I'm in the UK, using an ancient tempermental Potterton EP3000 boiler programmer, but I have a brand new Horstmann C27 which I'd like to replace it with.

Apart from the mains supply, there are three different cables going into the EP3000 backplate, of which one (from the boiler next to the programmer) has four sheathed wires plus an earth.

The other two cables (from upstairs where my hot-tank and pump are in an airing cupboard with the house thermostat just outside) have three sheathed wires. One also has an earth going in the backplate.

The programmer itself only has pins into terminals L N and 1 2 3 4 5 on the backplate, where terminal 2 is unused. Terminals 1 3 4 each have one wire, but terminal 5 is strapped to L as well as having another input wire. The backplate wiring looks more complicated than it probably is, because a few "passive" connections use terminals A B C D and a secondary N strapped to the first.

The C27 only has terminals E N L 1 2 3 4, where apparently 1=HW Off, 2=CH Off, 3=HW On, 4=CH On.

I don't really understand this wiring diagram inside the EP3000, which is all I've got to go on. It seems to say 1 and 3 control HW, with 3 being "on". For CH, 4 seems to be "on", but the diagram has lines to both 2 and 5 even though I have no connections on 2. It's "jumpered" to Program selection 16 (not 10).

enter image description here

Note that in the above, the circles represent actual contacts on the back-plate. The rest is just circuit diagrams for what goes on inside the programmer.

Can anyone confidently say how to connect the C27 just from the above information? I'm not a qualified electrician, and I wouldn't know how to identify what cables go where around my airing cupboard. I'm kinda guessing I'd just use "chocolate blocks" to replicate existing passive connections, and transfer the wires "as is" for terminals L N E 1 3 4. But I'm not certain of that, and I've no idea what to do with the existing terminal 5 and new 1.

In a nutshell, all I really want is for someone who knows about these things to tell me if I can safely discard the live wire on existing terminal 5 and connect the other one to new terminal 2. Also confirm that all the other connections simply go to the same numbers on the new programmer.

  • This link might help. This one too.
    – Tester101
    Commented Nov 30, 2011 at 19:15
  • @Tester101: The second of those links is what persuaded me to buy the C27 on eBay last week for £25, because it clearly shows it as a viable replacement for the EP3000. I have limited funds and electrical skills, so I'm not really keen on paying 2-3 times that just to have a professional come and wire it up for me. But neither of your linked charts make it obvious to me what connections I should make. Commented Nov 30, 2011 at 19:33
  • A picture of the wiring diagram you have for the EP3000 might help.
    – Tester101
    Commented Nov 30, 2011 at 20:54

1 Answer 1



enter image description here


I think the diagram you have, is showing the internal view of the programmer (simplified of course). In this image the blue rectangle represents the internals of the programmer, and the red lines represent the lines going to the boiler and power.

enter image description here

The lines with arrows are probably switches.

enter image description here

And if both HW and CH were on, it would look like this.

enter image description here

Because of this, 5 has to be linked to L to supply power to CH, like so.

enter image description here

But I'm guessing the new programmer supplies power to CH internally, so with the new programmer it should look like this.

enter image description here

Again, I could be wrong here as I've never worked on a system exactly like yours. After looking at the wiring diagram the arrows are either switches, or diodes. Though, diodes would look more like this ->|- .

Hope this helps.

  • I take it the implication there is I just transfer the wires from each letter/number to the same on the replacement. But my EP3000 has no wires on terminal 2, and the C27 has no terminal 5. That's my problem. Commented Nov 30, 2011 at 19:36
  • @FumbleFingers From what I saw in the charts I linked to in a comment the EP3000 had a link from L to 5, which doesn't seem to be required on the C27. It looks to me like you should be able to transfer the wires, but I could be wrong as I've never worked with these programmers.
    – Tester101
    Commented Nov 30, 2011 at 19:47
  • Well, my front-runner idea at the moment is just to connect the two wires currently on EP3000 t5 into C27 t2. But I'd surely like someone who knows a bit more to advise me, given that one of those two wires is Live. Commented Nov 30, 2011 at 20:33
  • Thanks, but I'm afraid that doesn't spell it out for me. All I want to know is which wires to connect to which terminals on the C27. Are you saying connect existing wire in 5 to new terminal 2, and forget about the fact that old 5 was also strapped to L? Commented Dec 1, 2011 at 4:35
  • @FumbleFingers Yes. I would think the wire on 5 should be moved to 2 on the new programmer, and the strap from L to 5 is not required on the new programmer. If I were you, I'd give a call to a local company that does this type of work. They may be able to verify this over the phone, at no charge.
    – Tester101
    Commented Dec 1, 2011 at 12:51

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