The first useful distinction is whether the system is producing no heat, or simply not keeping up.
If you see flames or suspect a fire because of smoke or a burning smell, get everyone outside, stay out and call 911 from a safe place. Do not stop to inspect the equipment or operate its shutoff. If the home is getting cold rapidly in severe weather, move occupants to a warm place and arrange urgent help. When there is no immediate fire or other emergency, electrical damage, repeated trips, flooding near electrical equipment or a fuel-fired backup vent problem still require qualified service. Do not approach unsafe equipment.
If air is moving and the home is still warming, the system may be operating normally, recovering slowly, or approaching the limit of available capacity under the current conditions. If indoor temperature keeps falling, you are dealing with a different problem.
If there is no useful heat at all, do not jump straight to a compressor, reversing valve, thermostat or refrigerant diagnosis. First record what the thermostat, indoor temperature, airflow, outdoor unit and backup heat are doing, then use qualified service if the home is not maintaining safe indoor conditions.
Little or no usable heat reaches the home
Record thermostat mode, setpoint, airflow, outdoor-unit state and auxiliary-heat state.
Do not name the failed part
The symptom still does not establish a compressor, board, valve, sensor or refrigerant fault.
The system runs, but indoor temperature drifts below setpoint
Record the rate and direction of the indoor temperature trend, not just how long the unit runs.
Conditions matter
Outdoor temperature, available low-temperature capacity, airflow, backup heat and building load all affect the result.
Register air feels less hot than furnace air
Heat-pump supply air can feel cool by comparison while still adding heat to the room.
Check the house, not your hand
Whether indoor temperature is holding or falling is more useful than hand-feel at one register.
Use the indoor temperature trend to separate two different problems.
A heat pump that runs continuously while holding the home near setpoint is not in the same state as a system that runs continuously while the indoor temperature continues to fall. Long run time alone does not prove that the system is undersized or failing.
Likewise, a system that takes a long time to recover from a large thermostat setback may be behaving differently from one that cannot maintain temperature under a steady setpoint. Preserve the schedule and control context before comparing capacity.
| State | Observe or ask | Boundary or next step |
|---|---|---|
| StateRECORD | Observe or askSetpoint and actual indoor temperatureWrite down both values and whether the gap is growing, stable or shrinking. | Boundary or next stepOutdoor conditionKeep the outdoor temperature from the same period so heating performance has context. |
| StateAlso record | Observe or askRecent setback or schedule changeA large recovery request can alter run time and auxiliary-heat behavior. | Boundary or next stepWhole home or one areaA room-specific problem can point toward distribution or zoning rather than whole-system capacity. |
Check what the thermostat and equipment are doing before treating the system as failed.
Record the thermostat mode, requested temperature, current indoor temperature and any display for auxiliary or emergency heat. If the system uses dual fuel or proprietary communicating controls, note that setup rather than assuming it behaves like a conventional thermostat.
A thermostat or reversing-control problem can produce confusing heating behavior, but terminal functions should come from the equipment and controller documentation, not a generic rewiring guess.
Heat-pump controls vary by equipment and topology. Have a qualified technician make wiring changes using the instructions for your exact equipment.
Use the heat-pump thermostat page for compatibility, auxiliary heat and changeover concepts.
Low airflow can look like a heating-capacity problem, but it still needs evidence.
At the homeowner level, the useful observations are simple: whether the filter is obviously restricted, whether supply registers and returns are obstructed, and whether airflow is weak everywhere or only in certain rooms. Do not turn those observations into a refrigerant or blower diagnosis.
For ducted systems, distribution matters because the heat pump can only deliver useful heating through the airflow the system and ducts support. Room imbalance, closed or blocked returns, and duct problems can change comfort without proving an outdoor-unit failure.
| State | Observe or ask | Boundary or next step |
|---|---|---|
| StateSafe observation | Observe or askFilter condition and visible obstructionsRecord what you can see without opening electrical or refrigerant compartments. | Boundary or next stepWhole-home versus room-specific airflowNote where the problem is strongest and whether it changed recently. |
| StateDo not infer | Observe or askLow refrigerant from weak heat aloneThat conclusion requires qualified measurements. | Boundary or next stepFailed blower from one weak registerDistribution and zone conditions must be separated first. |
Winter frost and defrost can interrupt heating briefly without meaning the system has failed.
