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.
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
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.
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)
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...
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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