Things You'll Need
Manifold gauge set
Electronic vacuum gauge
Container of nitrogen
Pulling a vacuum on a heat pump is essential whenever the sealed refrigerant system is opened or exposed to moisture or other contaminates. These impurities mix with the oil and refrigerant and cause an acid to form that deteriorates the components of the system. The moisture can also form ice at the point of restriction, blocking the refrigerant flow.
Locate the service valves and screw on the manifold gauge hoses. Connect the blue hose to the valve on the suction line, which is the larger tube leading to the compressor. Connect the red hose to the valve on the liquid line, which is the smaller tube leading from the condenser toward the evaporator.
Connect the electronic vacuum gauge to the suction port on the vacuum pump and screw the yellow hose of the manifold gauge set onto the vacuum gauge. Open the valves on the manifold gauge and turn the vacuum pump on.
Run the vacuum pump until the electronic vacuum gauge is registering about 500 microns. Shut the valves on the manifold off, and then turn off the vacuum pump. Turning the vacuum pump off before shutting the manifold valves will cause the the heat pump to suck the oil out of the vacuum pump, contaminating the system.
Disconnect the yellow hose from the electronic vacuum gauge and connect it to the nitrogen bottle. Open the valve on the nitrogen, then open the manifold valves to break the vacuum. Charge the unit to about five to 10 pounds of pressure. Shut the manifold valves.
Remove the yellow hose from the nitrogen bottle. Open the manifold valves and bleed the nitrogen off until the pressure is completely out. Releasing the nitrogen carries more of the moisture and contaminates out of the system. Screw the yellow hose back on the electronic vacuum gauge and turn the vacuum pump back on.
Repeat the step of evacuating the system to 500 microns. Turn the valves off and turn the vacuum pump off. Wait 10 to 20 minutes to see whether the unit holds the 500 microns. If the microns rise a small amount, that indicates more moisture in the system. Rising continuously indicates a leak.
If no signs of a leak or moisture are seen, remove the yellow hose from the electronic vacuum gauge and hook it to the refrigerant drum. Open the valve on the refrigerant drum. Open the manifold valves and break the vacuum with refrigerant. Turn the heat pump on and charge the heat pump.
Check the oil in the vacuum pump on every use, and change the oil often because the oil absorbs moisture and contaminants pulled out of the refrigerant system.