Central heating programmer wiring mysteries

[Deeteebee]

We've moved into a new (to us) house, with an old boiler, and an old programmer (Potterton EP 2002). The heating is erratic, which seems to be because the switches on the programmer are dodgy; so I want to replace it.

We're planning a renovation within the next year, which will involve moving the boiler, and destroying the wall where the thermostat is installed. So I've gone for a replacement programmer with a wireless thermostat/controls (Secure/ Horstmann C1727).

But of course... it doesn't use the same baseplate.

The old base plate is generously provided with spare terminals, for making connections - the new one not. And there's quite a nest of wiring there. Since this only has to last 6-9 months or so, my plan is to essentially leave the old base plate with all its connections in place, put a box over it, mount the new base plate on top, and just slave the connections from the old base plate to the new base plate.

(I've marked up the image to make the labelling of the key connectors clear. Everything else is spare terminals.)

This is the key for the old baseplate (you can see above that only 3, 4, 5 are in use, apart from power and earth):

And this is the key for the new one:

Clearly 3 and 4 on the old baseplate correspond to 3 and 4 on the new one.

But I'm not clear what 5 is on the old baseplate, and does it correspond to 2 on the new one?

Thanks for reading, and any advice,

David

 

[stem]

The Potterton terminal 5 is a link to the common connection of the central heating switch, which you can see from the diagram....

.....and it is linked to the L terminal with a wire connection.

With the new programmer this link to L is already made inside the programmer, so no terminal 5 is required.

So the wire link between the Potterton L and 5 is removed and discarded, and the remaining wire that is in terminal 5 goes to L [The original installer has used terminal 5 has been used as a convenient point to use for an L connection as it is connected to L by the link]

The other terminals used N L 3 & 4 are used identically with both devices. [1 & 2 which don't appear to be used in your case. Which is fine]

 

[Deeteebee]

Hi Stem - thanks so much for this reply. So I just need to connect 3 and 4 of the new base plate, to 3 and 4 of the old; plus L, N, and earth - is that correct?

 

[terryplumb]

Hi Stem - thanks so much for this reply. So I just need to connect 3 and 4 of the new base plate, to 3 and 4 of the old; plus L, N, and earth - is that correct?

What are you going to connect the other wire that is in terminal 5 of old programmer to ?

 

[Deeteebee]

My plan was to leave all the existing wires in place, just linking 3 and 4 on the new plate to 3 and 4 on the old, plus the power and earth lines. So whatever is already linked to live on terminal 5 still will be.

Is that a mistake?

(I have to say I'm a bit confused about what all the connections to the base plate are. There are five cables: the incoming power, which is fine; one cable coming directly from the boiler; and three others. I assume that two of these are to motorised valves, one for heating, one for stored hot water; but I don't know what the third cable relates to, and I don't know which is which.

The cable from the boiler has five cores; one which isn't used, three which are L/N/E; and one (black) which goes to 'D' in the photo above, where it is junction to a black lead from one of the other cables.)

 

[ianmcd]

The EP as you have discovered was marketed as a wiring centre and programmer all in one, so if you are leaving everything else in place just transfer the CH on and HW on to your new programmer along with a L&N, and leave the EP as a simple wiring centre

 

[terryplumb]

My plan was to leave all the existing wires in place, just linking 3 and 4 on the new plate to 3 and 4 on the old, plus the power and earth lines. So whatever is already linked to live on terminal 5 still will be.

Is that a mistake?

(I have to say I'm a bit confused about what all the connections to the base plate are. There are five cables: the incoming power, which is fine; one cable coming directly from the boiler; and three others. I assume that two of these are to motorised valves, one for heating, one for stored hot water; but I don't know what the third cable relates to, and I don't know which is which.

The cable from the boiler has five cores; one which isn't used, three which are L/N/E; and one (black) which goes to 'D' in the photo above, where it is junction to a black lead from one of the other cables.)

Ok that's fine. You don't need to worry about all the other terminals.

 

[Deeteebee]

Thanks Terry (and Ian and Stem).

 

[stem]

Hi Stem - thanks so much for this reply. So I just need to connect 3 and 4 of the new base plate, to 3 and 4 of the old; plus L, N, and earth - is that correct?

Yes

What are you going to connect the other wire that is in terminal 5 of old programmer to ?

Already covered.

the wire link between the Potterton L and 5 is removed and discarded, and the remaining wire that is in terminal 5 goes to L [The original installer has used terminal 5 has been used as a convenient point to use for an L connection as it is connected to L by the link]

 

[Deeteebee]

I was so clear on what I needed to slave through from the old base plate, that I forgot to link live to 2 on the new one. Since 2 is COM, that was an error :facepalm:.

Once I'd fixed that, all is working well. This is a nice unit - more convenient being able to adjust from the remote thermostat instead of having to crouching under the boiler to where the programmer was inconveniently installed; and even more convenient being able to do it from the phone (bluetooth). Thanks again everyone for your advice.

 

[stem]

Well spotted, I'd missed that too....