Suction Pressure Ranges
Low Range: Any system suction pressure that is below 55PSIG
Normal Range: Any gauge pressure between 56PSIG and 80PSIG*
• Some high effiicency residential products may have suction pressures that exceed
• 80PSIG Use the Manufacturer’s superheat charging chart to verify the correct
• charge level. Remember : Pressure is developed by the evaporator load!
• If the load is high, the pressure will be high.
High Range: Any suction pressure that is above 80PSIG
High Side Pressure Ranges
Low Range: Any high side pressure below 175 PSIG.
Normal Range: Any high side pressure between 176PSIG and 300PSIG*
High Range: Any high side pressure above 300PSIG.
Most residential and light commercial air conditioning systems have a high
side pressure approximately three to four times higher than the suction pressure.
Example Low Side = 70PSIG High Side = 210PSIG – 280PSIG
If the system is operating properly, these systems should exhibit normal
Subcooling and superheat levels.
Low Evaporator Load
Low system pressure, low superheat, and low subcooling indicate that the evaporator
Is doing all of the Latent Heat Transfer. There is not enough load to raise the suction
Pressure. With suction pressure low, the vapor density is low, causing low head pressure
And low Subcooling level.
Restricted Evaporator Circuit
A Restricted Evaporator Distributor Tube causes low suction pressure and high superheat. The high side pressure rises when the technician adds refrigerant to the system. Excessive charge causes greater Subcooling. This problem can be mistaken for a restricted metering device.
Restricted or Undersized Liquid Line
Low system pressure, high superheat, and high subcooling indicate that the evaporator is starving for refrigerant. High Subcooling indicates liquid storing in the condenser. The system has a significant temperature drop between the condenser outlet and the metering device inlet. Flash gas is likely to be present at the metering device.
Low System Level Charge
Low system pressure, high superheat, and low Subcooling indicate that both the evaporator and condenser are starved of refrigerant.
Restricted or Undersized Suction Line
Low system pressure, and temperature drop from the evaporator outlet to the compressor inlet that exceeds 4 degrees , causes both pressures to be low indicates a high resistance to vapor flow which should be corrected.
Restricted Suction Line Drier
Low system pressure and a temperature drop exceeding 3 degrees across the “suction-line” drier indicates that the drier is restricted.
Restricted Metering Device
Low system pressures, high superheat and normal to high Subcooling indicate a starved evaporator and flooded condenser.
Restricted Liquid Line Drier
Low suction pressure,high head pressure, high superheat and Subcooling indicate the condenser is flooded. High superheat and low suction pressure indicate a starved evaporator.
Restriction in Condenser
Low suction pressure and high head pressure, indicate that the refrigerant is stored in the condenser. High superheat shows the evaporator is starved, and high Subcooling indicates a flooded condenser. The key is the liquid line being colder than the ambient temperature.
High Evaporator Load
The superheat is high also suction and high side pressures are high. System is under high load conditions.
High System Charge Level
High suction and high side pressure , low superheat(flooded evaporator) and high subcooling (flooded condenser).
Bad Compressor Valves
High suction pressure and low high side pressure indicates bad compressor valves
High high side pressures and normal to high low side pressures indicate non-condensables (air, moisture) in the system.
Procedure for Superheat
1. Measure the suction pressure at compressor inlet
By measuring the suction pressure, you can
Find the evaporator saturation temperature.
2. Convert suction pressure to saturation temperature.
3. Using a digital thermometer, measure suction line 6 inches from compressor
4. Subtract temperature of evaporator from suction line temperature.
This is your Superheat.
Example: Suction pressure = 68PSIG R-22 Saturation temperature = 40*F
Suction line temperature = 50* F
The suction line is 10*F warmer than saturation temperature of evaporator.
Superheat Level is: 10*F
Procedure for Subcooling
1. Measure the high side pressure at the liquid line service valve
By measuring the high side pressure you can find the condenser
2.Convert high side pressure to temperature.
3. Measure the temperature of the liquid line at condenser outlet
4. Subtract liquid line temperature from the saturation temperature
Example: High Side pressure = 225PSIG R-22Saturation temperature =110*F
Liquid line temperature = 90*F
The liquid line is 20*F colder than the saturation temperature of the condenser.
Subcooling Level is: 20*F