Lesson $3$: Automatic Controls $/$ Lesson $3$ Assignments

Lesson $3$ Assignment $2$: Unit $14$ Service Call $4$

total of $40$ points. You have only one chance to submit your assignment for grading.

Service Call $4$

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 turns 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$.$ Use the diagram in Figure $14\mathrm{.}67$ for direction.

The technician installs pressure gauges to obtain the system pressures. New air$-$conditioning systems use R$-410$A$.$ If the system were standing at $80F,$ the system should have $235$ psig of pressure, according to the temperature$/$pressure chart for R$-410$A at $80F.$

Note

Care should be used in the standing pressure comparison. Part of the unit is inside, at $75F$ 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 $80F.$ 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 $70F.$

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