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Wiring new cylinder thermostat

Discussion in 'Plumbing and Central Heating' started by StrikeThree, 20 Nov 2011.

  1. StrikeThree

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    I am replacing a cylinder thermostat in a central heating system and would appreciate any wiring advice.
    The old thermostat is a Satchwell. It is very old, settings in Fahrenheit. It has three connections. One wire is yellow and black sleeved and is connected to the metal thermostat housing (so must be earth). The other two wires are coloured red and blue. Their terminals both go to 240V (wrt earth) when heating is on, both are at 0V when heating off.
    The new thermostat is a Horstmann HCT2. It has three terminals, com, 1 (call for heat), and 2 (satisfied). Given that the new thermostat doesn't have an earth terminal, can I use the three wires from the old Satchwell? If so, how would they be connected?
     
  2. gaswizzard

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    You only need com & 1 , i take it you have an S plan?
     
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  3. StrikeThree

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    Thanks for the reply.
    I'm not really sure, but I think it's an S plan. There's a motorised valve with water in and water out near the cylinder, and another of the same type near the boiler.
    So I should forget about the present earth wire and just use the red and blue to connect to com and 1 on the new thermostat. It doesn't matter whether red goes to com or 1. Is that correct?
     
  4. D_Hailsham

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    You don't need an earth wire, so make it safe at both ends

    Although it doesn't matter in practice which way round the wires are, it's better to have them the correct way round as it makes life easier if you have to fault-find.

    One wire will have 240V on it when you turn HW on at the timer; this should go to the Common terminal. The other wire connects to 1.
     
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  5. StrikeThree

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    Thanks for the help.
    Is it likely that the CH system, or part of it, is getting its earth from the hot water cylinder via the wire to the chassis of the cylinder thermostat that I am about to replace?
     
  6. D_Hailsham

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    Very unlikely. The earth wire is only there for the thermostat and the new stat does not need an earth. In any case the cylinder will be earthed via the copper pipes back to the boiler and any other component which has an earth wire, e.g the pump.
     
  7. StrikeThree

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    Many thanks.
    I've got the information I need now to make the replacement.
     

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