When working on a refrigeration system that uses a TXV you normally have a liquid receiver. This liquid receiver stores enough extra refrigerants to properly feed the TXV under any load condition. Charging this unit to a clear sight glass in most cases will ensure that you have added enough extra refrigerant to the receiver to properly feed the TXV. If a flooded head pressure control valve is being used, enough additional refrigerant must be added after filling the sight glass to ensure proper condenser flooding during low ambient conditions. The condensing unit manufacturer will normally tell you how much additional refrigerant this will require. Not adding this extra refrigerant may starve the TXV during low ambient conditions even if the unit worked fine while ambient temperatures were warmer.
A/C and heat pump condensing units don’t normally have liquid receivers. The extra refrigerant required to feed the TXV under heavy load conditions must be stored in the condenser coil itself. If a sight glass is used to charge one of these units the sight glass will only show that liquid refrigerant is leaving the condenser at the time of charging. As the load increases and the TXV throttles open, we may not have enough refrigerant stored in the condenser and a starved TXV will be the result. A lot of time may be spent looking for a leak that doesn’t exist. In the heating mode, we may not get the heat output the unit is rated for. The best way to charge a heat pump is by weighing the charge or by subcooling for A/C systems.
Subcooling is the amount of temperature below the saturated condensing temperature. The condenser contains mostly saturated vapor. An R22 A/C system with 226# of head pressure will have a condensing temperature of 110°F. The majority of the condenser will contain 110°F saturated vapor/liquid. Once all of the refrigerant has condensed to 110°F liquid, the refrigerant temperature will begin to fall below the SCT of 110°F. If the liquid line temperature is 105°F the system has 5 degrees of subcooling. The more liquid that is stored in the condenser, the more it will cool below the 110°F SCT. Higher subcooling indicates more liquid stored in the condenser. Lower subcooling indicates less liquid refrigerant in the condenser. Adding refrigerant will increase subcooling while removing refrigerant will reduce subcooling. The equipment model tag will normally indicate the required subcooling a system should be charged to. When a system is charged to the proper amount of subcooling, enough refrigerant is now stored in the condenser to properly feed the TXV under all load conditions. This method of charging is normally used in the cooling mode under normal operating conditions.
Weighing in the charge is the best way to charge a heat pump, especially when it needs to be charged in the heating mode. Most heat pumps use fixed metering devices for the heating mode so subcooling charging is not normally possible. Most heat pumps come pre-charged for a small amount of liquid line. The manufacturer will give a multiplier per-foot to determine how much extra charge needs to be added or removed. More refrigerant is added for longer line length while some refrigerant may need to be removed for shorter line lengths. Heat pumps are critically charged systems that require accurate charge calculation to be weighed into the system to ensure proper heating operation.