The next step is to release the test nitrogen and connect a vacuum pump to the system. The vacuum pump is used to remove essentially all gases and moisture from the refrigeration tubing. The vacuum pump typically connects to the refrigeration service valves on the outdoor unit. It is operated until the vacuum level inside the tubing drops to 50 microns. One micron is the very small absolute pressure needed to raise a column of mercury 1/1000th of a millimeter in height.
The outdoor unit of modern split system heat pumps is factory charged with refrigerant. The amount of factory-charged refrigerant corresponds to some maximum allowed length for the refrigeration tubing set between the indoor and outdoor units. Maximum allowed lengths in the range of 15 to 30 feet are common. If the refrigeration tubing set is longer, more refrigerant must be added to the system.
Once the refrigerant tubing has been brought to the required vacuum, the service valves on the outdoor unit are opened to allow the factory-charged refrigerant to fill the remainder of the refrigeration system components. The refrigeration system is now ready to operate.
Currently, the most commonly used refrigerant in residential heat pumps is R410a. Speculation exists that R410a will eventually be replaced by other refrigerants that have less effect on the atmosphere (e.g., lower global warming potential). These include carbon dioxide (CO2), propane and other hybrid mixtures. Given the amount of currently installed systems operating on R410a, it’s very likely that at least some future refrigerants will allow “drop-in” replacement for R410a, if and when as it is phased out.
Split system heat pumps have the advantage that no water or water-based antifreeze solutions are used in the outdoor portions of the system. There is nothing in the outdoor portion of the system that can freeze. All components containing water are housed in the indoor unit.