disconnect. If the outdoor unit has no power, only the indoor fan or aux strips run.
- Defrost control problems. A failed defrost board, defrost thermostat or coil sensor can leave the outdoor coil iced over solid.
- Failed run capacitor or contactor. The outdoor fan or compressor won’t start or hums and stops.
- Stuck or failed reversing valve, or a failed solenoid coil. The unit runs but delivers cool air.
- Low refrigerant charge from a leak. Heat output is weak, and you may see icing or low-pressure lockouts.
- Compressor failure. This is the least likely cause and the most expensive.
How to fix Carrier heat pump not heating: safe checks first
- Thermostat. Confirm it is set to HEAT (not COOL, OFF or AUTO with a wrong setpoint) and that the setpoint is a few degrees above room temperature. Replace the batteries if the display is dim or blank.
- Wait out defrost. If the outdoor unit is steaming and its fan has stopped, give it 10–15 minutes.
- Air filter. Replace it if it’s dirty, and make sure it’s installed with the airflow arrow pointing toward the blower.
- Registers and returns. Open supply registers and clear furniture or rugs away from returns.
- Outdoor unit surroundings. Gently brush off loose snow and leaves from the outside of the unit and keep about 2 ft of clearance. Do not chip ice off the coil. You’ll bend the fins. If it’s iced solid, switch to EM HEAT and call.
- Breakers and disconnect. Check the breakers for both the indoor and outdoor units. Reset a tripped breaker once. If it trips again, leave it off and call a technician.
- One reset. If the unit appears locked out, turn the thermostat off, wait 5 minutes, then return it to HEAT. Do this only once. Repeated resets can damage the compressor.
- Condensate line. In heat mode the indoor coil drains little, but a full float-switch pan can still shut the system down. Check that the drain line isn’t blocked.
- Panels. Make sure the indoor unit’s access panels are fully seated.
While you wait for service, use EM HEAT to stay warm.
How a technician will diagnose it
Use this list to sanity-check what a technician does and quotes:
- Reads stored fault codes from the Infinity/CĂ´r control or the defrost-board LED.
- Verifies thermostat wiring and configuration (O terminal, staging, aux lockout settings).
- Checks line and control voltage at the outdoor contactor, and tests the run capacitor’s microfarad rating.
- Measures supply/return temperature split. A typical heat pump rise is roughly 20–30°F, depending on conditions.
- Connects gauges or probes to check suction/discharge pressures, superheat and subcooling for charge. If the charge is low, they leak-search before adding refrigerant.
- Energizes the reversing valve solenoid and checks for a proper changeover. Confirms the coil is getting voltage and that the valve shifts.
- Tests the defrost sensor resistance and forces a defrost cycle from the board.
- Checks compressor amperage and winding resistance before condemning the compressor.
A quote for a compressor or reversing valve should be backed by these measurements, not by a guess.
Symptom, cause and what to do
| Symptom | Likely cause | DIY action | Technician job |
|---|---|---|---|
| Nothing runs, blank thermostat | Dead batteries, tripped breaker | Replace batteries; reset breaker once | Diagnose transformer, fuse or wiring |
| Indoor fan runs, outdoor unit silent | Outdoor breaker/disconnect off, contactor or capacitor fault | Check outdoor breaker once | Test and replace contactor or capacitor |
| Outdoor unit steams briefly, fan stopped | Normal defrost | Wait 10–15 minutes | None needed |
| Outdoor coil iced solid for hours | Defrost board or sensor fault, low charge | Switch to EM HEAT; clear loose snow | Test defrost controls; check charge |
| Unit runs, air is cool | Thermostat O/B setup, stuck reversing valve, low charge | Verify HEAT mode and setpoint | Check O/B wiring, test valve, leak-search |
| Weak heat, long run times | Dirty filter, blocked outdoor coil, low charge | Replace filter; clear around unit | Clean coil; measure charge |
| Outdoor unit hums, then trips off | Failed capacitor or compressor issue | Leave it off after one reset | Test capacitor and compressor |
| Fault displayed on Infinity control | Model-specific fault | Write the code down | Diagnose per Carrier service manual |
Carrier heat pump not heating repair cost
These are typical US ranges and include parts and labor.
| Repair | Typical cost |
|---|---|
| Thermostat batteries / filter (DIY) | $0–$40 |
| Service call / diagnostic | $90–$250 |
| Thermostat reconfiguration or rewiring | $100–$250 |
| Run capacitor | $150–$400 |
| Contactor | $150–$350 |
| Defrost sensor or thermostat | $150–$400 |
| Defrost control board | $300–$700 |
| Infinity/Greenspeed outdoor control board | $600–$1,500 |
| Reversing valve solenoid coil | $150–$350 |
| Reversing valve replacement | $900–$2,000 |
| Leak search + repair + recharge | $400–$1,800+ (depends on refrigerant type and leak location) |
| Compressor replacement | $1,800–$3,500+ |
| Outdoor coil replacement | $1,500–$3,000+ |
Check your warranty before approving major work. Many registered Carrier units carry a 10-year parts warranty, but labor is usually not covered. On older units, a compressor or coil failure often makes replacing the system the better value.
Related codes
- Carrier Heat Pump Aux Heat On Constantly: Causes & Fixes
- Carrier AC Not Cooling: Causes, Fixes & Repair Costs
- Carrier Furnace Blowing Cold Air: Causes & Fixes (for dual-fuel systems with a gas furnace backup)
- Carrier Furnace Short Cycling: Causes, Fixes & Costs
Parts & tools for this fix
- Air filter (match your return filter size)
- AA/AAA thermostat batteries
- Heat pump compatible programmable thermostat
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Frequently asked questions
Why is my Carrier heat pump running but blowing cool air?
Heat pump air feels cooler than furnace air. It usually comes out around 85–100°F, which can feel cool on your hand. If the air is close to room temperature or colder, the most likely causes are a defrost cycle in progress, a thermostat set up for the wrong reversing-valve logic (O vs. B), a stuck reversing valve, or low refrigerant. The last three need a technician.
Is it normal for my heat pump to stop heating and steam outside?
Yes, if it lasts only a short time. During defrost the outdoor fan stops and the unit melts frost off the coil, which makes steam. A defrost cycle usually lasts a few minutes up to about 10 minutes. Aux or emergency heat should keep the house warm meanwhile. If the outdoor coil stays caked in ice for hours, call a technician.
Should I switch to emergency heat if my heat pump isn't heating?
Emergency heat (EM HEAT) is a good temporary fix that keeps you warm until service arrives. It runs only the backup electric strips or furnace and locks out the heat pump. Expect higher electric bills while it runs, and don't treat it as a long-term solution.
Can I add refrigerant to my Carrier heat pump myself?
No. Refrigerant handling requires EPA Section 608 certification. Low refrigerant also means there is a leak, and the leak has to be found and repaired. Topping off without fixing the leak wastes money and can damage the compressor.