Most technicians in the HVAC field know the normal range of operation for the low-pressure side of an air conditioning system. This tends to be around 60 PSI to 85 PSI for R-22 and 105 PSI to 143 PSI for R-410A and is dependent upon operating conditions. On the high-pressure side of the system, there is a wide variation in pressure due to the wide outdoor temperature swing and due to the actual SEER (Seasonal Energy Efficiency Ratio) rating of the system being worked on. Because there is a wide variation on the high-pressure side of a system, technicians seem to focus on patterns seen on the low-pressure side of the system and unfortunately some use this as a short cut instead of checking the charge in a proper way. One such pattern is when the outdoor temperature is higher, the pressure measured on the low-pressure side of the system is usually higher. Because this pattern is noticed, it is confused with a charging method and unfortunately, there are technicians out there setting the refrigerant charge level in a system based on a guess of what pressure they think it should be at, during a given outdoor temperature. This couldn't be further from a real method and doing this will eventually cost the technician as well as the system and system owner.
If a system is charged in this manner, at the best, the system may work, and the technician may get lucky and be able to get by for a while. At the worst, the system's compressor may fail, the system may run for a long time at a low capacity, the electrical cost may be higher than it should be to run the unit, the lifespan of the system will be reduced, and after its all said and done, the technician still doesn't know how the system is really supposed to function. If you don't understand how the system really functions, then you have very little chance in troubleshooting a problem when one inevitably occurs. For a technician to grow in the HVAC field, one must know proper charging and troubleshooting methods to quickly and confidently service systems. In this article, we will be focusing on the charging methods.
Up until this point, we focused on pressures. However, we only check pressure in order to convert the pressure to saturated temperature. Remember that refrigerants have a known pressure/temperature correlation when the refrigerant is saturated (Saturated means both liquid and vapor refrigerant are present). While the system is running, the refrigerant is saturated in only two places, the evaporator coil and the condenser coil. We can measure the saturated temperature of the refrigerant in the evaporator (indoor) coil when we measure pressure on the low-pressure side of the system. This is done on the large vapor line. We can measure the saturated temperature of the refrigerant in the condenser (outdoor) coil when we measure pressure on the high-pressure side of the system. This is done on the small liquid line. All we need to do is to convert the low and high side pressures to saturated temperatures using a P/T chart, a P/T chart overlaid on a gauge face, a P/T app, or with a digital manifold gauge set. Below is an example of a P/T chart.
Let's look at the pressure/temperature correlation of R-410A on the P/T chart above. At 118 PSI, the saturated temperature of R-410A is 40°F. At 318.5 PSI, the saturated temperature is 100°F. If you measure 118 PSI on the low-pressure side of the system, then you know that the saturated temperature of the refrigerant flowing through the middle of the evaporator coil is at 40°F. This is a useful number when combined with the temperature of the tubing near the pressure port. The temperature on this tube will give you the temperature of the refrigerant running through the tube. This actual temp on the large vapor tube will be higher than the saturated temperature. The actual line (tube) temperature minus the saturated temperature will give you what's called Total Superheat. This is not only a charging method but also a measurement of how safely the refrigerant is entering the compressor. Below is an example of the total superheat on a running air conditioning system.
