Room thermostat

I have a Drayton mechanical room thermostat and I want to replace it with a programmable digital thermostat. The Drayton had three wires connected to its three terminals numbered 1,2,3. My CH system has a boiler indirect I think a HW CH programmer and a three port motorised valve that seems to be controlled by the room thermostat. the problem is that the Drayton seems to have seized up and will not cut off the heating . that's why i want to replace it. The new one I've bought has also three holes named NO,COM,NC. What wires do i connect to what terminal? Please help
I do know that the voltage between 1 and 3 is 230 V and also between 2and 3 is also 230 V.
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I can't answer the question directly, but can give you some pointers.
NO = Normally open NC = Normally closed COM = Common
This is a changeover switch in which, at low temperatures, NC is connected to COM. When the set temperature is reached, it changes over so as to connect NO to COM instead.
For your application, you only need 2 wires: * A live feed coming from the programmer when CH is on, and connected to COM. * A switched live, between NC and the motorised valve
Then, when the room gets up to temperature, the contacts will open, and the demand to the valve - on onwards to the boiler and pump - will be removed.
My guess is that the third wire going to the original stat is probably a Neutral - and the stat probably has a small internal heating element to increase its sensitivity. You don't need this wire in the new stat and MUST NOT connect it to the spare terminal - otherwise you'll have a dead short when the contacts change over.
The only hard bit is to work out which is which of the original 3 wires! Maybe someone here has experience of the Drayton stat and can tell you. Otherwise, you'll have to trace them to their sources and destinations with a continuity tester - with the power OFF of course!
Roger
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Add in the symbol:
N/C = No Connection and can be used as a loop through terminal.
Not to be mistaken with NC = Normally Closed.

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