Heat pump not turning on: check the outdoor unit first
A heat pump has two halves, indoors and outdoors. Knowing which half is dead tells you whether you have a power fault, a lockout or something for a technician.
If your heat pump will not turn on, find out which half is silent. The indoor air handler can run while the outdoor unit does nothing, and that is a different problem from a system with no power at all.
Do that first. It narrows the list fast.
Which half is not running?
Set the thermostat to heat and a few degrees above room temperature. Listen at the vents, then walk outside. If the indoor blower runs but the outdoor fan and compressor stay dead, the fault is on the outdoor side or in the signal reaching it. If both are dead, think power or the thermostat.
Wait out the restart delay
Many heat pumps hold the compressor off for a few minutes after a power blip or a shutdown, to protect it. During that wait the outdoor unit looks dead even though nothing is wrong. After any outage or breaker reset, give it at least five minutes before you decide it has failed.
Check the thermostat, then the breakers
Confirm the thermostat is set to heat and the setpoint is above the room temperature. Replace the batteries if it takes them. Then check two places for power: the breaker for the indoor air handler, and the outdoor disconnect box near the outdoor unit. Either can trip or get switched off without anyone noticing.
If a breaker trips again right after you reset it, stop. A breaker that keeps tripping is reporting a fault, and resetting it repeatedly can do damage.
Frost, ice and defrost mode
In cold, damp weather a heat pump may look inactive while it runs a defrost cycle. That is normal and ends on its own. A unit that is buried in ice and never recovers is a different story and needs a technician.
A safety lockout looks like a dead unit
Some systems shut themselves down after repeated failed starts or when a safety device trips. The unit then refuses to run until it is reset by power cycling or by a technician. If it locks out again after one reset, do not keep trying. Something is making it unsafe or unable to run.
What not to do
- Do not remove the outdoor unit's panels or touch the electrical components inside. Some parts can hold a dangerous charge even when the unit is off.
- Do not try to add or check refrigerant. Federal law requires EPA Section 608 certification, and it is illegal without it.
- Do not keep resetting a tripping breaker.
When to call a licensed HVAC technician
Call one if the outdoor unit stays dead after the restart delay, the thermostat and the breakers all check out, or the breaker trips again. Anything inside the cabinet is electrical and refrigerant work for a licensed professional.
Frequently asked questions
Why is my heat pump not turning on but the thermostat is working?
The outdoor unit may be waiting out a restart delay, the outdoor disconnect or breaker may be off, or the system may have locked out. Wait five minutes, then check the breaker and disconnect.
Is a frozen outdoor unit the same as not turning on?
Not always. Frost during cold, damp weather is normal and clears through a defrost cycle. A unit that stays iced over and never recovers needs a technician.
Can I fix a heat pump that will not start myself?
You can check the thermostat, the breaker and the disconnect. Anything inside the unit is electrical or refrigerant work and must be done by a licensed technician.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.
- ENERGY STAR. Air-Source Heat Pumps
- ENERGY STAR. Energy Savings at Home: Heating & Cooling