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N°1 – Frost protection of hot water heating coils – PB to HSC Level

In our European climate the 1st hot water heating coil of an air handling unit with fresh air inlets has to be protected by a frost protection thermostat.
This thermostat is activated when it detects temperatures lower than approx. + 5 [°C] (41 °F).


What other solutions can replace the installation of a frost protection thermostat on the 1st hot water heating coil of an air handling unit with fresh air inlets?

It would be possible to use glycols in the water of the heating system.
We could also install an electric frost protection heating coil for pre-heating the fresh air input.

As a general rule, in the case of risk of freezing, the frost protection thermostat:
– Stop 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

The frost protection thermostat (FPT) contains a capillary device whereby the expansion or fluid phase change can manipulate a contact for activating different protection or alarm functions.


Frost protection thermostats (FPT) sometimes have manual resets.
What is the use of fitting a manual reset frost protection thermostat?

The installation of a manual reset thermostat requires the visit of a technician to reset it (after removing, if necessary, the cause of the activation). This constitutes a guarantee that proper verification of the malfunction is carried out.


Give some examples of hot water system malfunctioning which could lead to the activation of the FPT of a hot water coil in an air handling unit.

We could imagine the following cases :
– The boiler breaks down
– The system pump stops
– A shut-off valve has been inadvertently closed
– The system has lost part of its water
– The air venting has not been properly carried out
– The temperature of the heating system water has been set at an insufficient level


In the air handling unit below, draw, with dotted lines, the electric connections between the FPT and the equipment it controls, on/off switches, closing or opening, starting up or shutting down.