Crankcase Heaters: When a Compressor Needs One and How to Test It

When a system sits off in cool weather, refrigerant migrates to the coldest part of the system. If that's the compressor, liquid refrigerant dissolves into the oil. At startup the oil foams, the compressor pumps oil out and can slug liquid, and bearings run without lubrication. A crankcase heater keeps the compressor warm enough that refrigerant doesn't collect there.

When one is required

It depends on the compressor and the system charge. Copeland's guidance:

• Residential single-phase scroll compressors (ZR and similar families): crankcase heaters aren't required when the system charge is at or below 120% of the compressor's refrigerant charge limit in the application bulletin. Above that, a heater is required. Copeland calls out long line set systems, where added charge pushes the total over the limit.
• Three-phase scrolls: required when the system charge exceeds the compressor's charge limit and an accumulator can't be piped to drain freely during the off cycle.
• Larger commercial scrolls have their own charge limits in their bulletins.
• Reciprocating compressors commonly need a heater or a pump-down cycle above much lower charges; check the bulletin.

Some outdoor units ship with a heater, some offer it as an accessory. Equipment makers' long line set instructions may call for one; check them. See mini-split line set length and charge for the same idea on ductless.

Find the charge limit for the exact compressor model in Copeland's application bulletin or the Copeland Mobile app, and add the line set charge to the factory charge.

Types of heater

• Belly-band (strap-on): wraps around the shell, low and near the oil sump. Copeland's bulletins say to mount it as low on the shell as possible, below the lower bearing pin welds.
• Insertion (well) heaters on some compressors.
• Self-regulating (PTC) heaters that lower their output as they warm.
• Board-controlled heaters that the outdoor control turns on only in cold ambient or during the off cycle. Some contactors and controls have dedicated crankcase heater terminals.

Testing a crankcase heater

• Is it energized? Many heaters are wired to line voltage through the contactor's normally closed contacts or a control, so they run when the compressor is off. Check for voltage at the heater leads with the compressor off.
• Continuity/resistance: with power off and leads disconnected, an open reading means a failed heater.
• Amps: clamp the heater lead; a working heater draws current roughly matching its wattage at line voltage.
• Feel: after a few hours off, the bottom of the compressor shell should be noticeably warmer than ambient.
• Ground: check each lead to ground. A grounded heater can trip the breaker and get blamed on the compressor. See AC breaker keeps tripping.

Copeland notes crankcase heaters must be properly grounded.

Startup after a long off period

Where a heater is fitted, Copeland recommends turning it on for at least 12 hours before starting the compressor. On a new install or after the disconnect has been off for days in cold weather, power the unit (thermostat off) and let the heater work before calling for cooling or heat.

A hard start kit won't fix a compressor that's stalling on liquid. HVAC School warns against adding one to a compressor that's stalling from liquid migration; fix the migration instead.

FAQ

Sources

• Copeland AE4-1312: Application guidelines for 1.5 to 6.75 ton Copeland Scroll compressors (crankcase heat)
• Copeland: Crankcase heaters for scroll compressors (technical information)
• Copeland AE4-1374: three-phase crankcase heat and heater location
• HVAC School: Field guide to crankcase heaters