Gas pressure problem

I have installed a new boiler - ( taking into account everything posted on the news group :) ) The old boiler rating plate was 100,000 BTU, fed from 22mm pipe. The new boiler 80,000 BTU, is fed in exactly the same way. ( except small run of 15mm at the last 1/2 m )
The hob pressure now fluctuates when the boiler lights, whereas it didn't with the old boiler. I can't think why.
The input gas pressure is well over 20mbar (minimum required by boiler) The burner pressure is 13mbar ( as spec ). The boiler takes the correct amount of gas ( from timed meter reading ).
The boiler is about 4m from the meter. The hob is fed from a T down to 15mm over a run of about 12m. T is about 1m from the boiler.
I cant obviously re-fit the old boiler and see whats going on. One thought is that the old boiler simply never switched off and we never noticed it.
Presumably the hob has no local regulation - I havent checked if there's any pressure test points on it. Hints welcomed.
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Whichever is simplest. Separate the hob and boiler supplies and running back to the meter with each. The reason why you now have problems is that the old boiler was probably set to a lower burner pressure, drawing less gas.
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Run both appliances from a 22mm pipe with 15mm branches as close to each as is possible.
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Wrong. Run two separate runs back to the meter.
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snipped-for-privacy@hotmail.com (robf) wrote in message

I would treat the boiler installation as if it was a completely new installation. i.e. Don't assume the pipe sizing is ok from the old boiler.
Just becuase it is a lower powered boiler does not mean it's initial start-up is less demanding.
You need to work out the pipe sizing for the boiler *and* the cooker. This may be covered in the Gas Fitting FAQ? If not some googling should show the calc's needed. If not then let me know and i'll post an example.
This calculation will show you exactly what size piping you can use. I personally wouldn't bother running seperate feeds from each appliance back to the meter. This has been debated a few times and i don't see how it makes any difference where the bottleneck is?
You do need to be a bit careful about this as a gas hob could extinguish when the boiler lights up. The hob will still be switched on...
Let us know how you got on.
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On Mon, 22 Dec 2003 04:59:50 +0000, Reader wrote:

Firstly: Could you find out what the pressure at the meter outlet is during boiler operation. "Well over 20mb" implies well over 21mb at the meter which might be a problem. I take it that the inlet pressure measured at the boiler was whilst the boiler was working? Is the main control valve fully turned on?
It is quite likely that from the sort of sizes and lengths you have there should be no problem with the gas pipe sizing.
There could be a problem with the main govenor. Are there any other valves on the pipe work other then the Main Control Valve?
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Thanks all.
The drawings shown do represent the pipe.
I made a manometer and tested the pressures on the boiler.The burner pressure is correct, 12.9 mbar in spec( read 13mbar on manometer ).
I think I made the mistake of measuring the Static pressure on the main side of the boiler rather than the 'running pressure'
The static pressure blew the water out of the manometer so I assumed it was wellover the 20mbar needed by the boiler.
I will recheck this with the boliler running. But in essense, I have changed very little pipework, and in theory, the boiler should be taking less, although I agree, somone may have turned down the wick on the old boiler to hide the problem.
Old boiler Marathon floor mount - new boiler Ideal Classic ff380.
Both standard non condensing basic boilers. Shame I never measured the consumption before ripping the old one out.
I will let you all know.
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Just have two separate runs back to the meters. All sorted.
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On Tue, 23 Dec 2003 09:39:49 +0000, IMM wrote:

I don't know what sized manometer but the 30 mb type generally only lose their water if they are also inadvertently inclined. The smaller 20mb type (which I doubt the OP has) are not really suitable for gas supply checking only for appliance work.

My patience has worn very thin.
1) There is no need IMHO to repipe the gas installation at the OP's home, nor in proabably several million homes in through the UK. There might be any number of problems with the gas pipes but the general pipe sizing on this layout is not likely to be one of them.
2) Point two if there is a problem with the gas installation (and I can think of several the OP might have) then taking the cooker supply pipe to the meter outlet will NOT solve the OP's problem. Indeed he will have gone to considerable effort and be no better off.
3) Many of the problems the OP might ave a fairly serious problems with Transco's kit and will need their attention before they become worse.
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If standing and working pressures are fine indicating the pressure regulator is fine, then have two runs back to the meter. Although the regulator could be sticking and jumping.
As a matter of course all CH gas runs should be a dedicated supply only for the boiler. I have too much experience of problems when the CH line is teed off. Many of the old area gas boards would only have a dedicated supply for the CH boiler.
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Why? As long as the pipework is sized to the appropriate standard (BS6891 IIRC), that should be that.
Part of it is reproduced on the Copper Development Association web site.
Provided that Transco's equipment is working correctly and the pipework is sized to account for the flow and pressure drop, pipe lengths and sizes, then there is no reason to install a separate pipe all the way back to the meter for the boiler.
I suspect that the only reason that this is done is because the people doing the work are incapable of doing the arithmetic. The customer pays in terms of labour, materials, disruption and damage to decorations.

.andy
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The maintap has to be turned on slowly.
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Sometime after the tenth pint, becoming inadvertently inclined and losing one's water are not uncommon.

Just keep an eye out for the next email offering to extend your manometer by a few inches.

For the first few pints, standing is not normally a problem.
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On 24 Dec 2003 01:49:45 GMT, snipped-for-privacy@cucumber.demon.co.uk (Andrew Gabriel) wrote:

Very good :-)
.andy
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On Wed, 24 Dec 2003 01:49:45 +0000, Andrew Gabriel wrote:

ROFL.
My Email filter says "inches" = 94.4% chance of junk mail "extend" = 98.2% chance of valid mail
So it might just get through. 8-;
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On Mon, 22 Dec 2003 12:46:59 +0000, John Stumbles wrote:

from above after removing the variuos bit that make up the hob burner. A standard 6mm rubber hose can just be persuaded to become precariously attached to the injector.
On some models the test point might be part of the inlet connection. On other models where the controls are mounted on a removable fascia the test point might be lying below that fascia.
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