Direct answer: If the thermostat is calling for heat but the outdoor unit never starts, the most common causes are a thermostat setup issue, a tripped breaker or outdoor disconnect, a severe airflow problem that led to a shutdown, or an electrical fault in the outdoor section that is not a safe DIY repair.
Most likely: Start by confirming the thermostat is in heat mode with the setpoint raised, then check the air filter, indoor airflow, the indoor and outdoor breakers, and the outdoor disconnect before blaming the heat pump itself.
A heat pump in heating mode normally needs the outdoor unit to run. If the indoor blower runs but the outdoor section is dead quiet, separate the easy power-and-settings checks from the pro-only electrical faults right away. Reality check: in very cold weather, some systems will lean on auxiliary heat for stretches, but the outdoor unit should not stay off all the time unless something is wrong or the system is in a short defrost-related pause. Common wrong move: resetting breakers over and over without finding out why they tripped.
Don’t start with: Do not start by ordering a capacitor, contactor, or control board. On this symptom, lost power and setup mistakes beat bad parts more often than homeowners expect.
Air comes from the vents, often lukewarm at best, but outside there is no fan sound, no compressor hum, and no startup attempt.
Start here: Check thermostat mode and setpoint first, then verify both the indoor and outdoor breakers and the outdoor disconnect are fully on.
The house warms some, but electric backup heat seems to be doing the work while the outdoor unit stays off.
Start here: Treat this as an outdoor-unit problem first, not a thermostat-only problem. Confirm power to the outdoor section and look for ice, fault lights, or a recent breaker trip.
The system seemed normal in cooling, but now the thermostat calls for heat and the outdoor section never starts or only pauses briefly.
Start here: Double-check thermostat programming for heat pump operation and make sure the thermostat was not set up as a conventional furnace system.
You may hear a click or short hum, then nothing, or the breaker trips again when heat is called.
Start here: Stop resetting it repeatedly. That points to an electrical or compressor-side fault that needs a technician.
If the thermostat is not actually sending a proper heat-pump call, the indoor blower or auxiliary heat may still run while the outdoor unit never gets the signal it needs.
Quick check: Set the thermostat to Heat, raise the setpoint several degrees, cancel any schedule hold confusion, and confirm it is configured for a heat pump rather than a conventional furnace/AC setup.
A tripped breaker, pulled disconnect, or loose service switch can leave the indoor section alive while the outdoor unit is completely dead.
Quick check: Check the main panel for both indoor air handler and outdoor heat pump breakers, then look at the outdoor disconnect box near the unit to make sure it is present and fully seated.
A badly clogged filter, blocked return, or iced coil can push the system into poor operation where backup heat runs but the heat pump side does not recover normally.
Quick check: Inspect the filter, make sure supply and return grilles are open, and look for heavy frost or solid ice on the outdoor coil.
If settings and power are correct but the outdoor unit stays silent or only clicks, the problem is often in the contactor, capacitor, defrost/control circuit, compressor protection, or wiring.
Quick check: Listen for a click at the outdoor unit when heat is called. If you hear a click but no proper startup, or if the breaker trips, stop at diagnosis and call for service.
A surprising number of no-heat calls come down to thermostat mode, schedule overrides, or a thermostat set up for the wrong equipment type.
Next move: If the outdoor unit starts after correcting settings, the problem was likely thermostat mode, schedule logic, or setup. If the thermostat still shows a heat call but the outdoor unit stays off, move to power and airflow checks.
What to conclude: You want to rule out a false call for heat before touching anything else.
A heat pump that is starved for airflow can ice up, run poorly, or fall back on auxiliary heat, which makes it look like the outdoor unit has failed when the system is really protecting itself or struggling.
Next move: If the outdoor unit starts and heating improves after restoring airflow, the restriction was likely part of the problem. If airflow is decent and the outdoor unit is still dead quiet, check power next.
What to conclude: This separates a basic maintenance problem from a true outdoor-unit no-start problem.
Heat pumps usually have separate power feeds for the indoor air handler and the outdoor unit. One can be on while the other is off.
Next move: If restoring power brings the outdoor unit back, monitor it closely. A breaker that trips again is a fault, not bad luck. If breakers and disconnects are on and the outdoor unit is still silent, the problem is likely beyond a basic homeowner reset.
At this point you are not trying to repair live electrical parts. You are looking for field clues that tell you whether the system is locked out, iced up, or failing to start under load.
Next move: If you only found a temporary lockout after a power interruption and the unit resumes normal operation, keep watching for repeat failures. If you hear clicking, humming, repeated failed starts, or see heavy ice, schedule service instead of forcing more resets.
Once basic settings, filter, and power checks are done, the remaining causes are usually not good DIY territory on a heat pump.
A good result: If emergency heat keeps the house safe until service arrives, you have limited further strain on the outdoor equipment.
If not: If even emergency heat does not warm the house, or breakers continue to trip, treat it as an urgent HVAC service call.
What to conclude: The practical next move is controlled temporary heat plus a clean service call, not random part replacement.
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Only temporarily if auxiliary or emergency heat is taking over. In normal heat-pump heating, the outdoor unit should run. If it stays off all the time, something is wrong or the system is in only a brief pause.
The usual reasons are wrong thermostat mode or setup, lost power to the outdoor unit, or an outdoor electrical fault. Start with thermostat settings, filter, breakers, and the outdoor disconnect before assuming a failed major part.
One reset is reasonable. If it trips again, stop. A repeat trip points to a real fault such as a short, hard-start problem, or compressor issue, and repeated resets can make things worse.
Not on this symptom unless you are trained and have already confirmed the fault safely. Those parts live in a high-voltage area, and this page's safer homeowner path is to stop after the basic checks and call for service if power and settings are correct.
Heavy ice is not a normal homeowner-fix condition. Shut the system off at the thermostat and arrange service. Ice can point to airflow trouble, defrost problems, or refrigerant issues, and forcing operation can damage the system.
A dirty filter usually does not cut outdoor power by itself, but it can create poor airflow, icing, and backup-heat behavior that makes the system look like an outdoor-unit failure. It is still one of the first things worth checking because it is common and safe.