A customer calls to say that the central air-conditioning system in their small office building will not cool. When the service technician arrives, he turned the thermostat to COOL and sets the thermostat to a temperature that is lower than the present temperature in the space. The indoor blower motor starts. This indicates that control voltage is present and will send power on as requested for cooling. Only recirculated air is coming out of the ducts. The technician checks the outside unit, notices that it is not running, and removes the control panel cover to see what controls will actually prevent the unit from starting, which will give some indication of what the possible problem may be. The technician installs pressure gauges to obtain the system pressures. New air-conditioning systems use R-410A. If the system were standing at 80°F, the system should have 235 psig of pressure, according to the temperature/pressure chart for R-410A at 80°F.

Care should be used in the standing pressure comparison. Part of the unit is inside, at 75°F air, and part of the unit is outside. Some of the liquid refrigerant will go to the coolest place if the system valves will allow it to. The gauge reading may correspond to a temperature between the indoor and outdoor temperature but not less than the cooler temperature. Some systems use fixed-bore metering devices that allow the system pressures to equalize and the refrigerant to migrate to the coolest place in the system.

In this example the pressure in the system is 105 psig, the refrigerant is R-410A, and the temperature is about 80°F. A typical pressure control setup may call for the control to interrupt the compressor when the system pressure gets down to 42 psig and to make the control start the compressor when the pressure rises back to a temperature corresponding to 70°F.

Required:
What is the problem and the recommended solution?