Explosive Ignition Suprima 120L
- Thread starter Graham Powell
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Good point about re-checking the gas working pressure. I have come across Baxi boilers which were very susceptible to poor supply preesure (in that case caused by problems with the mains) but not previously a big Suprima. That said, what's the supply pipe size and lenght between boiler and meter? Could be that a long run is amplifying supply pressure drop in some situations - although how is a bit of a stretch!
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I'll describe the whole setup.
There are in fact 3 gas boilers and a couple of gas hobs being fed from the single meter. The last to be installed was the boiler for the pool. The gas feed for this is taken directly after the meter and runs for about 100 feet in that standard yellow plastic pipe. It then goes to copper with the last two feet or so as 15mm.
Initially we suspected gas pressure, but this has been measured many times and it would appear that however many of the other appliances are on, it makes no difference to the ignition of the pool boiler.
Last night it started to go wrong again. Had been working faultlessly for three days.
The more I look and listen to the problem, I always seem to come back to a temperature problem. The ambient temperature around the boiler is 30 degrees C. I have strapped a temperature probe to the PCB and have been monitoring the rise and fall as the boiler goes through its various phases. With the boiler temp setting to about two thrids, the PCB sits at about 55 degrees C. If I take the side casing off, this drops by about 10 degrees. After the couple of bangs last night I left the PCB assembly totally exposed at the 45 degree position. This obviously keeps the PCB at the ambient room temperature. It then was OK.
The measured DC bias on the flame detector input was -6VDC dropping to about -30VDC when alight. Strange thing though, that leaving the probes connected, the flame detector did not work correctly. This is different from before.
A couple of things I will probably change:
1: The electrode gasket, is in good condition apart from the fact that it is now in two halves. Careful position though seems to make a good seal.
2: When the pool heat exchanger no longer requires heat, motorised valves shut off. This means that the pump, which is controlled by the boiler, is pumping against a dead end for the 3 minute run on time controlled by the PCB. The boiler temperature rises as it can't be pumped away. So maybe this ties in with the 6-7 minute off time I originally used for making it go wrong.
I was thinking of adding another motorised valve across the flow and return which turned on when nothing else required heat. That way the pump would never pump to nothing and any excess heat from the boiler could be pumped away.
Is this a good idea, or does it not really matter that the pump can hit a dead end for a few minutes.
Before I call out Potterton, I need to make absolutely sure that they can't blame some other part of the system for the fault. We have had numerous conversations with the technical dept at Potterton and although very helpful, one gets the feeling that once you have got past the "replace the PCB" or "replace some other component", their knowledge of the real in depth workings of this thing is limited.
I could really do with a copy of the circuit diagram of the PCB
There are in fact 3 gas boilers and a couple of gas hobs being fed from the single meter. The last to be installed was the boiler for the pool. The gas feed for this is taken directly after the meter and runs for about 100 feet in that standard yellow plastic pipe. It then goes to copper with the last two feet or so as 15mm.
Initially we suspected gas pressure, but this has been measured many times and it would appear that however many of the other appliances are on, it makes no difference to the ignition of the pool boiler.
Last night it started to go wrong again. Had been working faultlessly for three days.
The more I look and listen to the problem, I always seem to come back to a temperature problem. The ambient temperature around the boiler is 30 degrees C. I have strapped a temperature probe to the PCB and have been monitoring the rise and fall as the boiler goes through its various phases. With the boiler temp setting to about two thrids, the PCB sits at about 55 degrees C. If I take the side casing off, this drops by about 10 degrees. After the couple of bangs last night I left the PCB assembly totally exposed at the 45 degree position. This obviously keeps the PCB at the ambient room temperature. It then was OK.
The measured DC bias on the flame detector input was -6VDC dropping to about -30VDC when alight. Strange thing though, that leaving the probes connected, the flame detector did not work correctly. This is different from before.
A couple of things I will probably change:
1: The electrode gasket, is in good condition apart from the fact that it is now in two halves. Careful position though seems to make a good seal.
2: When the pool heat exchanger no longer requires heat, motorised valves shut off. This means that the pump, which is controlled by the boiler, is pumping against a dead end for the 3 minute run on time controlled by the PCB. The boiler temperature rises as it can't be pumped away. So maybe this ties in with the 6-7 minute off time I originally used for making it go wrong.
