Crankcase heaters are a critical but often overlooked component in air conditioning and refrigeration systems. Their function is simple but vital: to prevent liquid refrigerant from migrating into the compressor crankcase during the off-cycle. A missing or failed crankcase heater is a leading cause of premature compressor failure.

This guide covers why they're necessary, how they work, how to install them, and how to diagnose them in the field.

The “Why”: Understanding Refrigerant Migration

When a system shuts down, the refrigerant vapor will naturally travel to and condense in the coldest part of the system. More often than not, this “cold spot” is the compressor. This phenomenon is known as refrigerant migration.

This issue isn't limited to cold climates. Consider a split system in a hot climate like Florida:

• An outdoor unit cycles off in the late afternoon.
• The indoor unit is in a scorching hot attic, keeping the evaporator and suction line warm.
• As the sun sets, the outdoor condensing unit cools rapidly, becoming the coldest part of the system.
• Refrigerant migrates from the warm indoor coil, through the suction line, and settles in the cooler compressor crankcase.

When liquid refrigerant floods the crankcase, it mixes with and dilutes the compressor oil, drastically reducing its ability to lubricate.

The Consequences: From Foaming to Catastrophic Failure

The real damage occurs at the next startup. The sudden drop in crankcase pressure causes the liquid refrigerant to boil violently, turning the oil into a useless foam. This event has two destructive outcomes:

1. Bearing Washout: The foamy mixture is forced out of the crankcase, starving the crankshaft, bearings, and other precision parts of essential lubrication.
2. Slugging: A “slug” of incompressible liquid refrigerant and oil is drawn into the cylinders or scrolls. This can cause immediate, catastrophic damage, including broken valves, damaged pistons, or shattered scroll sets.

Experience has shown that compressors filled with liquid may cause excessive start-up noise or trip the internal overload protector several times before running. A crankcase heater reduces these issues, leading to fewer customer complaints about noise or dimming lights.

Special Compressor Considerations

A Special Note on Scroll Compressors

Modern scroll compressors have engineered “compliance” mechanisms that allow the scrolls to separate slightly, helping them process small amounts of liquid refrigerant. However, they are not invincible.

According to manufacturers like Copeland, due to a scroll's inherent ability to handle liquid, a crankcase heater may not be required if the total system refrigerant charge is below a specific limit. It is crucial to consult the manufacturer's technical data for the specific compressor model to find these charge limits. If the system charge exceeds the specified amount, a crankcase heater is still required regardless of the system type.

Some scroll compressors are also designed to stall if they take in too much liquid as a protective measure.

Warning: Adding a hard start kit to a compressor that is stalling due to liquid migration is not recommended. The increased starting torque can override the compressor's self-protection, forcing it to start against a slug of liquid and leading to a broken scroll set.

A Note on Variable-Speed Compressors

Variable-speed compressors often do not require a traditional external crankcase heater. The variable frequency drive (VFD) that controls the compressor typically has a programmable feature that utilizes the motor windings for heating. The drive sends a small amount of DC current through the windings, generating up to 50 watts of heat.

Types of Crankcase Heaters

1. Wrap-Around (Belly-Band) Style

This is the most common type. It consists of a flexible band heater that straps directly around the outside of the compressor housing, near the bottom.

•  Pros: Easy to install and replace on-site without opening the refrigeration circuit. Provides even heating.
•  Cons: Less efficient than internal heaters. 

2. Immersion (Well) Style

This heater is inserted directly into a dedicated well or port in the compressor crankcase, heating the oil directly.

• Pros: More efficient and provides faster, more direct heating of the oil.
• Cons: Can only be used on compressors specifically designed with a well. Replacement can be more involved.

Voltage and Wattage

Heaters come in various voltage and wattage ratings (e.g., 24V, 120V, 240V, 480V) with power outputs typically ranging from 40W to 120W. It is critical to match the heater's voltage to the system's power supply and its wattage to the compressor's size. Using an incorrectly sized or voltage-matched heater will either be ineffective or cause it to burn out.

Installation and Sizing Guidelines

Installation Procedure

A proper installation is crucial for the heater to function correctly and safely.

1. Select the Correct Model

Choose the appropriate heater based on the compressor's size and the required wattage as specified in the manufacturer's application guidelines.

2. Positioning is Key

Install the heater as low on the compressor shell as possible without covering or being affected by the compressor shell bottom weld. The clamp mechanism should ideally be positioned over the vertical shell seam weld.

Ensure that the heater is installed perfectly horizontally and not at an incline.

3. Ensure Full Contact

The heater must make snug, direct contact with the compressor housing along its entire length for effective heat transfer.

Do not place the heating portion of the band over any other weld projections or seams on the compressor shell. This can create hot spots and lead to premature heater failure.

4. Secure the Heater

Close the locking mechanism and tighten the screw to the manufacturer's specified torque, typically 20-25 in-lb. This ensures adequate contact and prevents heater burnout.

If there is excess clamp bracket, it may be trimmed. Ensure no sharp edges are left that could damage wiring.

5. Safety and Labeling

WARNING: Crankcase heaters must be properly grounded.

