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UK based. Worcester-Bosch combi-boiler (Greenstar 25si combi Mk IV). Pin timer.

Our problem is we can only get the central heating to turn on by setting the timer wheel (and obviously the thermostat).

The boiler has an off/timer/on switch and it doesn't matter if the switch is on timer or constantly on. We still can only get the heating to come on by setting the pin wheel to "on" at the time we want the heating on.

Hot water (on demand) works perfectly.

Thermostat seems OK, too - if not a little stiff to change temperature. The heating seems to regulate when on.

Any ideas what might have broken?

Thank you!

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    Has it ever worked? If it once worked and now doesn't, what was changed just before it went TU? - If it never worked, then the wiring may be incorrect. Duffers often mess up this kind of circuit. - If it once worked, then the wiring is probably correct and the off/timer/on switch has failed. – A. I. Breveleri Oct 2 '16 at 21:49
  • Thanks for the quick response, much appreciated :) I'm not sure if it's ever worked properly - the boiler (according to the previous owner) had only been installed about 3-4yrs ago... – Kinnectus Oct 2 '16 at 22:11
  • What is the model? W-B makes several sizes of combi. – A. I. Breveleri Oct 2 '16 at 23:02
  • It sounds like the "on" position on the switch doesn't bypass the timer....how many terminals does the timer have? – ThreePhaseEel Oct 3 '16 at 1:18
  • Edited question with boiler model. A quick look I believe the timer is an MT10 and that these can fail - some people having replaced theirs more than three times. I think, as you say, the switch for the "off/timer/on" (timer override?) might be faulty. Is it worth upgrading to a digital? It seems they're quite interchangeable, which is good and wholly unexpected, I must say. The installation manual shows a simple step-by-step and it appears there is a single connector from programmer to boiler. It's just that the boiler entire main cover has to be removed to get to the timer securing screw. – Kinnectus Oct 3 '16 at 6:15

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