how to replace cylinder thermostat ?

Please can someone just advise me below? While trying to work out what's going wrong with the 3way valve on our central heating, I've been fiddling with the hot water cylinder thermostat a lot, and it's become stiff. (no tittering...)
Is it a simple job to replace it, eg 2 or 3 wires, standard codes?
Current one looks like http://www.toolstation.com/search.html?searchstrG346
and has a single white cable going into a 10-way honeywell connection box in the airing cupboard.
The thermostat cable has 3 wires, red, black & grey.
regards
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wrote:

Think of it as a simple changeover switch...one wire will be connected to the "common" switch contact whilst the other two are connected to the "normally open" and "normally closed" contacts.
Probably best to use a multimeter to work out which is which as there are *possibly* different conventions in use with regards to colour coding.
sponix
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keith wrote:

WD40?
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wrote:

R2D2?
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In an earlier contribution to this discussion,

It doesn't really matter about the knob being stiff to turn - as long as the stat turns on and off at the indicated temperature. You shouldn't want to change the setting very often - haven't changed mine for years!
If you *really* want to change it, it's quite straightforward as long as you get the right sort of stat, and wire it like for like. With a Y-Plan system, it's *vital* that the stat is of the change-over type - and not just an on/off switch. I don't know whether the Toolstation one is ok - it doesn't give enough information.
The stat needs to have 3 connections - common, demand and satisfied. When the tank temperature is below the stat setting, common is connected to demand. When the tank gets up to temperature, the contacts change over and common is then connected to satisfied. Without this connection, the CH won't work after the HW gets hot.
When I said wire it like for like, I meant in terms of function. Forget wire colours and contact labels - they may be different between the old and new stat. Demand and Satisfied are sometimes labelled NO (normally open) and NC (normally closed) or vice versa - so you'll need to work out which is which.
--
Cheers,
Roger
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Thanks, that's good. I can use my multimeter to work that out, I was worried about whether it had anything other than just a switch in it, sounds like it's pretty standard to just be a c/over switch.. cheers
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