WARNING: Never apply power to the heater in free air or before it is installed on the compressor. This will cause it to overheat and burn out.

The presence of a crankcase heater should be clearly indicated. Apply caution labels or markings on or near the compressor to alert future technicians.

Control Strategies & Wiring

How a crankcase heater gets its power is just as important as the heater itself. Here are common methods:

Constant Power (Always-On) 

The heater is wired to a constant power source and remains energized whenever the main disconnect is on. This is common in commercial systems.

Contactor Control

A common wiring method for crankcase heaters, illustrated below, often causes confusion. It appears that the lead connected to T1 lacks voltage due to an open contact. However, a closer inspection reveals that the compressor run wire shares the T1 terminal block with the second crankcase heater lead. This allows the second lead to draw the necessary operating voltage from the compressor windings. This setup also ensures that the crankcase heater deactivates when the contactor closes.

When replacing a single-pole contactor with a double-pole contactor, technicians often encounter an issue with the crankcase heater. Unlike a single-pole contactor, which provides continuous voltage on one leg, a double-pole contactor interrupts both L1 and L2 voltage lines when open. This means wiring a double-pole contactor identically to a single-pole contactor will prevent the crankcase heater from functioning. The solution is to connect a short jumper wire from L2 to T2. This allows voltage to pass through L2 to T2, mimicking the continuous voltage supply of a single-pole contactor.

Auxiliary Relay on the Contactor 

This auxiliary relay is a secondary electrical switch that works in tandem with the contactor to control the operation of the crankcase heater. Its primary function is to ensure the crankcase heater is only active when the compressor is off and to de-energize it when the compressor is on.

Internal Winding Heat

An older method used dedicated control circuits with separate capacitors to send a small amount of current through the compressor windings themselves, generating heat internally.

Module-Controlled 

On some modern systems, crankcase heaters are powered directly by the unit's control module and do not require separate field wiring.

Discharge Line Thermostat 

When the discharge line temperature drops, the switch closes and energizes the crankcase heater. When the compressor is running, the thermostat opens and cuts off the crankcase heater. This is very common on Trane units. Also note that some Carrier units use an air thermostat in the outdoor unit.

Ambient Temperature Switch

The temperature switch must be exposed to ambient temperatures that will turn the crankcase heater on or off based on a designed temperature threshold.

SureSwitch 

The White-Rodgers SureSwitch by Copeland has dedicated terminals for crankcase heater connection.

Operational Best Practices

• Pre-Startup Heating: On a new install or a system that has been off for an extended period, it must be energized for a minimum of 24 hours before the initial start. The most critical start for a compressor is its first one, and this requirement should not be compromised. The reason is not simply to warm the oil, but to gently heat it to drive off any liquid refrigerant that has migrated into it, ensuring a high concentration of pure oil is available to lubricate bearings and surfaces on startup.
• Continuous Operation: The crankcase heater must remain energized during all compressor off-cycles to be effective. It should always be wired in a way that it turns on whenever the compressor is switched off.

Field Diagnostics & Troubleshooting

A failed crankcase heater often goes unnoticed until the compressor fails. Here’s a simple diagnostic procedure:

1. Visual Inspection

• Check for obvious signs of failure like cracks, burned spots, or frayed electrical leads, especially where the wires enter the heater body.
• On units with sound blankets, peel the blanket back. They can trap moisture and accelerate corrosion and failure.

2. Temperature Check

• Use a thermal imager or infrared thermometer. When power is applied to the heater, the compressor shell should feel slightly warm to the touch—not hot, but noticeably warmer than the ambient temperature. Use caution when touching energized equipment.

3. Electrical Testing

Always disconnect and verify all power before performing resistance checks.

• Voltage Check: With the circuit energized and the heater supposed to be on, carefully check for proper voltage at the heater's leads. If there's no voltage, the problem is in the wiring or control circuit, not the heater itself.

Voltage Check

• Resistance (Ohm) Check: Disconnect the heater from its power source. Ensure 0V from each leg to ground. Remove the wires for the crankcase heater. Set your multimeter to the ohm scale (Ω) and measure the resistance between the two leads. An operable heater will show a specific, measurable resistance, while a failed heater will show “OL” (Open Line), indicating a break in the heating element. 

Resistance check

• Short to Ground test: With the crankcase heater power disconnected, check for any continuity to ground. A reading other than OL could indicate a short to ground, which could trip breakers in a way similar to that of a shorted compressor.

Short to Ground Test

You can determine the target resistance for a heater using the formula: Resistance (Ω) = (Voltage² / Wattage)

Here is a quick reference chart for common heaters:

The Final Word: A Critical Component, Not an Optional Extra

In an industry focused on cost and efficiency, the crankcase heater has sometimes been omitted by manufacturers, perhaps to meet higher efficiency standards, perhaps to cut costs. However, it is one of the most valuable and legitimate “upsell” items you can offer a customer. Installing a crankcase heater is a low-cost insurance policy against a multi-thousand-dollar compressor replacement. It’s a mark of a thorough technician who understands the science of the system and is dedicated to its long-term reliability.

—JD Kelly