I was thinking of adding another motorised valve across the flow and return which turned on when nothing else required heat. That way the pump would never pump to nothing and any excess heat from the boiler could be pumped away.
Is this a good idea, or does it not really matter that the pump can hit a dead end for a few minutes.
Before I call out Potterton, I need to make absolutely sure that they can't blame some other part of the system for the fault. We have had numerous conversations with the technical dept at Potterton and although very helpful, one gets the feeling that once you have got past the "replace the PCB" or "replace some other component", their knowledge of the real in depth workings of this thing is limited.
I could really do with a copy of the circuit diagram of the PCB
Well the overrun is there to cool the boiler, so it should be able to pump somewhere! I expect the Mfrs Instrs only call for a manual bypass valve but I would also expect potty to go for it if there isn't one at all.
I see no connection at all though between lack of bypass and its going bang if off for 10 mins rather than 2! ..A thought ...does the fan stay on during overrun - ?
Not happy about the gas supply. What diameter is "that standard yellow plastic pipe"? Also don't like 2ft of 15mm. Some boilers insist on a greater diameter pipe at the boiler end. A thin pipe behaves a bit like an electrical inductance. At the instant the gas valve opens you will get a drop in pressure because of the drop across that pipe and its failure to maintain the pressure which has just collapsed at the gas valve. If your corgi man doesn't know how to test for a transient pressure drop at the input, you won't see it. Normal methods don't show it because the resistance of the test point is too high. And forget digital electronic pressure meters, they average the pressure over time between display updates.
Ig you're in electronisc you'll have cans of freezer spray - the type used for finding bad connections. Try that, it'll cool the pcb nicely. Opposite would be a hair dryer!
Have you replaced the HT lead - they often break down a bit without showing a spark anywhere else. Not sure if the S120's is one of the high resistance core ones or not.
I see no connection at all though between lack of bypass and its going bang if off for 10 mins rather than 2! ..A thought ...does the fan stay on during overrun - ?
Not happy about the gas supply. What diameter is "that standard yellow plastic pipe"? Also don't like 2ft of 15mm. Some boilers insist on a greater diameter pipe at the boiler end. A thin pipe behaves a bit like an electrical inductance. At the instant the gas valve opens you will get a drop in pressure because of the drop across that pipe and its failure to maintain the pressure which has just collapsed at the gas valve. If your corgi man doesn't know how to test for a transient pressure drop at the input, you won't see it. Normal methods don't show it because the resistance of the test point is too high. And forget digital electronic pressure meters, they average the pressure over time between display updates.
Ig you're in electronisc you'll have cans of freezer spray - the type used for finding bad connections. Try that, it'll cool the pcb nicely. Opposite would be a hair dryer!
Have you replaced the HT lead - they often break down a bit without showing a spark anywhere else. Not sure if the S120's is one of the high resistance core ones or not.
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Just in case everybody was worried that I might have been launched into outer space by the explosive ignition, I shall put your minds at rest. The saga continues but just possibly may becoming to some sort of conclusion.
Since the last posting, the 2 foot of 15mm pipe has been replaced with 22mm. There is now only 2 inches of 15mm into the boiler. This may have made a slight difference, but did not cure the problem. Next the burner was changed. Absolutely no difference. Then I had a suggestion from another newsgroup that fluctuations in the mains voltage could have an effect on the flame detection circuit. The mains voltage does indeed vary between about 233V to 241V, but could not really detect any pattern between volts and explosive ignition.
So now I conclude that the only way explosive ignition can occur is that unburnt gas has got into the flue before enough gas has passed over the sparking electrodes. A previous posting describes the mechanical construction of this boiler, and how I thought this was possible. Fortunately this construction provides for some simple experimentation. I have now placed a carefully hand crafted piece of right angled pipe under the burner to deflect some of the incoming air/gas mixture to the other side of the burner (where the electrodes are). The effect so far has been dramatic. To date I have had no explosive ignition. A few more weeks are required to confirm. And what's more, the consistency of the ignition has now improved. Whereas before, even when the ignition was not explosive, the loudness of the resulting pop would vary from virtually nothing to something of concern. But now I can hardly hear the ignition at all. I daresay that in a couple of weeks, I shall be proved wrong, but so far the results are very encouraging and could prove that the Suprima 120L either has a design fault or requires such careful setting up that it is more by luck than good judgement it works correctly.
