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N°2 – Installation of capillary devices for frost protection thermostats – PB to HSC

Protection of air handling units N°2 – Installation of capillary devices for frost protection thermostats – PB to HSC

The capillary device of frost protection thermostats must cover all the surface area of the heating coil.



Question  

What would be the best position of the FPT capillary:
– at the fresh air inlet side of the coil?
– at the heated air outlet?
Justify if possible your choice.

The FPT can only be installed after the 1st heating coil. We will study the reason for this in the

next question.




Question  

Explain why the FPT set at approx. + 5 [°C] (41°F) cannot be installed in front of the heating coil.
Explain why, even when installed after the heating coil, it still ensures anti-frost protection.

The FPT cannot be installed in front of the heating coil because, being cooled by the cool air intake, still not heated,  it would risk stopping the AHU even though the coil is properly fed with hot water and there is no risk of freezing.
The AHU must be able to operate whatever the outside temperature.
When installed after the heating coil, the FPT simply registers the proper working of the heating coil.
If in extreme cold conditions the temperature of the exhaust air from the coil is higher than 5 [°C] (41 °F), it means it is well supplied with hot water and there is no risk of freezing.



Question  

Indicate the usual functions controlled by the FPT in the case of risk of freezing?

As a general rule, in the case of risk of freezing, the frost protection thermostat:
– Stops the fan blower motor
– Stops the air return fan motor
– Closes the fresh air inlets and exhaust air outlets
– Activates the opening of the hot water coil regulation valve
– Sets off an alarm

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