EcoGuide · Heat Pumps · Published 2026-10-01 · 1,300 words
Heat Pump Not Turning On? 7 Checks Before You Call Anyone
The Short Answer
When a heat pump will not start, the cause is usually one of four things: the thermostat is not actually asking for heat or cooling, the unit has no power, it is sitting out a deliberate delay, or a setting is locking it out in cold weather. All four can be checked from inside the house in a few minutes.
First, work out which part is not running. Go outside and look and listen at the outdoor unit, then check whether air is moving at the vents (that is the indoor blower). The checks below follow that split.
| What you notice | Start with |
|---|---|
| Nothing runs, inside or outside | Checks 1 and 2: thermostat call and power |
| Indoor fan blows, outdoor unit silent | Checks 2, 3 and 4: outdoor power, delay, lockout |
| It starts a few minutes later than expected | Check 3: compressor protection delay |
| Only in cold weather, and aux heat runs instead | Check 4: outdoor temperature lockout |
| Started right after a new thermostat | Check 7: wiring and setup |
| Thermostat clicks, nothing happens at the unit | Thermostat clicks but nothing happens |
1. Is the Thermostat Actually Calling?
Check the mode (Heat, Cool or Auto, not Off) and the setpoint. For cooling, Trane’s first check is to “make sure that the set temperature is lower than the current temperature”; for heating it must be higher.
Most smart thermostats show when they are calling. On an ecobee, an orange flame or ring means it is calling for heat and a blue snowflake or ring means cooling; a white icon means it is not calling. A thermostat also waits until the room is a little past the setpoint before it calls; ecobee’s default for this differential is 0.6–1°F. If you are a degree away, give it a moment.
The cleanest test is the thermostat’s test mode. On an ecobee: Main Menu → General → Settings → Installation Settings → Test Equipment, then Heat Pump Heat Stage 1 On, and check that the outside unit is on and its fan is running.
2. Power: Breakers and the Outdoor Disconnect
Carrier’s homeowner checks: make sure the circuit breaker has not tripped and the disconnect switch is in the “on” position. The disconnect is the box on the wall next to the outdoor unit. The indoor air handler usually has its own breaker too, so check both.
Be careful with a tripped breaker. Trane warns that resetting the breaker and restarting the unit could cause damage and suggests calling a professional. At the very least: if it trips again after a reset, leave it off and call a technician. A breaker that keeps tripping is protecting something.
3. A Normal Delay: Compressor Protection
Most thermostats will not restart the compressor straight after it stops. ecobee says it will not start a new cooling cycle until at least 5 minutes after the last one ended, to prevent short cycling. Honeywell’s FocusPro has a Compressor Protection setting of up to 5 minutes.
Some thermostats tell you this is happening: Honeywell shows “Wait” or a flashing “Cool On”, and Nest shows “Delayed”. Wait five minutes after any power cut, setting change or mode change before deciding it is broken.
4. Cold Weather: The Outdoor Temperature Lockout
Many thermostats stop calling the heat pump below a set outdoor temperature and use the backup heat instead. On an ecobee this is the Compressor Min Outdoor Temperature threshold, which ecobee says is 35°F by default. Below it, the outdoor unit simply is not asked to run.
That is often deliberate: heat pumps have a cut-off or balance point, and ecobee points to the heat pump manual or the specification sticker on the unit for the right number. If someone set the lockout too high, the heat pump sits idle while expensive aux heat does the work. If the lockout was lowered or disabled, ecobee suggests going back to the manufacturer’s figure or the default.
It works the other way in cooling: ecobee notes it is generally not recommended to run an A/C unit below 60°F outside, so a cool spring morning is a poor time to test cooling.
5. Winter Only: It May Be Defrosting
In cold, damp weather the outdoor unit regularly runs a defrost cycle, and for a few minutes it can look and sound unlike normal heating. Carrier says a light, thin layer of frost, generally less than 1/4 inch, is normal and should clear during defrost. See heat pump icing up for when ice is a real problem.
6. Airflow: Filter and Clearance
A blocked filter or a smothered outdoor unit makes a heat pump work badly and can trip its safety controls. Carrier recommends at least 24 inches of clear space around the heat pump, and Trane suggests changing the filter every 1–3 months. Clear leaves, snow and anything stacked against the unit.
7. Right After a New Thermostat: Wiring and Setup
If the problem started with a thermostat swap, check the install before anything else. ecobee’s steps: pull the thermostat off the wall, gently tug each wire to make sure none are loose, then compare the wiring screen (Installation Settings → Equipment → Wiring) with a photo of the wires. If they do not match, use Reconfigure Equipment.
If the system runs but heats when you ask for cooling, that is a different fault: see heat pump heating instead of cooling.
When to Call a Technician
Carrier lists the jobs for a professional: contactor, capacitor or damaged wiring when the unit will not start, and fan motor or electrical faults when the outdoor unit will not run. Call if a breaker trips again, if the outdoor unit hums but its fan does not turn, or if the thermostat is clearly calling (test mode on) and the outdoor unit stays silent. What the click at the unit tells you can help you describe it.
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Frequently Asked Questions
Why is my heat pump outside unit not running but the inside fan is blowing?
Check the outdoor disconnect and its breaker, wait five minutes for the compressor protection delay, and check whether a cold-weather lockout is active (ecobee’s Compressor Min Outdoor Temperature defaults to 35°F). If the thermostat is calling and the unit still will not start, call a technician.
Why won’t my heat pump turn on in cold weather?
Many thermostats stop using the heat pump below a set outdoor temperature and switch to backup heat. Check the lockout setting against the cut-off temperature in your heat pump’s manual.
Should I reset the breaker on my heat pump?
Carrier lists a tripped breaker as a homeowner check, but Trane warns that resetting and restarting can cause damage. If it trips again after one reset, leave it off and call a professional.
How long should a heat pump take to start?
After a power cut or a change of mode, many thermostats wait up to 5 minutes before starting the compressor to protect it. That delay is normal.
Related Guides
- Heat pump heating instead of cooling — the O/B setting
- Thermostat clicks but nothing happens — contactor or capacitor
- Aux heat won’t turn off — when the backup heat takes over
- Smart thermostat troubleshooting — every fix guide by symptom
