Explosive Ignition Suprima 120L

[Graham Powell]

This Potterton Suprima 120L is connected to a heat exchanger which in turn heats a swimming pool. The explosive ignition started after about 18 months and gradually got worse. The Corgi engineer contacted Potterton and they suggested getting the PCB upgrade kit, which consists of all the electronics and the electrodes. This seemed to solve the problem for about 2 weeks after which the problem returned. Potterton next suggested we change the gas valve, this made no difference. The new PCB has now been set up, which basically means the high and low fan speeds have been set. Still no difference. The burner has been checked for damage, there is none and it is as clean as it will ever be. The fan appears to work. You can unscrew half of it and watch it spin using the test switch on the back of the PCB.
This problem is not there all the time. If the boiler is in a continuous heat cycle, gas on for about 2 minutes, off for about 2 minutes, this will continue for several hours with perfect ignition every time. However it appears that if the boiler is off for some period of time, say 10 minutes, the next time it comes on, bang! and up to a 2 foot flame shoots from the flue. Of course it does not do it every time, which is why this problem is so elusive.
One thing that is noticeable is that when you listen to the ignition sequence, there is a variable gap between the end of the sparks and the gas lighting. Certainly, when there is a large gap (approx 1 second) you are virtually guaranteed a large bang. Small intervals can sometimes result in a bang.
One must not forget that this boiler did work for 18 months, but I am not yet convinced that this boiler ignition system is 100% reliable.
When the ignition sequence initially starts up, the fan turns on. At this point you can often smell unburnt gas coming out of the flue.
So the questions as I see it are:

1: When the gas valve shuts off, there must be gas left in the pipe between the valve and the fan. Is this gas somehow creeping up into the burner and not being cleared out completely from the flue the next time the boiler starts?

2: Is the flame detector shutting down the sparks before it should.

3: Because it seems to happen more after the boiler has been off a little while, is it an issue with temperature. Are there components on the PCB that are temperature sensitive?

4: Could it be a condensation issue? When the boiler turns off the fan stays on for another 5 seconds sucking in cold air to the burner.

I would like to know how this gas valve works. I read in another posting that it is a 2 stage operation. As far as I can see this is just a simple solenoid valve that turns on when the sparks start. I have measured the volts across the coil and it is correct (200Vdc).

Any clues much appreciated.

Graham

 

[croydoncorgi]

If you are seeing a variable delay between fan start and burner light-up, with a bang when the delay is longer, the boiler is filling with gas before ignition, so when it does light it WILL go with a BANG! It is PROBABLY NOT a gas valve problem as such because the valve is an 'all-or-nothing' type which either opens or doesn't. The fan is variable-speed, with a feedback arrangement to control the gas-mix, so POSSIBLY there's a problem with the mix (too weak / rich to light initially). Has your engineer checked the pressure-sensing tube between gas valve and fan assy?
What about the ignition electrodes and HT cables?
Can you hear the ignition circuit working (click - click - click) BEFORE the boiler actually lights up?

 

[Graham Powell]

The start up sequence is as follows. First the fan starts. Initially with a slight woosh and then dies down to a lower speed. After 10 seconds you can hear the sparks start. These last for about a second or so. The solenoid on the gas valve opens at the beginning of the sparks. So up to this point everything would appear normal.
The ignition cables and electrodes were changed a couple of months ago when the PCB was changed and they still appear to be OK. The gasket underneath the electrodes is on its last legs though, but appears to making a good seal.

Tell me about this pressure sensing tube. I assume that this is the small tube that goes from the gas valve into the fan chamber. What is this actually measuring? I don't think this has been checked. Is this something I can check before the engineer comes back next week. He is also in contact with Potterton and I think they are stumped as well.

The only thing I have yet found that makes this nearly repeatable is that if I turn the demand for heat off for about 5-6 minutes, the next time it comes on the likelihood of a bang is greatly increased. In the on-off, on-off situation with approximately a 4 minute cycle, always seems OK.
I have just had a thermocouple strapped to the side of the burner to monitor the temperature at which certain events happen. With my initial boiler temperature setting, the outside of the casing was going from 82 degrees to 72 degrees during the cycle. If I let it drop another 10 degrees, the probability of the bang increases. However I don't think this is a temperature issue as I have now run the boiler at a lower temperature. In this case the casing went from 70 degrees to 62 degrees. In the on-off mode this gives OK ignition. So it may be the time for which the boiler is off that is significant. Testing continues at the lower temperature.

