Heating and Air Conditioning

Figure 19: Pressure-Temperature Relationship Conditions Equivalent To 30 MPH Or 1750 Engine RPM:

The system should be operated for at least 15 minutes to allow sufficient time for all parts to become completely stabilized. Determine if the system is fully charged by the use of test gauges and sight glass if one is installed on system. Head pressure will read from 180 psi to 220 psi or higher, depending upon ambient temperature and the type of unit being tested. The sight glass should be free of bubbles. Low side pressures should read approximately 15-30 psi, depending on the ambient temperature and the unit being tested. The type of control and component installation used on a particular system will directly influence the pressure readings on the high and low sides, Fig. 18 .


The high side pressure will be affected by the ambient or outside air temperature. A system that is operating normally will indicate a high side gauge reading between 150-170 psi with an 80° F ambient temperature. The same system will register 210-230 psi with an ambient temperature of 100° F. No two systems will register exactly the same, therefore, allowance for variations in head pressures must be considered. Refer to Fig. 19 for the most important normal readings likely to be encountered during the season.

Ambient Temp. High Side Pressure
80 150-170
90 175-195
95 185-205
100 210-230
105 230-250
110 250-270