2 Apr 2016
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United Kingdom
I bought a C17 to replace my aged brown/cream digital Drayton Tempus Seven. The Drayton backplate has 7 teminals: E, N, L, 1, 2, 3, 4; and there are 3 wires connected to N, L, and 4. The C17 has an industry-standard backplate of 6 terminals: N, L, !, 2, 3, 4 and a separate earth terminal. The Drayton works fine with wires attached N, L, 4 but when I connect to these terminals on the C17 it won't work. Everything looks fine: I can programme the settings and the red "on" light appears but the boiler won't fire up even on 24hrs (continuous) . Any help would be very much appreciated. There are only these 3 wires.

The CH setup is a Vokera Combi Unica HE and it is connected to the Drayton digital timer via a Honeywell HC60NG relay module (which is bound to a Honeywell analog wireless thermostat).
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Many thanks. The reason I have not done this is because the installation leaflet which comes with the C17 gives two typical wiring layouts for combi boilers and both diagrams are labelled "Terminals L-2 should not be linked". Is my 3-wire set-up untypical?
No it's not untypical, but the manufacturers can't show every possible wiring diagram. You need the link between L and 2 as shown above. Your old programmer had the link from live to common internally.
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The link is not there because plenty of combination boilers do not require it. Fitting the link on those would damage the boiler.
I owe apologies all round. I leapt straight to the example wiring diagrams and missed the comment elsewhere in the instructions: "The CentaurPlus has voltage free contacts. A link L-2 is required for mains voltage applications". Have inserted this link and all tickety-boo now. Sorry to have wasted your time.

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