 

[drboilerfix]

You need to make sure Potterton supplied you with the latest PCB for this model. The latest pcb is 5106568 and it has an upgraded ignition unit on it. It is also very important that the electrode leads are connected correctly onto the ignition unit as there is a right and wrong way for this, check instructions that came with the parts. Also the PCB should have asmall sticker on it that says iss 2.
This particular boiler has always had ignition problems.
The other obvious thing to check is the electrodes themselves that they are clean, try cleaning them up with some emery paper. All the best.

 

[croydoncorgi]

I suspect (but do not actually know) that the pressure sensing tube monitors the airflow through the fan inlet and adjusts the gas valve output pressure to set the correct gas / air mixture. Given the way gas valves work, it's quite unlikely to fail intermittently. However, if the sensing tube was partly blocked or a seal was missing or damaged, the effects would be unpredictable.
Advice already given on cleaning electrodes and making sure wires are right way round should be taken first.

 

[Graham Powell]

The first board fitted when the boiler was new was:
5106584 Issue 02 dated 30/02

The board fitted now is:

5106568 Issue 03 dated 26/04

Strange that the later board has a lower number.

I am worried about this business of fitting the electrode connections the right way. The HT connections for the spark are not polarity sensitive. The leads for the flame detection however are polarity sensitive. One lead is connected to the chassis the other presumably is the signal.
These two wires connect to J1 on the PCB and this plug can only be connected one way. However, in the picture showing the wiring for this upgraded PCB, it shows the earth wire Grn/Yel being connected to the pin on J1 closest to the HT Out transformer. This is opposite from the way the plug connects to J1. So which is correct. Have I got to swap the two wires in the J1 plug to make it look like the picture.

 

[croydoncorgi]

Silly question...
but when the 'upgraded' PCB was installed, was a wiring loom change fitted at the same time?..... I believe there should have been.

 

[Graham Powell]

The new electrodes and associated wiring were installed. It turns out that the diagram showing the wiring of the flame detector is reversed and therefore wrong. Just out of curiosity, using the old wires, I made up a cable with the wiring as per the diagram. As expected the flame detector did not work. So what happened during the ignition was that the sparks started, the gas ignited, but because the flame detector was not working, the sparks continued for 3 seconds and then the gas went out. This then proves that when the flame detector is wired correctly it does actually work, and it is the flame detector that is shutting off the sparks.
What I have been seeing is that the bang occurs when the boiler has been left off for a period of say 6-10 minutes. So I conclude that this is either a time or temperature problem with some part of the electronics. So next I attached a voltmeter to the flame detector input just to see what was being measured. When the boiler is off I see 200VAC. At the point of ignition (flame detected) this drops momentarily to 194VAC and the stabilizes at 196VAC. I have tried to trace out the circuit in this area and there are a lot of high value resistors, so any marginal connections on the J1 socket are going to affect the operation of the flame detector. So far today the boiler has operated without any bang. If this J1 connection does turn out to be the problem (need about two weeks of no bangs to give me confidence), then possibly what is happening is that when the sparks start, there is sufficient gas around the electrodes to partially ignite, trigger the flame detector early and turn off the sparks. Meanwhile unburnt gas is being pumped straight into the flue. Next the partialy lit gas ignites the rest of the gas resulting in the bang. I know this all sound a bit unlikely, but something along these lines must be happening. And something like this is quite plausible when you look at the construction of the boiler. The fan blows gas in under the right hand side of the burner, the flue sits on top on the right hand side of the burner, but the electrodes are above the burner on the left hand side. So it seems to me that it would be quite easy for the unburnt gas to be blown straight up into the flue before any of it has wafted across to the electrodes. I am looking at this whole problem from the point of view of an electronic engineer rather than a gas engineer so there may well be things I don't understand.

Is it possible to get hold of circuit diagrams for the PCB. My guess is that the holy grail may be easier to obtain.

 

[kevplumb]

look on the bright side saves getting fireworks

 

[htgeng]

look on the bright side saves getting fireworks

yeh. wonder how much all these 'parts' came to? should have cut your losses mate and ripped out that potty and replaced with a quality condensing machine, esp as its likely running 24/7 to heat your pool. The monthly gas bill is....????

