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Anyone understand combi boiler control systems?

Von: newshound (newshound@fairadsl.co.uk) [Profil]
Datum: 07.06.2010 12:37
Message-ID: <8740g8Fb74U1@mid.individual.net>
Newsgroup: uk.d-i-y
I'm baffled by my Halstead Gold / Wickes 90. It's about five years old and I
am not particularly keen to replace it at the moment.

Central heating works OK, so pump and fan, and their sensors are fine. Gas
valve expects 3 - 20 volts, starting at about 10. Starts up fine and
modulates up to 18 or so, water gets to the right sort of temperature, no
sign of boiling in the primary HX etc.

With CH off, DHW goes through proper start up sequence, lights burner with
11 volts on gas valve and runs for 10 seconds. Then the voltage on the gas
valve jumps up to 19.8 and shortly afterwards the ignition lock-out cuts in.
This is not the ultimate trip from a thermocouple on the primary HX outlet
pipe, it's the trip which comes in if sensors indicate that the fan or pump
might have failed. An obvious theory, suppose the DHW heat exchanger is
blocked, then the primary water will get too hot, this gets sensed by the
DHW thermistor. I'd expect this to modulate down the gas valve, but perhaps
it has a trip level as well. But, the water in the diverter valve isn't
getting particularly hot in 10 seconds of running; a few seconds after the
trip (stopping the flow as soon as it trips), the thermistor resistance is
down from 10k to 8k, corresponding to about 35 degrees. Not a faulty
thermistor, I have changed it anyway, and both test out OK.

Obviously the Pactrol control board? Except that I have a spare board, and I
get *exactly* the same symptoms with that.

If you switch the CH on and let it warm up, then operate DHW the diverter
valve actuates to give precedence to DHW and it functions OK, modulating at
15 to 18 volts or so on the gas valve depending on what you have set the
temperature on the front panel, and giving a decent flow of nice hot water
(so I think the secondary HX, which I have descaled, is reasonably OK

I'm speculating that high resistance at one or other of the connectors
between the thermistor and the control board might persuade it to give the
gas valve full voltage a few seconds after startup, and then some monitor in
the software is saying "gas voltage at top limit, better trip". It's not
particularly easy to "rewire" this connection because of the multi-way
connectors, but I might have a go at this next. In the good old days there
would have been a preset on the board somewhere for setting these
parameters, but I suppose these days it is done within the software?

Or am I missing something obvious?

Steve


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