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[rylandsian]

I have taken out a faulty switchmaster 1151 controller and fitted a Horstmann centaur plus c27. The wiring should have been straight forward.
switchmaster wiring was, 1 2 3 4 5
CH live neutral water not used.

Horstmann wiring is now, 1 2 3 4 5 6
neutral live not used not used water ch

Basically nothing works now except boost on the hot water. It should have been straightforward, but going from a mechanical timer to a digital unit seems to be a problem. I think it is a simple wiring error, but am not sure what to do now.
Thanks in advance (hopefully)

[latchkey]

If I understand you are switching from a mechanical time to digital and you are lacking a wire. I am not familiar with that model but it does sound like a wiring issue. Could be as simple as your grounding wires not being correct. Sorry for the lack of help, not really sure what to tell you other than to recheck the wiring.

[homeimp12]

It does sounds like it might be a ground wire, but sometimes these changes are not as straightforward as we thing. If the wiring is older, the wires could be considerably difference in color and connection. I have found that a wiring diagram is the only way to combat this, or find an electrician.

[ericmark]

I would hope from date this is all cured by now. Normally only look in electric section. However many of these programmers have dip switches and turn keys to set them up for the different systems. With some you can't stop water from heating. The user guide does refer to setting for system type. From the instructions it seems there is a blue circuit link on this model to set it up. Remove for gravity domestic hot water heating.
The earth (we don't call it ground here that's what they call it in USA) is only it seems to allow interconnection and is not required for the controller. It does however need both live wires line and neutral to work it's not battery powered. The battery does maintain the time with a power failure but you will lose display and it will not function. I hope you don't need this as getting a little cold now.