Vokera Syntesi 35 DHW firing then cutting out

31 Mar 2003
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United Kingdom
Hi, have a Vokera Syntesi 35 which must be 18 months old. Its been running okay since we moved in although the DHW is not red hot and runs cold if you turn the tap/shower on too fast.

Trouble now is the DHW won't run. It will fire up and then within 15 secs drop down to an idle burn (if that makes sense?).

I've turned the dial up to max and that now runs the DHW to a warm heat but the water flow is down to a trickle.

I've searched for this and can't find any answers - any help would be greatly appreciated - have the handbook here for ref but can't find anything that relates to it.

Any help greatly appreicted.

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A 35kW boiler should have been giving you very good water - all other things being equal.
If turning the knob makes a difference then the first thing I'd be checking would be the DHW flow temp sensor. But without knowing what resistance it should be, not so easy.
Always a contender too is the diverter/diaphragm/flow switch, whatever's fitted - that you'll have read about.
thanks for the reply! So a blocakge wouldn't cause the problem as it would be consistant throught the heat range?

There's two thermistors - one on the main heat exchanger and one on the DHW heat exchanger manifold.

As the heating is working fine would you expect it to be the DHW switch?
Getting in to guessworks here cos I don't know how it works. It knows the water is flowing, fine. Then it should start the pump (almost certainly). If pump doesn't get switched on it will shut down because the main h/e will get v hot v quick. Once running, the DHW flow sensor would be key.
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cheers Chris - I've unplugged the thermistor on the DHW and it produces an error on the control board, so I've ordered a new one (£6) to see if its malfunctioning.

The pump and everything else seems to be fine so we'll see.

Thanks for your advice

dhw heat exchanger blocked, check flow in red hot, flow out cold on 22mm pipes from main heat exchanger, if only 18 months old was system power flushed

lots articles on this do a searchon dhw
Agree with above. Check Benchmark log book. Bet the system has dirty water that has blocked the HW HE
Yes I should have said that - I got the idea that the problem had come up suddenly so was thinking that something might have "bust".

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