Compressor heaters, commonly called crankcase heaters, serve one essential purpose: they keep liquid refrigerant from pooling in the compressor’s oil reservoir when the system is off. This prevents a cascade of mechanical problems that can shorten a compressor’s life or destroy it outright. They’re found in air conditioning units, heat pumps, chillers, and commercial refrigeration systems.
Why Refrigerant Migrates When the System Is Off
While a compressor is running, the mechanical work it performs generates enough heat to keep refrigerant in a gaseous state inside the compressor housing. The problem starts when the system shuts down. As the compressor cools, refrigerant vapor throughout the system naturally moves toward the coldest point, and that’s often the compressor crankcase. Once there, falling temperatures and pressure cause the vapor to condense into liquid and dissolve into the lubricating oil sitting at the bottom of the crankcase.
The longer the system stays off, the more refrigerant accumulates. During extended shutdowns, large amounts of refrigerant can transfer from the low-pressure side of the system to the high-pressure side as temperatures equalize. The result is a crankcase full of oil that’s been heavily diluted with liquid refrigerant, which sets the stage for serious damage when the compressor starts back up.
The Damage Diluted Oil Causes
Liquid refrigerant is heavier than compressor oil, so it settles beneath the oil layer at the bottom of the crankcase. When the compressor starts, the sudden drop in crankcase pressure causes that trapped refrigerant to boil rapidly. Because the boiling happens underneath the oil, it churns the oil into a violent foam, pulling tiny oil particles up and out of the crankcase. This is called oil foaming, and it strips the crankcase of the lubricant that bearings, pistons, and other moving parts depend on.
The consequences get worse from there. The foamy mixture of oil and refrigerant gets pumped through the compressor’s cylinders and into the discharge line. A compressor is designed to pump gas, not liquid. When liquid slugs of refrigerant and oil pass through the cylinders, the incompressible fluid hammers against valves, piston rods, and bearings. This is known as slugging, and it can crack valves, score bearing surfaces, and bend connecting rods. Meanwhile, oil that’s been pumped out of the crankcase coats the inside of refrigerant lines and heat exchangers throughout the system, reducing efficiency. The compressor, now trying to pump a denser-than-normal mixture, draws excessive electrical current, which can overheat the motor windings or trip safety switches.
How Crankcase Heaters Prevent This
A crankcase heater applies gentle, continuous warmth to the compressor’s oil sump. By keeping the oil and lower compressor housing a few degrees warmer than the coldest point in the system, the heater eliminates the temperature gradient that draws refrigerant toward the crankcase in the first place. Refrigerant only migrates to and condenses in the coldest spot, so as long as the compressor isn’t that spot, the oil stays clean.
The heater runs whenever the compressor is off. In many systems it’s wired so that it energizes automatically the moment the compressor shuts down. When the compressor starts running again, a thermostat opens and cuts power to the heater, since the compressor’s own operating heat takes over. Some modern systems use temperature sensors at the compressor, the indoor coil, and the outdoor unit. If the compressor temperature isn’t a specified amount above the lower of the other two readings, the heater turns on. Once the compressor is warm enough relative to the rest of the system, the heater shuts off to save energy.
Types of Crankcase Heaters
Two main designs are used. Belly band heaters wrap around the outside of the compressor shell near the bottom, transferring heat through the casing and into the oil inside. They’re common in residential air conditioners and heat pumps because they’re easy to install and replace without opening the compressor. Insertion (or immersion) heaters slide directly into the crankcase through a port in the compressor housing, heating the oil from inside. These transfer heat more efficiently and are typical in larger commercial and industrial compressors where the oil volume is greater and external heating alone may not be sufficient.
Preheat Requirements Before Startup
If a system has been powered off for a long time, or if you’re commissioning a new installation, the crankcase heater needs time to drive accumulated refrigerant out of the oil before the compressor can safely run. Copeland, one of the largest compressor manufacturers, specifies a minimum of 12 hours of heater operation before starting the compressor. Some field guidelines call for 24 hours on new installs or systems that have been de-energized for extended periods. Skipping this preheat period risks oil dilution and bearing damage on the very first start.
This is why technicians often emphasize that you should never flip the breaker on for your outdoor unit and immediately call for cooling after a long power outage or seasonal shutdown. The heater needs time to do its job first.
What Happens When a Crankcase Heater Fails
A failed crankcase heater doesn’t cause an immediate breakdown. The system will still run normally for a while, which is why the problem often goes undetected. Over time, though, each startup cycle sends a slug of diluted oil and liquid refrigerant through the compressor. You might notice the compressor making unusual noises at startup, or oil stains appearing at refrigerant fittings as excess oil circulates through the system. Eventually, repeated slugging and poor lubrication lead to valve damage, scored bearings, or motor burnout. Many premature compressor failures that look like bad luck are actually the result of a crankcase heater that quietly stopped working months earlier.
Technicians can check heater operation by feeling the bottom of the compressor shell when the system is off. It should be noticeably warm to the touch. An ohm reading across the heater’s terminals confirms electrical continuity. On systems with external belly band heaters, a visual inspection can reveal cracked or corroded elements.