Auto A/C Terms - Glossary
High Pressure Relief Valve

High side pressures are typically between 2 and 2 and one half  times the ambient air temperature as expressed in degrees Fahrenheit (degrees F). However, refrigerant overcharge or extreme humidity can cause those pressures to rise excessively, and when that happens, the high pressure relief valve will discharge refrigerant in order to lower pressures.

The most common pressure relief valve is known as a 440 blow off switch. Typically located on the rear head of the compressor, the 440 blow off valve will discharge refrigerant when pressures increase to 440 psi. Those valves were designed to reseal themselves when high side pressures are reduced.

Sometimes pressure relief valves are located on the liquid line receiver drier. They are sometimes referred to as a fuse. Those valves will also release refrigerant to atmosphere when pressures rise too high.