Anyone understand combi boiler control systems?

[newshound]

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

[Arfa Daily]

That's extremely similar to a fault that my daughter's combi suffered a while back, and which Geoff, the expert on here helped me to track down.

It was being caused by an erroneous reading from the PHX temperature sensor. Measuring at the connector on the PCB where this sensor came in, you could see a significant jump in the voltage, as the boiler cut back out. It turned out that the actual connector into the sensor itself was intermittent internally to the sensor body, and I managed to get a temporary fix by wedging it with a thin piece of cardboard. Obviously, as you've replaced the sensor, it isn't going to be the exact same problem, but Geoff said that it was common for the loom itself to give problems with breaks at connection points. I managed to prove the point when I was sorting out the diagnosis on my daughter's one, by disconnecting the sensor plug at the PCB, and hanging a pot across the pins, set to a suitable resistance to 'fool' the control processor into thinking that the sensor was present, and indicating that the set temperature had not yet been reached. Obviously, you can't leave it running like that for long before you would get a genuine overheat condition, but long enough to prove that the boiler stays lit.

Arfa

[newshound]

Thanks, that's *very* interesting. Oddly enough, another test I tried earlier was to put a resistance decade box in place of the thermistor and fire it up with 10k, then drop it 1k at a time in an attempt to simulate what the thermistor might be doing. But it still tripped in a similar way (I didn't have the gas valve voltage monitored at the time). All of which seems to point at a fault at the loom to PCB connection. Time for a bit of surgery, I think.

Steve

Engineers will go without food and hygiene for days to solve a problem. (Other times just because they forgot.) And when they succeed in solving the problem they will experience an ego rush that is better than sex--and that includes the kind of sex where other people are involved.

Lifted from

[geoff]

In message , Arfa Daily writes

I thought that the Finest did have a pot to set the DHW temp, but you're not going to resolve the problem by twiddling with this, you should leave it alone as it would have been correct before the fault (as shown by both boards showing the same fault)

The reason it modulates up to full and then down is because the temperature sensor is after the HE and it needs to be hot enough to give useful feedback. The NTC should be about 2.5 to 3k at "hot"

are you sure that the CH is working correctly

give individual wires on the loom a tug check for elasticity- the insulation is crimped as well as the wire, it is easy to miss a broken wire

What is the flow like?, Is the pump working

The fact that you've descaled the HE doesn't mean that you don't still have a blockage

[newshound]

Geoff

Many thanks for the thoughts. Will hunt for bad wires first (I've done the obvious check, of course), and failing that, dig out the data logger again and do some more monitoring!

Steve

My understanding is that the control system should ramp the gas up to full (or to the appropriate "control" level) over a period of (say) 10 to 30 seconds, presumably because it is taking readings all the time and wants to make sure everything is working OK. In this case, the gas valve voltage starts at 11 volts for ignition, and after a few seconds flips straight to

19.8 or something like that faster than an analogue meter can follow. When I was investigating a previous problem you could see that the ramp wasn't actually linear, it was a step function going up a few tenths of a volt every second or so (the numbers might be out by a factor of 3, but that was the basic shape of the function (as you might expect would be generated by a digital system).

Well no, but at least it works. While the DHW doesn't work at all.

Good point, I will do that check. But I'm almost certain it's worth by-passing the suspect pair of wires. I'm also slightly suspicious whether the connectors are gripping the "pins" on the thermistor/NTC and the board sufficiently: I had a fault before at the thermistor end (on the CH thermistor).

Well, it's operating the flow switch, and seems to be pumping hot water round the rads at the normal sort of rate. (I have been tempted to add a flowmeter to that circuit).

Agreed. But when I can keep the gas alight while running DHW, e.g. after running the CH for a bit, and leaving it "running", I get a decent flow of hot water through the DHW. When I had a really blocked secondary HX I was only getting tepid water, and then the gas modulated down because the thermistor was sensing high temperature on the primary side.