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First Stage of
protection:
The first stage consists of an Out-side frost thermostat located in a
weather proof box, and situated on a north wall so as not to be subject to
sunshine or indeed any extraneous heat sources (i.e. do not locate it over
boiler-house door or external vents etc).
The function of this thermostat is to override automatic control time
functions on all circulating pumps when the outside air temperature fall
below 3ºC.
So, the following pump circuits will
start:
1-
Constant temperature pumps serving air-handling plant, heat exchangers etc.
2-
Radiator Circuit pumps.
3-Chilled
water pumps.
4-Any
other circuit which may be considered at risk of freezing.
In some situations
cooling and heating valves on AHU’s will need to
drive open.
Note: Circulating water will be less inclined to freeze as the heat exchange
surfaces will draw heat from the building back into the circulating water
(provided the building internally is not too cold).
Second stage of
protection:
An immersion thermostat located in the return
water header to the boiler, will override the automatic time control and
start the burner on sensing a return water temperature of 5ºC. This will heat
the return water to approx 7/8ºC. before switching
off again.
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