A heat pump that does nothing at all is one of the easier problems to narrow down, because the list of causes is short and several of them take a minute to check. Start at the thermostat and work outward. If you get to the end of the homeowner checks, call and tell us where you stopped.
First: what exactly is not running?
A split heat pump has two halves. The indoor air handler moves air through the house. The outdoor unit runs the compressor and the fan. Before anything else, notice which half is silent:
- Nothing indoors and nothing outdoors. Power or thermostat. Start with the checks below.
- Indoor fan runs, outdoor unit silent. The outdoor side is not getting power or a start signal, or a part in it has failed.
- Outdoor unit hums but the fan does not spin. Almost always a failed capacitor or fan motor. Turn the system off so the compressor is not straining.
Checks you can do yourself
- The thermostat. Is it set to HEAT or COOL, with a setpoint that should call for it? Is the screen blank? A blank screen usually means dead batteries or no power from the system. Replace the batteries first.
- The breakers. A heat pump normally has two: one for the indoor air handler, and a separate larger one for the outdoor unit. Check both in the panel. Reset a tripped breaker once. If it trips again, stop and read why an AC or heat pump keeps tripping the breaker.
- The outdoor disconnect. A grey box on the wall near the outdoor unit. Someone may have pulled the handle or flipped the switch during yard work.
- The air handler door. Many units have a safety switch behind the panel that cuts power when the door is not seated. If someone changed the filter recently, push the panel firmly into place.
- The condensate float switch. In cooling season, a clogged drain lifts a float switch that shuts the whole system off. If you see standing water in the drain pan, that is your answer. Our article on heat pump leaking water inside covers clearing it.
- The filter. A badly clogged filter does not usually stop a heat pump from starting, but it can trip safety limits that stop it soon after. Replace it if you cannot see light through it.
What it usually is when those checks pass
- A failed run capacitor. The most common single part failure on outdoor units. The unit hums, tries, and does not start. An inexpensive part and a short visit.
- A failed contactor. The switch that sends power to the compressor and fan. Pitted contacts stop power getting through.
- A blown low voltage fuse on the control board. Usually caused by a short in the thermostat wiring. Replacing the fuse without finding the short just blows the new one.
- A failed defrost board or control board. Less common, and the board usually shows a fault light that points to it.
- Damaged thermostat wire. Rodents chew low voltage wire in attics and crawlspaces more often than people expect.
In winter: running but not heating
If the unit runs but the air is cool, that is a different problem. The heat pump may be in defrost, which is normal for a few minutes on a cold morning, or it may be low on refrigerant or stuck in cooling mode. Our article on a heat pump not heating goes through those.
When not to keep trying
If the outdoor unit hums without the fan turning, if a breaker trips twice, or if you smell something burning, turn the system off. Repeated start attempts on a failed capacitor strain the compressor, and that is the part you do not want to replace. See compressor replacement cost for why.
What we do
We test the power at each stage, the capacitor under load, the contactor, the low voltage circuit and the board, so the repair is the part that failed and not a guess. Our diagnostic is a flat $69 and it is credited toward the repair. If you have no heat or cooling in serious weather, see emergency and same-day repair. More on the equipment is on our heat pump services page.
Frequently asked questions
Why is my heat pump not turning on at all?
The most common causes are a thermostat with dead batteries, a tripped breaker, an outdoor disconnect that was switched off, an air handler door safety switch, or a condensate float switch that shut the system down.
My heat pump outdoor unit hums but the fan will not spin. What is wrong?
Usually a failed run capacitor or fan motor. Turn the system off. Letting it keep trying to start puts strain on the compressor.
Does a heat pump have its own breaker?
Usually two: one for the indoor air handler and a larger one for the outdoor unit. Check both, and reset a tripped breaker only once.
Can a clogged drain stop my heat pump from turning on?
Yes. Many systems have a float switch in the drain pan that shuts everything off when water backs up. Clearing the drain line usually restores operation.
Why does my heat pump run but blow cool air in winter?
It may be in a normal defrost cycle, which lasts a few minutes. If the air stays cool for longer, it may be low on refrigerant or stuck in cooling mode and needs a technician.
How much does it cost to fix a heat pump that will not turn on?
Our diagnostic is $69 and credited toward the repair. Common fixes like a capacitor, contactor or fuse are inexpensive. A control board or compressor costs more, and we quote before any work.