Dimension/Flow of gas pipes


I am considering having a new central heating boiler.
A guy giving a quote said that he may have to upgrade part of the pipe work run from the meter to the boiler from 22mm to 28 mm to given sufficient flow/pressure at the boiler?
Also Transco (or whoever they are called this week) are repiping the supply to houses on the road where I live
This involves putting new plastic pipes in the old cast iron pipes - obviously reducing the diameter (and the flow/pressure at the meter ???)
The diameter of the new plastic pipe in to my house appears to be about 24 mm.
If someone previously had a certain flow/pressure at their boiler - won't the installation of the narrower pipe in the street effect that?
Can someone please explain how this hangs together please in layman terms I am not really technical.
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Judith wrote:

No
The gas meter governor looks after your supply.
--
Adam



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On 28/03/2012 17:22, Judith wrote:

That may be the case. It will depend on if the maximum gas rate required by the new boiler is larger than that of the last boiler.
Its actually not too difficult to do the sums (although I have noticed lots of gas fitters don't actually do them and just work by rules of thumb). The basic idea is that once gas is flowing in the pipes, it will leave the main governor at the meter at dynamic pressure of 21 mbar above atmospheric (quite a low pressure in reality). It needs to arrive at the appliance at 20 mbar or more. So the pipe work can lose up to 1 mbar.
Each metre of pipework will offer a bit of flow resistance. So the narrower the pipe or the longer the run, the greater the pressure drop. A swept bend will have the same resistance as 0.3m of straight pipe, a 90 degree elbow will be about the same as 0.5m.
So the first job to do is work out the total effective pipe length (i.e. allowing for the bends / elbows etc), then you need the gas rate of the boiler in m^3 per hour. With multiple appliances you need to do the sums in sections working out the combined loss at each branch etc.
So for example using the full 1 mbar drop available, a 6m 15mm pipe will be able to deliver 1.9 cubic metres of gas per hour. A 22 mm pipe the same length will supply up to 5.8 cubes, and a 28mm pipe will do 12.
BS 6891 (and others) has the full details of the calculations and tables of the various pipe discharge rates.

Yup.
So a bit less inside...

It may but its unlikely. The pipe supplying the property will deliver the gas at a higher pressure than that used inside the property. Hence it can shift more gas per hour for a given size of pipe than those in your house. The installers will ensure that whatever they do, they will not compromise the pipes ability to deliver enough gas cope with the maximum that your meter can handle.
(its the job of the main governor (the round metal thing in the pipe before your meter) to reduce and regulate the supply pressure to that required in the house)

--
Cheers,

John.

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On Wed, 28 Mar 2012 17:58:53 +0100, John Rumm

Ah, I wondered why gas pressure was expressed as a few mb when atmospheric pressure is around 1 bar, so it's the differential pressure that is expressed.
But that has now got me thinking. According to my hall barometer the atmospheric pressure can change +/- considerably more than 21 mb, so does the governor also prevent my gas hob from sucking all the air in the kitchen into the gas network...
--
Graham.
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Graham. wrote:

Yes, exactly. It maintains 21mBar over atmospheric.
--
Tim Watts

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Pity, because I had a great idea for a gas FIT for flatulent sufferers starting next Sunday.
--
Graham.
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wrote:
<snip>
Thanks - good explanation, which I could actually follow (just :-)
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Another explanation in the form of a presentation <http://www.copperinfo.co.uk/webcasts/domestic-gas-pipework-pipe-sizing/p layer.html>
or
<http://tinyurl.com/cdcc9c4
--
Alan
news2009 admac myzen co uk
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The old gas pipes will probably have been sized for town gas. This needed bigger pipes than natural gas. It is unlikely to have much effect unless you are drivel and want two or more boilers in parallel to get more hot water or heat a pool.

The gas is a lower pressure after passing the regulator at the meter. If there is a longish run they will do some of it in bigger pipe so the pressure doesn't drop below what the boiler wants. If they run it outside make sure they put steel trunking in too to stop the pikeys pinching the pipe.

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