by Ron Walker
Non-Condensables
One of the more difficult AC system problems to diagnose are non-condensables in the system. Non-condensables are always caused by improper system evacuation during an installation or repair. These poor procedures leave air, nitrogen, or moisture in the system.
Non-condensables occupy condenser coil space that is normally used to condense refrigerants. Because of this wasted condenser space, the proper amount of heat cannot be rejected, causing a rise in condenser temperatures/pressures, higher compression ratios, and system inefficiencies.
A system with non-condensables will have poor cooling and high humidity and will appear to be operating normally, making identification of this problem difficult. If a system has subcooling, superheat, and system pressures/temperatures that simply don’t make sense, suspect that non-condensables are in the system.
Refrigeration Principles at Work
Pure refrigerants inside a container have a saturation temperature that is equal to the ambient temperature surrounding the container. For example, a bottle of R-410a refrigerant sitting in an 80°F room. Once the bottle equalizes at 80°F and all of the heat transfer is complete, it will have a pressure of 235.8 PSI. (Refer to your pressure temperature (PT) chart.)
Checking for non-condensables requires that all of the refrigerant be pumped into the condensing coil. The heat in the condenser must be rejected so the air leaving the condenser is the same temperature as the entering air. Once the leaving and entering air temperatures are equal, the pressure inside the condenser should correspond to saturation temperature equal to the outside air. If the pressure is too high, this indicates non-condensables!
The Procedure (Note - You must have a discharge line pressure port must be available for use.)
These symptoms and procedures also pertain to a system with mixed refrigerants.
Non-Condensables
One of the more difficult AC system problems to diagnose are non-condensables in the system. Non-condensables are always caused by improper system evacuation during an installation or repair. These poor procedures leave air, nitrogen, or moisture in the system.
Non-condensables occupy condenser coil space that is normally used to condense refrigerants. Because of this wasted condenser space, the proper amount of heat cannot be rejected, causing a rise in condenser temperatures/pressures, higher compression ratios, and system inefficiencies.
A system with non-condensables will have poor cooling and high humidity and will appear to be operating normally, making identification of this problem difficult. If a system has subcooling, superheat, and system pressures/temperatures that simply don’t make sense, suspect that non-condensables are in the system.
Refrigeration Principles at Work
Pure refrigerants inside a container have a saturation temperature that is equal to the ambient temperature surrounding the container. For example, a bottle of R-410a refrigerant sitting in an 80°F room. Once the bottle equalizes at 80°F and all of the heat transfer is complete, it will have a pressure of 235.8 PSI. (Refer to your pressure temperature (PT) chart.)
Checking for non-condensables requires that all of the refrigerant be pumped into the condensing coil. The heat in the condenser must be rejected so the air leaving the condenser is the same temperature as the entering air. Once the leaving and entering air temperatures are equal, the pressure inside the condenser should correspond to saturation temperature equal to the outside air. If the pressure is too high, this indicates non-condensables!
The Procedure (Note - You must have a discharge line pressure port must be available for use.)
- With the compressor running, close the liquid line service valve. When the suction side gauge reaches 0 PSIG, disconnect power from the system.
- Make sure there is no frost just past the liquid line service valve. If there is frost, tighten the service valve. If the frost continues, the valve must be replaced.
- Make sure the system power is off!
- Disconnect the compressor power leads from the contactor.
- Measure and record the outdoor air temperature.
- Reconnect power to the unit and run the condenser fan until the leaving air temperature is equal to the outdoor temperature.
- When the temperatures are equal, read the system’s high side pressure.
- Using your PT chart, compare the saturation temperature of the condenser to the outdoor air temperature. They should be very close, or equal.
These symptoms and procedures also pertain to a system with mixed refrigerants.