We are in the process of getting estimates to replace our evaporator/blower of a 2 piece unit. When asked about the process out of curiosity the HVAC guy told us since the the system holds pressure and the refrigerant/condensor is new he will just "park" the refrigerant in the compressor, then cut the line to the evaporator, attach the new one and open the the compressor valves to get the system up and running. Is this possible? Can a compressor hold all the refrigerant of an HVAC system?

The Pump Down Method

Yes, this is in fact true and its called pump down. Whenever a new condensing unit is purchased it is pre-charged at the factory with refrigerant for a line set up to 25 feet. This is done all the time by HVAC technicians when they have to open the system and work on the line set or evaporator coil. I have done this many times and it is quite common in HVAC. I would do this the same way as your HVAC guy.

The process of doing this may sound complicated but it is easy for an HVAC technician. There are two valves at the condenser, one valve for the suction line and one valve for the liquid line. The HVAC technician closes the liquid line valve and turns the air conditioning on. The compressor pumps all the refrigerant into the condenser coils. The HVAC technician watches his gages carefully and when the pressure gets to zero on the gages he quickly closes the suction line valve to trap the refrigerant in the condenser and then turn the compressor off. This saves using refrigerant unnessarily because the same refrigerant in the system can be reused for the new evaporator coil or air handler. After the new evaporator coil or air handler is installed the HVAC technician evacuates the new part of the system and when completed with the evacuation process opens the valves to allow the refrigerant into the new part of the system. To make sure the charge is correct he uses his gages and the superheat method or subcooling to ensure the proper amount of refrigerant is in the system.

Commercial Systems use automatic pump down before shutting down to prevent liquid slugging in compressor and oil migration. Photo is pump down valve and pump down solenoid.

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