Wonder why I bother trying to explain energy efficiency sometimes.

 

[croydoncorgi]

Your analysis is interesting but maybe not the only possibility.
In practice, it's reasonable to position the electrodes away from the main airflow - that way the initial flame is less likely to blow out or get lifted off the burner.

There are other factors: eg. the fan is variable speed - so maybe the speed control is not holding it down sufficiently during the first part of the ignition sequence, so burner lights - flame detected - flameout - gas flow continues - reignite attempt with too much gas - Bang! I think this less likely because with the fan too fast there would be insufficient gas around the electrode to light at all. OTOH, if airflow is the only control on the throttle setting in the gas valve, running the fan too fast at the wrong moment might create the bang by itself.

Seems to me that the most likely cause is poor ignition in the first place, so that gas / air mix builds up too much before the spark actually ignites the burner at all. Once the gas mix catches, the flamespeed etc. is usually enough to propagate the flame rapidly across the whole width of the burner without any problem. A scenario where the burner only partly lights, there's a delay for unburned gas to accumulate and THEN the flame propagates and causes a bang seems unlikely.

Just in case you did not know: flame detection usually works by the rectification effect of a flame. Present AC on the detector electrode then sense the DC bias between electrode and ground. Usually simple to do and very reliable.

 

[Graham Powell]

1:
There probably is an opportunity here to enter the firework business. Perhaps I shall loosen the wall mountings and see if the boiler will launch itself into outer space.

2:
The pool is in an insulated brick building. So heat loss is minimal, especially if you keep the air temperature at or just above the same level.
With the boiler off, water temperature drops by about 0.5 degree C per day. Of course if I lived in Australia I would not need a boiler in the first place.

3:
But does this explain the variable delay between the end of the sparks and the bang. The sparks will only stop if the flame detector turns them off. Have now had another day of bang free operation. All I have done is to scrub up the electrodes a bit and ensure the flame detector connections were making properly. At the moment I certainly cannot provoke the problem by setting up the conditions I used before. The outside air temperature has risen over the last couple of days, so maybe that is significant.

When you don't know the answer anything is possible.

 

[croydoncorgi]

Seems to me that you are one of very few users that have the means and skills to actually log what's going on.

I'd be interested in the timing of the sequence: switched-live on - volts and amps on fan (DC!) - power to gas valve - HT to electrode - DC bias sensed on detector electrode, and whether the thing actually banged or not (sense pressure peak in flue??), recorded for a large number of burner cycles. If flame presence could be independently sensed and timestamped eg. by photodetector - even better.

Given the number of boilers including this model that suffer from detonation for no generally-known reason, Potty should already have done it! I reckon that if someone could actually collect a big chunk of data from a boiler susceptible to the problem, an obvious pattern (eg. long delay between HT on and flame sense followed by bang) would point strongly to the real underlying cause.

 

[ChrisR]

SOmething else to look at would be the gas supply to the boiler. Other boilers are known to need very rapidly-reacting regulators (on the meter) if explosive ignition is to be minimised.
What size is the gas supply between the meter and the boiler, how long is it and what other appliances are connected? How far from the boiler is the size reduced? Does it make any difference if another appliance such as 2 rings on a hob, is ON when the boiler fires?
Has anyone REALLY checked the gas pressure at input when it fires? Capitals because a normal check won't give a true reading.

If you think of the supply pipes as inductive and resistive, with a regulator which needs a (variable because it's mechanical) bias current to give a stable output, you can see that the pressure (like voltage) at the boiler can be anything but stable & from a low impedance. Then there's another pressure regulator in the gas valve in the boiler. The CMRR for that isn't too bad but it's a mechanical device, with a delay in response to a step function. So when the fan comes on or changes speed, the amount of gas you get is unpredictable. If it's too small, gas can build up between clicks so firing is late and loud, which of course it would also be if you got too much.
You need a corgi bod who's used to diagnosing this sort of problem to look at it - and it has to misbehave to order!

 

[PEDANTICVINDICTIVEMAN]

I would have got Potterton to it ages ago and paid them once, then every time it went back to banging fetch them out without paying until "they" had fixed the problem, after all it's their pile of shiitte.