I have a old SM300 timer on a combi boiler which is broken and i have a new ST9100C programmer here. There was only 3 wires on the SM300, brown, blue and earth which turned the boiler on and off on a timer. Which terminals do i connect these to on the ST9100C? i have N L 1 2 3 4 and the earth block.

Not possible, the old earth must be a phase, unless there's at least another wire.

Post what wires are connected to which number, a pic showing the connections would be better.

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On the SM300 backplate the blue went to number 1, the brown to number 4 and the earth to the earth block. and there is N L 1 2 3 4 and the earth terminal on that back plate.

You need an L & N for the clock.

1 & 4 are the switching pair the same as the St 9100.

What is the boiler make/model

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worcester 24cdi, i have just moved into the property. On the SM300 the on/off button worked but the timer dial didn't move.

So 1 should go the LS in the boiler, and 4 should go to LR in the boiler.

Plus you also need the L&N to run the clock

Is there a room stat fitted.

I would change the ST 9100 for the Honeywell DT 92E programmable room stat.

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No room stat, so their should be 2 more wires?

In that case the DT92E would be perfect.


Yes there must be an L&N for the old and new.

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I'm stuck with the ST9100C for now, i have connected LS and LR from the boiler to the positions you said above, but get no display on the ST9100C? will it need the other 2 wires?

You need the L&N or it wont work, but neither would the old one

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It explains why the timer didn't work on the old one then, the on/off worked. Wonder who wired it up, anyway i guess i need to find some cable with another 2 wires or run another cable from the boiler.

You need a 230vac supply, ideally it would be from the same fused spur feeding the boiler.

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The emtpy blocks on the boiler are NS & ST8, which position do they go on the ST9100?

Ok follow this exactly.

Wire NS to N in the 9100.

Wire LS to L in the 9100.

Wire LR to 4 in the 9100.

Link L to 1 in the 9100.

and forget st 8 you are working in st 12.

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