Replacing heating programmer

[gac]

It's a job I've been meaning to do for ages, replace my ancient and noisy Sangamo M5 24 hour programmer with a Horstmann C27 7 day model. Probably not the best quality, but I don't need anything complicated as it's only a basic gravity HW/pumped CH system, which I don't think has any external thermostats (other than the one on the boiler itself which has nothing to do with the programmer). So, armed with the Horstmann wiring guide which has simple instructions on which wires to replace when going from Sangamo->Horstmann, I took the front off. Only to find I have a connection which isn't referred to in the new guide. So I'm not 100% sure whether to ignore this connection and simply tape over the end and leave it disconnected, or whether it needs to go somewhere.

Terminals 1, 3, 4, 5, 6 are all present on my existing controller so they're no problem to convert over to the new controller following this guide, but I also have a wire leading to terminal 8; this seems to suggest a connection to a room thermostat but I'm 99.9% sure there isn't one. Obviously I don't want to tamper with anything and risk damage I thought I'd ask on here as I know there's a few friendly heating engineers who might fancy giving me a spare 30 seconds of advice

Installation guide for old programmer

Installation guide for new programmer

Feature comparison and "direct replacement" wiring guide

Picture of my current backplate (numbering is 1-8, bottom-top)

Thanks in advance!

[Lazy Daisy]

You made a good choice

I don't know if it's any help but mine looks like this behind

that's just with a boiler, hot water cylinder upstairs and a thermostat for the radiators.

[Hairy Chris]

The wire on terminal 8 is the live for the pump which would be switched by a room thermostat if you had one as shown in the info for the old time clock.

Connections:

Red Live to terminal marked L on new time clock.

Black Neutral to terminal marked N on new time clock.

Brown live on old terminal 1 goes to terminal 3 on new time clock. (boiler live)

Brown live on old terminal 8 goes to terminal 4 on new time clock. (pump live)

The two Blue neutrals go to the terminal marked N on the new time clock.

The earths all go together on the earth terminal.

The Brown live that goes bewtween terminals 1 and 6 on the old time clock is redundant.

Hope this helps.

[gac]

Sort of helped Daisy, although this heating system has been in here for years so I have no idea about any of it! And yeah, I thought terminal 8 should be a thermostat, but as I say I've never found one...the only place it could be is inside the false wall that boxes in the heating pipework which would be THE most stupid place to put it (not impossible though).

And thanks a LOT for your reply Chris, I tried making sense of all the wiring diagrams but couldn't for the life of me figure out why terminal 8 had been missed out. I'll try wiring it up your way and give it a try once I've labelled everything up (as if it doesn't work first time I can at least put it back as-is and look at it when I have more time)

[gac]

You're a legend Chris, all seems to work fine! Except I cracked one of the wall plugs by screwing it in too tight so I need to unwire, drill another hole and re-fasten it. But electrically, everything seems to be good, I just need to work out the timings so I can have hot water without central heating sometimes; the new programmer seems to either control the two channels totally independently, or totally dependently, neither of which quite suit my system as I can have hot water alone, or hot water and central heating. Minor thing though, just needs some tinkering with the programming now. Thanks again for your help (and you Daisy)

[VWD]

With the wall plug- you might be lucky and find that a screw screwed in a little way will let you remove wall plug and replace it with a new one- sometimes it might take a few tries. Or try a slightly larger screw .

Edited 13 September, 2012 by VWD

[Hairy Chris]

Happy to help