Do you have a low pressure control installed on the condensing unit?
Then you don't need a thermostat!
Most of the commercial refrigeration units I repaired, never had thermostats.
They maintained the desired case temperature by pressure!
As the refrigeration equipment cools the product, the evaporator pressure slowly decreases.
Remember, lower temperature always translates into lower pressure on properly operating equipment.
Setting the low pressure control to open at a desired pressure also obtains a desired temperature.
Sure, a thermostat is more desirable. But it also means additional electrical work.
Wires must be pulled between the thermostat and the condensing unit.
Sometimes it's not worth the extra trouble or expense.
What are some of the negatives when using a pressure control to maintain the correct temperature?
Anything that affects the pressure affects the product temperature.
Expansion valves must have a minimum operating pressure. They must have sufficient high side pressure to allow the valve to meter correctly.
Suppose you have a dairy display case operating on a low pressure control. All is well and the case temperature stays within acceptable limits.
However, cooler weather arrives and you get a call. "The case is too warm."
You check and find no apparent reason. The equipment is operating perfectly. But the condensing unit doesn't run enough to cool the product.
The condenser fan is moving cooler air across the condenser. Cooler air means lower high side pressure.
Lower high side pressure causes a loss of expansion valve operating pressure and abnormally low evaporator pressure.
Insufficient refrigerant is being metered into the evaporator. Less refrigerant, means lower pressure.
Lower pressure causes the low side pressure control to open.
The result is a warm case.
The condensing unit is off because the evaporator has been starved for refrigerant!
Install a condenser fan cycle control.
A condenser fan cycle control simply cycles the condenser fan on and off with pressure. The fan stays off until enough pressure is available to properly feed the expansion valve.
In cooler weather, the condenser fan only runs when necessary. In warmer weather the fan runs continually.
Would I recommend using pressure for a refrigeration control system?
Only if installing a thermostat is not an option.
However, pressure control of case temperature works great, when properly installed!
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