Heat Pump Outside Unit Not Running: Where to Look

The two breakers, then the disconnect block

A heat pump has two breakers, one for the indoor air handler and one for the outdoor unit. A tripped breaker often does not look off, it sits limply in the middle, so push it firmly all the way off and back on.

Then look at the wall beside the outdoor unit. There is a grey box there with a handle or a block that pulls straight out, and that is the disconnect. It is what a technician pulls before working on the unit, and it is what gets left out, or pushed back in upside down, which is the off position.

Pull it right out and look at it. Some have cartridge fuses inside that blow, and a blown fuse there gives you exactly this, a unit with no power while the breaker looks fine.

Then the low voltage fuse indoors

The outdoor unit is switched on by a signal from the indoor board, so if that signal never arrives the unit sits there with mains power and does nothing.

On the indoor air handler’s control board there is a small blade fuse, usually three or five amps, the same kind a car uses. A broken stripe through the window means it has blown, and a shorted thermostat wire is normally why.

If the thermostat itself is also blank, that is the same fuse talking: a blank thermostat and where its power comes from.

Power off, then see whether the blade turns freely

Pull the disconnect block out first, so the unit cannot start while your hand is near it.

Then reach through the top grille with a long stick and give the fan blade a push. If it spins freely and keeps going, the motor is not seized and the fault is almost certainly the capacitor. If it is stiff, gritty or will not move, the motor bearing has gone.

A capacitor is a cheap part and a dangerous one to handle, because it stores a charge long after the power is off. This is the point where most people stop and make a call, and that is a reasonable place to stop.

Leaving it humming does real damage. A motor that cannot start draws heavy current and cooks itself, so switch it off at the breaker until it is fixed.

Give it five minutes, then look at the contactor

First, wait. Most systems hold the compressor off for up to five minutes after any interruption, and a lot of people diagnose a dead compressor during that wait.

If it still never starts, the usual suspects are the contactor, which is the relay that switches the compressor on, and the run capacitor that gives it the push to start.

Contactors fail in two ordinary ways. The contacts pit and burn until they no longer pass current, and insects get inside and stop them closing, which is a real and common thing rather than a joke.

The third possibility is the compressor protecting itself. If it has been struggling it shuts down on its own overload and restarts when it has cooled, which gives you a unit that works in the morning and not in the afternoon.