Gas supply 15mm into 22mm

25 Oct 2007
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United Kingdom

Just had a new Combi boiler fitted instead of a conventional boiler as an in place replacement. As with other people have been told I needed a 22mm gas feed. What I don't understand is that I have a 15mm pipe coming from my main gas supply to which my plumber has attached a 22mm pipe which runs to the boiler.

How does this give the extra supply? Surely it will still be a 15mm supply so I might as well have used the original 15mm supply pipe.

Shouldn't he have upgraded the 15mm pipe from the gas box too?

Any help would be much appreciated.
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With out trying to describe pipesizing and gas rates/volumes, as long as the 15mm that is left in will allow the correct volume of gas and doesnt give more than 1mbar drop accross the whole pipe run between meter and inlet of boiler in theory that length of 15mm could be next to the meter, at the boiler connection of even in the midddle of the run.

However, some manufacturers state a dedicated 22mm supply, if the boiler gas union is for 22mm then thats what the feed should be and not reduced before the boiler.

Personally we would have changed the bit near the meter also but without seeing the job who knows why its been done that way.

All of the above is CORGI stuff anyway.
its always better to have the gas pipe getting smaller away from the meter but as lcgs correctly states, the position of the 15mm doesn't matter (apart from the reducers).

However, last year myself and my dad changed my parent's boiler and that was a 15mm feed. I worked out that you could only get about 4 meters of pipe before the 15mm was too small.

So hopefully your boiler is close to the meter, else you might bet burner issues on full load.
sanj. was it yourself that recently gave a very detailed insight into this with the drawings of the flow and turbalences the reducers cause.
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Thanks lcgs,

The 15mm pipe runs from the meter. This pipe has then had an elbow attached so it now goes in two directions.

The 15mm bit continues on its original course off to the cooker. At the elbow it goes off into a 22mm pipe that then goes off to the boiler but then gets reduced back down at the gas inlet on the boiler to 15mm. So it seems a bit pointless (not knowing about flow rates etc) having a chunk of 22m in the middle.

The boiler technical manual says do not use less than a 15mm pipe but does not state a dedicated supply. The Gas inlet spec says 15mm. Its a Vaillant ecotec plus 831.

One more question if I may. What is the score with regards to running copper pipes on the exterior of houses (which I now have)? Apart from looking unsightly is there any danger of them cracking from the cold and leaking? Isn't it better to have them running in side?
with out seeing it or knowing the lengths of each section of pipe, and from your description of the initial section of i would speculate that this pipe sizing arrangement will be incorrect from the meter especially as the cooker tee is before the 22mm section that then runs to the boiler.

If you could give a rough drawing of the gas run with pipe lengths i'd be able to tell you for deffinate.

External pipe runs are common and are fine, clips should be UV resistant and give clearance from the wall. if your in an area where copper theft is likely re consider this route.

Im urrently on a blocks of flats where all the gas runs are 28/22 and 15mm copper externals. Cant see them lasting till after christmas especially with the good price for scrap.
I would suggest a 28mm from the meter, as you have not explained what other appliances are connected to the gas (if any).

Well, you have... a cooker.

Proper pipe sizing is required as over a 1mbar drop on the installation is against regs.

yeah i posted some stuff about reducers! some of the plumbers seem a little unconvinced still.

Anyway getting back on topic. It sounds like the gas pipe is too small. If you post your lengths then we can judge.

ie meter > elbow > 0.5m>elbow >4m etc
I have sent an image to the admin guys to post to this forum.

In words

From the meter a 15mm pipe approx 3m long.
a tee at 2.5 meter.
22mm pipe runs off for .5m drops approx 1.5m below ground level (three storey house)
turns a corner around the side of the house.
Runs the width of the house approx 8m
Drops another 1.5 m and then comes inside to the boiler.

That might make sense when the picture gets posted.

I feel nervouse about the metal being outside now knowing about the probability of theft.
because he's refering to a meter box i presumed NG altough i know LPG meters can be sited the same
because he's refering to a meter box i presumed NG altough i know LPG meters can be sited the same

If its natural gas it way undersized.

It would need 28mm upto the split and 22mm from there to the last 600mm at the boiler.
Sanj.varah wrote

yeah i posted some stuff about reducers!some of the plumbers seem a little unconvinced still

You failed to answer my final question about velocity and why the tables showed inconsistency's. :rolleyes:
28 from the meter to the mentiond tee position and 22mm to the boiler.

a quick one for the installers, what size sleeve would you use for the 22mm when it enters the house?

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