Digital Roomstat

I want to replace my central heating roomstat with a digital unit. My current unit has 4 wires going into it one being an earth. Can anybody recommend a replacement - I have noticed that some only cater for 2 wires.
Thanks
Steve
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wrote:

connect all the wires to it. ..
SJW A.C.S. Ltd.
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On Sat, 21 Feb 2004 11:02:59 -0000, Wooden wrote:

Presumably a normal mechanical room stat. So those 4 wires are L, N, Sw L and E. A mechanical stat has a heater in it to make it more sensitive to temperature changes, hence it needs the N.

In which case they are battery powered and just have a contact that closes when calling for heat. Just connect the L and Sw L to the terminals. The L should only be live when the CH programmer is in ON mode.
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In an earlier contribution to this discussion,

That's precisely right. As far as the operation of your central heating is concerned, all you need is an on/off switch - requiring just a live and switched live connection. The earth was provided because the old stat had metal bits which could become live under fault conditions and the neutral provided a return path for the built-in accelerator heater circuit. Neither of these is required by the new stat.
When I recently replaced my old mechanical stat with a Honeywell CM 67, I did the following: The chosen position for the new stat was slightly different from that of the old stat, so that the old wire wasn't long enough. I therefore terminated the old wire in a junction box inside a cupboard, and took a new wire - with only 2 conductors (connected to the appropriate cores in the old cable) - from the junction box to the stat. Works a treat!
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wrote:

It would be prudent to check what the third wire is currently used for.
It could be connected to a Fully Motorised Zone Valve to close the valve when there is no requirement for room heat
More normal Spring return Zone Valves do not require this.
Michael Chare
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In an earlier contribution to this discussion,

I suppose this is true, if you have a valve which is motored in both directions - in which case you would presumably use the changeover contacts in the stat, and there would also be a "heating not required" feed from the programmer. My impression though is that most valves - whether 2-port or 3-port - have a spring return which operates when the power is removed.
In my case (3-port valve with spring return) the third wire to the old stat was *definitely* a neutral for the accelerator heater. I know because I took the stat to bits after I had removed it!
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On Sat, 21 Feb 2004 14:32:54 +0000 (UTC), "Michael Chare"

Agreed - my Sunvic valves are this type requiring a changeover contact in the room stats. I bought them in about '79 by choice - not having any faith in springs....
Geo
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As suggested I have disconnected the earth & third wire & wired up the new stat - all seems to work ok. I am now concerned about the 3rd wire re: further discussions, however the motorised zone valves are spring returns so I am keeping my fingers crossed :-) Thanks for all the help.
Steve
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My old mechanical stats (sunvic I think) had four wires + earth, which were live in, two changeover live outputs to drive a non spring motorised valve, and finally a "night setback" connection, which enabled a small heater (a resistor) to crudely drop the temperature of switching by 10 deg F. This last was a neutral connection, with the other end of the resistor connected to the live in terminal.
If you have this type look for two resistors, one as normal connected across the main contacts, and the second with its own external terminal at one end.
If this is the case you can ignore and insulate this connection, as your new digistat will do the job far better.
Charles F
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