I am now waiting for the thought police to come and take me away.
Since the last posting, the 2 foot of 15mm pipe has been replaced with 22mm. There is now only 2 inches of 15mm into the boiler. This may have made a slight difference, but did not cure the problem. Next the burner was changed. Absolutely no difference. Then I had a suggestion from another newsgroup that fluctuations in the mains voltage could have an effect on the flame detection circuit. The mains voltage does indeed vary between about 233V to 241V, but could not really detect any pattern between volts and explosive ignition.
So now I conclude that the only way explosive ignition can occur is that unburnt gas has got into the flue before enough gas has passed over the sparking electrodes. A previous posting describes the mechanical construction of this boiler, and how I thought this was possible. Fortunately this construction provides for some simple experimentation. I have now placed a carefully hand crafted piece of right angled pipe under the burner to deflect some of the incoming air/gas mixture to the other side of the burner (where the electrodes are). The effect so far has been dramatic. To date I have had no explosive ignition. A few more weeks are required to confirm. And what's more, the consistency of the ignition has now improved. Whereas before, even when the ignition was not explosive, the loudness of the resulting pop would vary from virtually nothing to something of concern. But now I can hardly hear the ignition at all. I daresay that in a couple of weeks, I shall be proved wrong, but so far the results are very encouraging and could prove that the Suprima 120L either has a design fault or requires such careful setting up that it is more by luck than good judgement it works correctly.
I am now waiting for the thought police to come and take me away.
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5 years on and my fix has proved a success. No more explosive ignition. All you need to check now is the electrodes. The flame detector is very sensitive and requires the electrodes to be changed probably about once per year.
There is a glaring misaprehension in some of your previous postings!
You seem to be thinking that the flame detection may have had something to do with the noisy ignition.
All that relates to your problem is the time the spark takes to ignite the gas.
That depends on the gas/air mixture and the timing of the spark relative to the gas being turned on. The sparking starts first.
The spark has to be strong enough to ignite the mixture, this is a function of the voltage, usually about 14 kV and the electrode gap, usually 3-4mm.
And in your case if the electrode is getting fully illuminated by the gas mixture.
Your solution implies the later problem was the cause. Exactly why I have no idea without seeing your boiler.
Tony
You seem to be thinking that the flame detection may have had something to do with the noisy ignition.
All that relates to your problem is the time the spark takes to ignite the gas.
That depends on the gas/air mixture and the timing of the spark relative to the gas being turned on. The sparking starts first.
The spark has to be strong enough to ignite the mixture, this is a function of the voltage, usually about 14 kV and the electrode gap, usually 3-4mm.
And in your case if the electrode is getting fully illuminated by the gas mixture.
Your solution implies the later problem was the cause. Exactly why I have no idea without seeing your boiler.
Tony
All this proves what a pile of crap the 120L was I hate them!!! 
GP has mentioned the electrodes.Without reading all the posts and with the risk of repeating what someone else has said,there are modified (shorter) electrodes for this boiler.Part No 5108474
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So 5 years later all the work time and expense has been the customers, and not the manufacturers, didn't they do well, also with the diy pipe adaption, if the boiler blew up would the house insurance be void, even if the adaption had nothing to do with it, as I said in 2005-
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.
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.
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I am well aware that the explosive ignition and the electrode problem are different. The explosive ignition is cured by improving the gas flow. The flame detector issue just causes the gas to go out after it has been on for a couple of seconds. Cleaning the electrodes works up to a point but changing them every year keeps everything fine. There has never been a problem with the gas igniting, just keeping it on.
One could take that argument further now we know that there was a modification to the ignition electrodes.
In view of the perceived hazards of explosive ignition, it could be strongly argued that the new ignition electrodes were a "safety modification" which was required because the original boiler design was flawed. As such they should have been fitted free to anyone whose boiler was affected.
Tony
In view of the perceived hazards of explosive ignition, it could be strongly argued that the new ignition electrodes were a "safety modification" which was required because the original boiler design was flawed. As such they should have been fitted free to anyone whose boiler was affected.
Tony
If you are now telling us that there was never any explosive ignition then either you dont realise what causes that or you have not noticed that you headed your post with "explosive Ignition".
For the record explosive ignition occurs when the boiler first ignites and NOT when it goes off as a result of no flame detection.
Tony
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yep, pretty much as I see it.
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