Residential heat pump systems use refrigerant for heating and cooling. During the cooling cycle refrigerant flows from the compressor through the condensing coil and to the evaporator coil as a liquid in the small copper line. In the evaporator coil, the refrigerant boils causing the coil to cool. The refrigerant then flows back to the compressor as a gas. In the heating cycle a valve, in the condensing unit, reverses the direction of the refrigerant flow. A heat pump uses a second stage electric heat strip in extremely cold temperatures.
Turn the heat pump off at the thermostat.
Remove the service valve caps from both of the refrigerant lines. The service valves connect the outside condensing unit to the refrigerant lines that lead to the inside air handler.
Connect the low pressure gauge hose, found on the left side of a set of refrigerant manifold gauges, to the service valve found on the large refrigerant line.
Connect the high pressure gauge hose, found on the right side of the manifold, to the service valve found on the small refrigerant line.
Connect the middle hose of the manifold to a jug of refrigerant. Open the jug's valve.
Measure the temperature of the air inside of the house, called the ambient air, with a thermometer.
Find the ambient air temperature on a refrigerant pressure temperature chart. Subtract 20 degrees from the ambient air and find the corresponding refrigerant pressure. (see reference 3 for a PT chart)
Turn the thermostat to cool.
Check the manifold gauges suction side pressure. If the pressure measures within 4 pounds per square inch gauge (psig) of ideal then do not add refrigerant.
Turn the handle on the low side of the manifold clockwise to open the valve. This charges the system. Leave the valve open for 60 seconds. Then close the valve.
Allow the system refrigerant pressures to equalize after charging. This takes 30 to 60 seconds. Recheck the gauges. Add more refrigerant if needed.
Disconnect the manifold hoses from the service valves once the system has charged to within an acceptable level (+/- 4 psig). Replace the service valve caps.