The crankcase heater.

A crankcase heater is a low wattage element wrapped around or inserted into the base of the compressor. It keeps the oil in the compressor sump slightly warmer than the rest of the system.

That sounds trivial. It prevents a specific and expensive problem.

What it prevents

When a system sits idle and cold, refrigerant migrates toward the coldest point and condenses there. If that point is the compressor sump, liquid refrigerant collects in the oil.

On start-up, pressure drops suddenly and that refrigerant boils out of the oil violently, taking the oil with it as foam. For a few moments the compressor runs with poor lubrication, which is exactly the condition bearings do not survive repeatedly.

A crankcase heater keeps the sump warm enough that refrigerant goes somewhere else.

Why it matters less here, and still matters

Phoenix nights are mild compared with most of the country, so migration is a smaller risk than it is in a cold climate. Not every system here has a heater fitted.

Where it matters most locally:

• Systems that sit idle for long stretches, which is every snowbird house
• Heat pumps, which cycle differently
• Cooler months, and the handful of genuinely cold Valley nights

The practical consequence

Do not kill the breaker to the outdoor unit for the season. If the system has a crankcase heater, it is powered through that breaker even when nothing is running, and switching it off for months defeats the entire point.

If the power has been off for a while, manufacturers typically want the breaker back on for several hours before the compressor runs. Switch the breaker on, leave the thermostat alone, and come back later. Worth remembering before the first hot day of the year.

Where somebody has already run it, that is not a disaster, it is just wear you did not need to add.