During the heat pump defrost cycle, the water to the fan coils can temporarily become colder.
Anti-cold-air function
Many fan coil controllers stop the fan when the water temperature drops below a certain limit.
Why it happens
The goal is to avoid the feeling of cold air drafts in the space during defrost.
Pipe sensor
The operation is usually based on a temperature sensor on the coil or the supply line.
Diagnosis
If the fan stops only during defrost and starts again afterwards, this may be normal operation and not a fault.
