It’s simple. No one wants to be wrong.
Defrosting is a necessary evil, literally warming up the equipment designed to chill the air.
The punishment for under defrosting is painful. The evaporator stops working, and the ice that is built up needs to be removed, sometimes manually. It can also cause damage to the evaporator.
The punishment for over defrosting is latent, or hidden. It shows up in the power bill.
When an operator is faced with these two options, the choice is clear. Program the system so that under defrosting is avoided.
New Systems Over-Defrost
Over-Defrosting also occurs when commissioning a new system. The refrigeration system is used to control the “pull down” of the building, which mainly involves making sure that the moisture in the area and concrete floors has been removed prior to freezing the space. Here is the recommendation from IIAR:
Phase II–Hold at 35°F for concrete cure
35°F shall be maintained in rooms that will operate below freezing until the concrete contractor or engineer verifies that the concrete has cured and it is safe to proceed with Phase III. In lieu of concrete curing verification, the following can be used to indicate that the concrete has cured:
• Verification that doors have remained closed and other sources of moisture have not been present, and
• Room humidity has stabilized, and
• The evaporator unit(s) have cycled through at least two (2) defrost cycles and the evaporator coils have remained dry and reasonably free of frost for at least 24 hours.
During Initial Commissioning of refrigeration systems, operators are encouraged to over defrost in order to keep the construction schedule on time. Sometimes those settings stay.