The crankcase heater warms the compressor oil sump so liquid refrigerant does not collect in the oil while the compressor is off. A failed heater can cause foaming oil and liquid slugging at start-up, leading to compressor damage. It should be warm when the compressor is idle.
This is a small part with a big job. Compressors that sit outside, or that start after a long outage, depend on it to avoid damaging their bearings.
What it does
The heater is a band or insert on the crankcase that draws low power whenever the compressor is off. It keeps the oil slightly warmer than the coldest point in the system, so refrigerant stays a vapor and does not migrate into the oil. When the compressor starts, oil is ready to lubricate.
Signs it is failing
- Compressor shell cold when off.
- Noisy or rough start after standing.
- Oil foaming in the sight glass, if fitted.
- Breaker trip from a shorted heater.
- Heater band loose or damaged.
- Repeated compressor failures after outages.
Common causes of failure
| Cause | What happens |
|---|---|
| Burnt-out element | No heat to the sump |
| Loose band | Poor contact with the shell |
| Power interruption | Breaker off or wiring fault |
| Moisture damage | Corrosion in the connection |
| Wrong rating | Too little heat for the compressor |
What you can safely check
- Feel the shell carefully. After power has been on for a while, the crankcase should feel warmer than the room.
- Check the disconnect. Heater needs power even when the compressor is off.
- Look at the band. Is it tight and clean?
- Ask about outages. Did the box go through a long shutdown?
This part is on live power or connected to the pressurized refrigerant circuit. Do not open the electrical box, bridge terminals or loosen fittings. Only a qualified technician should test or replace it.
What a technician does
- Tests the heater for continuity and ground faults.
- Checks power supply and controls.
- Replaces the heater or band.
- Warms the compressor before restart after a shutdown.
- Checks oil level and condition.
Keeping it healthy
- Leave the disconnect on except during service.
- After long shutdowns, ask about warm-up time before starting.
- Report cold compressor shells.
- Include heaters in yearly service.
Switching off the disconnect to save energy defeats the heater. Start-up after a long off period can then be damaging. Keep the service invoice and any pressure or temperature readings in your equipment file, because comparing them with later visits is the easiest way to spot a slow change before it becomes a failure. A short note on the date, the outdoor weather and what the box was doing when you noticed the problem is also useful.
When to call for service
Call if the compressor is cold when idle and the unit has been on for hours, or if you plan to restart after a long shutdown. Ask the technician how long the sump should warm before you switch the unit on.
Repair or replace
Heaters are replaced and are inexpensive compared with the damage they prevent. If a compressor has been damaged by liquid slugging, the heater may be only part of the story. See power outage and storm recovery.
Related reading: the parts and components guide, the compressor and the suction accumulator.
Questions about this part
Do all compressors have one?
Many do, especially outdoor units. Ask your technician.
Should I switch it off in winter?
No. It matters most when the weather is cold.
How long should it warm before start-up?
It depends on the compressor. Ask the manufacturer or technician.