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efficiency/capacity loss using fan cycling control

I'm working with a 3.5 tonne split unit needing to run year round with outdoor ambient temps going down to 32F or lower. There is a fan cycling control installed to maintain head pressure as well as a crankcase heater. This is for a computer server room and according to my calculations the heat load is around 50% of total capacity based on room size and power consumption.

When outdoor ambient temps are around 70F and above, all is well. The room is able to reach and maintain 71F with apparent consistent compressor cycling patterns (there's a temperature recording device in the room).

However, when outdoor temps drop, even a few degrees, the system is no longer able to maintain the temp and it appears the compressor is now constantly running; or at least is no longer cycling with any regularity/apparent pattern.

When outside ambient returns to 70F and above, operation returns to normal.

Is this the expected operation when using a fan cycling control to maintain head pressure?


  • Eugene Silberstein_2Eugene Silberstein_2 Member Posts: 349
    Very strange....

    When the outside temperature drops below 70 degrees, you say that the compressor runs constantly and the system is not able to maintain the desired temperature in the computer room.

    While the compressor is running constantly, what is the condenser fan motor doing?

    As the outside ambient temperature drops, the system becomes more efficient and should be able to maintain the desired computer room temperature much easier.

    It sounds more like a fan control issue than anything else.

    Just keep in mind... As the outside temperature drops, the rate of heat gain on the space decreases and the rate at which the air conditioning system can reject heat increases. So this definitely sounds like a control issue...
  • TechmanTechman Member Posts: 2,144
    Fan cycle

    What were your hi side readings ? Did you have a full liquid line at the metering device during the fan cycling?
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