When boiler fires hob flames go down

Hello all, am desperately worried that I've made an error somewhere. As is the way, I thought I'd gone by the book, but it appears I may have over looked something. If it was anything other than gas then I wouldn't be 'too' concerned, but...
I very recently replaced the gas hob that was in this house when we purchased it. At the same time I removed some of the old gas piping (had very small leaks behind the oven and was way out of spec). The old gas pipe didn't have an isolation valve and had a very small leak. In the new system I fitted an isolation valve, replaced the hose connection with a soldered joint and removed about 5 straight through connectors and elbows to replace with a nice clean straight run.
The problem now is that when the combi boiler fires for water or heating the hob flames get smaller. On individual rings it's not really noticeable, but on the large centre ring it's quite significant.
I've checked the isolation valves on the boiler and hob and they're both open full, I've also checked the valve on the gas meter and that too is fully open. There aren't any other gas appliances in this house.
Have I done something wrong? and what can I do to rectify this? Thanks as ever for any and all advice.
Seri
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Firstly if its a fixed hob this should be fitted by a registered CORGI fitter, but presume you knew that. I must admit we do notice a slight decrease in flam on the gas cooker when the boiler comes on but think its due to the fact the boiler has a greater requirement for gas and is comes before the hob in the pipe work. Don't think its anything to worry about, unless your hob goes out everytime your boiler starts but that's only my opinion.

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Not true. It is quite legal to replace a gas appliance in your own premises provided you do it competently.
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It doesn't!! He has to "competent".
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In article
wrote:

It sounds like the pipe supplying the hob and combi is inadequate. The real danger would be if you set the hob to a gentle simmer and the flame goes out when the combi cuts in.
--
Tony Bryer SDA UK 'Software to build on' http://www.sda.co.uk
Free SEDBUK boiler database browser
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just turned all 4 burners and the fish kettle/griddle burner up full, then turned the heating up full and left the upstairs and downstairs hot taps running (perhaps a tad over kill, but why take chances).
I then went around each burner turning it down to the smallest possible flame (with the rest still on full), I left each burner on the lowest setting for about a 1 minute each and tried blowing over them (hey, not scientific, but still better than nothing). Anyway, no burner went out, so I'm assuming this is no great shakes. Correct?
Thanks again
Seri
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In article
wrote:

Not quite the right test IMO: you need to have the hob burners on the lowest possible setting *before* you fire the boiler. The risk is that the combi firing up drops the pressure at the hob to below what is required to sustain a flame.
--
Tony Bryer SDA UK 'Software to build on' http://www.sda.co.uk
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Okay, I just repeated the test again, except this time I let the boiler extinguish before turning the individual burners down, then let it fire again. I also tried turning all the boilers down to minimum and firing the boiler.
No flames extinguished, but when all 4 burners and the central fish kettle/griddle burner were on full the boiler firing had quite a noticeable effect on the flame size.
The gas piping from the meter to the boiler is 22mm copper and to the hob is 15mm copper. I'm speculating that the piping to the hob needs to be replaced with 22mm the same as the boiler. Thanks again for all your help, it's really appreciated.
Seri
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Have a look at, http://www.ukcopperboard.co.uk/public/howcanwehelp/inst_tips3_contents_list . shtml and click on pipe sizing. What you should aim to achieve is a 1 mb or less drop in pressure from the meter to the appliance, any more than that and you are asking for problems, the main one being incomplete combustion, which creates carbon monoxide.
Andy

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together this:

Although there are plenty of others here who will be able to advise you better, I haven't actually seen a hob that needs 22mm. I think if anything you need to upgrade the run from the meter to the hob tee off up to 28mm. I would advise you to call in a CORGI registered installer at this point.
--

SJW
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Okay, when I commisioned the hob I tested the static pressure drop using a manometer (0 mbar) but after reading some of the PDF's on that extremely informative site (thanks Andy) I've realised that I never tested the dynamic pressure drop. Am going to get the manometer out now and test the dynamic pressure drop to see if it's more than 1.25mbar. Thanks again for all the advice
Seri
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Okay, after my testing I'm now more flummoxed and concerned than ever.
I just ran through some tests at the meter test point using a manometer.
The standing pressure with the boiler and hob isolation valves closed is 24.8mbar
The standing pressure with the boiler and hob isolation valves open is 24.8mbar
When the boiler fires the pressure drops to 9mbar, and whilst the boiler is heating this stays at 9mbar. When the boiler stops the pressure returns.
Am I right in assuming that this means there is a problem with meter govenor and I need to call transco out? Thanks again for all this advice.
Seri
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On Wed, 15 Dec 2004 13:47:13 -0800, Seri wrote:

That just shows there is a reasonable supply of gas before the main governor.

And that was at the meter outlet. There is something very wrong with the meter, governor or service pipe. Time to call TRANSCO 0800 111999
--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at www.diyfaq.org.uk
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A reasonable "pressure". The supply is the flow, which is not there.

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If the hob pipe goes right back to the meter, then either:
1. Check the supply pipe size to the hob. If undersized then replace a part of it with 22mm. 2. Get Transco in. Sounds as if the regulator/supply is suspect.
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A really big thank you to the group.
I just had Transco in and they've confirmed there's a problem with the supply, they changed the regulator but that's not the problem. It seems that when they replaced all the pipes in this street about a month back, they may have put a kink or some slight blockage in our supply by accident.
Either way, our gas is now capped off, and Transco are supposed to be out to dig up out street again in a little while.
So, once again, a huge thank you to everyone on the group for helping me troubleshoot this and find a solution.
Thanks again
Seri
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On Wed, 15 Dec 2004 10:53:57 -0800, Seri wrote:

I take it that you've read the FAQ. ? It might be that the old cooker was more tolerant of pressure variations. You need to find out where the big changes of pressure are coming from. It even be that the main governor is failing. It is also just as possible that you have inadequate installation pipework.
--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at www.diyfaq.org.uk
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Take the hob supply right back to the meter.
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