Troubleshooting the System Charge

The system refrigerant charge is either over or under charged if it isn’t correctly charged.
For a system undercharge, depending on the severity, you will have most or all the following symptoms:
1.      The saturated suction temperature is less than expected after converting the suction pressure to its saturated temperature with the PT chart. For instance, common coil temperature for A/C would be 45 degrees F. On a system using R-22, your gauge reads 50 psig, you’re only running a 36 F coil.
2.      Your system has a high suction superheat. After measuring the suction line surface temperature to be about 58 degrees, subtract the saturation temperature found in symptom one. In this example 58 – 36 = 22 degrees of superheat. Too high for a low saturated temperature.
3.      Low liquid sub-cooling. Find the saturation temperature of the condenser pressure and subtract that number from the actual liquid line temperature. Less than 5 is low, could be zero.
4.      Low saturated condenser temperature. Its 85 F outside so you add 30 degrees to get 115 F. Looking up that pressure on the PT chart you read 242 psig, but your gauge reads 190 psig, which is 98 degrees F, low.
5.      Low system capacity. Unit won’t cool the room or takes a very long time to do it. Perhaps only works at night.
6.      High compressor discharge superheat. Measuring the discharge line about 6 inches from the compressor you see 138 F, subtracting the saturated condensing temperature found above, 138 – 98 = 40 F. That’s above the 35 recommended.
7.      Compressor body is warm, not sweating, or even hot.
8.      The system could be cycling on the over temperature protection or low pressure cut-out.
9.      Oil may smell burnt if running in this condition long enough.
10.   Evaporator frosting or ice on the suction line for a very low charge.
Symptoms of overcharge, depending on the severity, you could have most or all the following:
1.      High saturated condensing temperature. When you converted your condensing pressure to temperature it was 40 degrees higher than the ambient. Too high.
2.      High saturated suction temperature. You were expecting a 45-degree coil, but after converting the pressure with a PT chart you find its running 50 degrees.
3.      Low system capacity, here again the unit will not do the job because over or under charge, the system doesn’t function correctly.
4.      Compressor amps will be high. This is usually from liquid getting back to the compressor.
5.      Liquid sub-cooling (the actual liquid line temperature minus the condenser saturation temperature) will be greater than 15 degrees.
6.      Low suction superheat. More noticeable on a system with a fixed metering device it will also cause a drop for a TXV if its severe enough.
7.      Compressor may be noisy and vibrate.
8.      Compressor starts with a lunge due to partial seizing caused by liquid damage and oil loss.
The above numbers were for example purposes and not from a real unit although the limits mentioned are good practice.