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Crankcase Heaters and Compressors | HVAC Refrigeration . Hello, I have a question about crankcase heaters. I am aware that they keep the refrigerant from saturating in the compressor when not running. However, if the refrigerant is R-22, it becomes saturated at approximately 40 degrees. We know the a/c is only operating in warm ambient temperatures. When it is off for the evening hours & the ambient temps stay above 40 degrees, why would one need to have a crank case heater? Why do we need it on during this temperature range, or are crankcase heaters designed to be activated only during the colder months? Also, if one installs unregulated crank case heaters (always operating), can this be a negative factor for the compressor life?

Crankcase Heaters and CompressorsCrankcase Heaters and Compressors

Question Asked by Steven Snyder
Some people reading this are scratching their heads and wondering what we are talking about. Most people have no idea what a crankcase heater is? This is a technical question that a student or first year HVAC technician would ask.

Crank case heaters can be found on all kinds of HVAC compressors. Especially HVAC compressors that are expected to be in service during low ambient conditions. Like heat pumps and even some air conditioners.  I’ll take the questions about Crankcase Heaters and Compressors and answer them separately. This assumes that you have a very good understanding of refrigeration and electrical components for HVAC. This will include understanding compressor contactors and basic relays and switches used in HVAC. The way you asked the question it looks like you may have that knowledge. Of course, a lot of the information mentioned can be found on this site.

Crankcase Heaters and Compressors - Winter Operation

  • are crankcase heaters designed to be activated only during the colder months? This depends on the type of systems you are talking about. I’ll describe the different types. Modern chillers that utilize advanced microprocessor controls control the crankcase heaters automatically when the temperature of the sensor (thermistor) falls below a certain temperature. The temperature setting is manually determined by the manufacturer, the customer, or the chiller start-up technician. This is done when the chiller is brand new and first started. Many chillers have a pump down cycle specifically to eliminate any refrigerant in the compressor. When the chiller is satisfied it goes through a shutdown sequence. This includes purging the refrigerant from the compressor so it doesn’t become saturated and cause problems with liquid slugging. Refrigerants will migrate to the compressor when the right conditions exist. A pump down solenoid/valve will prevent the refrigerant from migrating to the crank case of the compressor. Even with the pump down cycle some chillers still have crank case heaters.

Residential Condensers

  • On the other end of the spectrum, we have the standard residential condenser. It is not as sophisticated as the microprocessor controlled chiller. In the residential condenser, the crankcase heater typically stays on 24/7. Some others have a clicks-on switch that closes when the temperatures falls below the clicks-on set point. The clicks-on activates the crank case heater protecting the compressor from liquid slugging problems. You will notice some air conditioners with a clicks-on on the suction line. If you follow this circuit out on the ladder or pictorial diagram you will find it activates a relay or contactor. This is for the crank case heater on the compressor.

Refrigerant and Oil

Crankcase Heaters and Compressors

This chiller compressor operates in low ambient conditions and utilizes a crankcase heater for compressor protection

  • I think your question is alluding to the fact that you are missing a key in the knowledge of compressors and crank case heaters. The crank case heater not only keeps the refrigerant warm and above the refrigerant boiling point it also keeps the oil in the compressor warm. This is so any refrigerant trapped in the oil can boil off. As the oil migrates through an air conditioner or heat pump refrigeration circuit it absorbs refrigerant. The oil eventually finds its way back to the compressor. In the off cycle, if the temperature falls below the saturation point of the refrigerant (Whether it is R-22 or other) liquid will form inside the crankcase. This is what we are trying to avoid. The crankcase heater keeps the oil above the saturation point of the refrigerant otherwise, there are problems with liquid slugging. Additionally, migration of the refrigerant from a warm place to cooler place needs to be addressed. In the winter the house will be warm or warmer than the outside where the condenser is located. We want to prevent the migration of refrigerant to the condenser. We do this with a crankcase heater or a solenoid valve located in the liquid line. In addition to all that an accumulator, in the case of heat pumps and systems designed to run in low-ambient conditions, helps prevent liquid slugging of the compressor.

Seasonal On or Off?

  • Finally, your last question. If one installs an unregulated crankcase heater (always operating), can this be a negative factor for the compressor life? This has already been answered above. Some low SEER HVAC condensers keep the crank case heater on 24/7 no matter the ambient temperature. Or any temperature for that matter even when it is very hot outside. You will notice in these units that there is a single pole contactor. Ever ask yourself why these units have a single pole contactor? One big reason is because the circuit for the crankcase heater is permanently on. This is done with a single pole contactor through the non-switchable side of the contactor. It is important to note that if you size the crankcase heater improperly the heater may be too hot for the oil inside the crank case. That can lead to premature failure of the compressor. It is best to follow all manufacturer’s guidelines when installing a crankcase heater on any compressor. Additionally, if you work on an HVAC system where the compressor has a crankcase heater be careful. An active crank case heater will cause burns to your skin.

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Do you have a heat pump instead of an air conditioner? See our comprehensive heat pump category for help with heat pumps:

Furthermore, additional heat pump resources here:

Related Links: Learn more about Control Circuits for Air Conditioning & Heating (opens in a new window)

I hope that answers your questions about Crankcase Heaters and Compressors Steven.

High Performance HVAC

Crank Case Heaters and Compressors

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