Light frost on the outdoor coil can be normal in winter operation. During defrost, the coil warms and visible steam can appear. That temporary state is different from an outdoor coil that remains heavily iced after defrost and is accompanied by lost heating performance.
Record whether frost clears after defrost, whether the outdoor unit resumes normal operation, and whether the home continues to hold temperature. Persistent ice crosses the normal-behavior boundary, but it still does not identify a specific failed component.
| State | Observe or ask | Boundary or next step |
|---|---|---|
| StateCan be normal | Observe or askFrost forms and later clearsObserve the full defrost cycle rather than judging from one moment. | Boundary or next stepSteam appears during defrostVisible vapor can be normal as accumulated frost melts. |
| StateEscalate | Observe or askIce does not clear normallyPreserve photos and the timing pattern for qualified diagnosis. | Boundary or next stepHeating performance keeps fallingDo not keep treating persistent icing as normal frost formation. |
Backup heat changes what you need to check, but there is no universal outdoor-temperature rule.
Some systems use electric auxiliary heat, some coordinate a furnace or other backup source, and some are designed without a separate backup heat source. Record what your system actually has and whether auxiliary or emergency heat is active, unavailable, locked out or manually selected.
A control that brings backup heat on at a certain outdoor temperature is a system-specific design choice. Do not assume that one changeover temperature applies to every heat pump or that backup heat must always run whenever the weather becomes cold.
Read the Emergency Heat guide before changing settings or assuming the heat pump has failed.
When the house falls behind, compare the building load with the heat the system can actually deliver under those conditions.
The relevant capacity is not the nominal tonnage printed in a product family name. Heating output changes with outdoor conditions and the exact matched equipment. A cold-weather performance question therefore needs the exact equipment identity, the home's heating load or credible load evidence, and the backup-heat strategy.
Do not reuse the old system's tonnage as proof that the replacement or current system is correctly sized. A heating problem can come from design, installation, controls, distribution or equipment, and the symptom alone cannot rank those causes reliably.
Use the cold-climate page to understand low-temperature capacity and backup design, then return to diagnosis if the home still cannot maintain temperature.
Before the service visit, write down what the house, thermostat and equipment were doing at the same time.
For a not-heating call, bring a short record of what the house, controls and equipment were doing at the same time.
- Temperatures
Setpoint, actual indoor temperature and outdoor temperature
Include whether the indoor temperature was stable, recovering or still falling.
- SYSTEM
Indoor and outdoor model numbers
Exact model numbers matter for capacity, controls and matched-system matching.
- Operation
Airflow, outdoor-unit state, defrost behavior and backup-heat state
Record only observations, not a guessed diagnosis.
- Evidence
Fault codes, photos, video and recent service history
A fault code helps the technician narrow the problem but does not automatically identify the failed part.
- Recent changes
Thermostat schedule, controls, filter, renovation or equipment work
Changes close to the start of the symptom can be important clues.
Move to the repair page when the task has become professional diagnosis or repair.
Common questions when a heat pump is not heating.
Why is my heat pump running but the house is getting colder?
That is different from long run time with stable indoor temperature. Record the indoor temperature trend, outdoor conditions, airflow, auxiliary-heat use and exact equipment models if known. The remaining cause can involve building load, available capacity, controls, distribution, installation or equipment and should not be reduced to one guessed part.
Why does the air feel cool even though the heat pump is on?
Heat-pump supply air can feel much less hot than furnace air while still heating the room. Whether the home is maintaining or recovering toward setpoint is more useful than hand-feel alone.
Does frost on the outdoor unit mean it is not heating correctly?
Not by itself. Winter frost can be normal and may clear during defrost. Ice that persists after defrost, especially with declining indoor temperature, needs troubleshooting.
Should I turn on emergency heat?
Do not use Emergency Heat as a universal troubleshooting step. First establish what backup heat your system actually has and why the control is calling for it. Use the dedicated Emergency Heat guide for that question.
Can I tell which part failed from the symptom?
Usually not. A not-heating symptom can narrow the possibilities, but a compressor, control board, sensor, reversing valve, blower or refrigerant problem should not be treated as confirmed without the measurements or service evidence required for that exact system.