15mm or 22mm gas pipe to combi-boiler?

8 Oct 2005
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United Kingdom
I have had 3 estimates for replacing my old combi-boiler with either the Vaillant ecotec plus 824 or 831.
All of the fitters said that I would need a new 22mm gas feed from the meter, as the 15mm currently installed was not sufficient.
Looking through the Vaillant eco-tec range leaflet on their website, I see that both of these boiler have a 15mm gas inlet. Only the 837 boiler has a 22mm inlet.
So what is the need for 22mm?
Is it new regs or something?
Ta for any help.
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It is needed to allow an adequate gas flow to the boiler.
the last meter or so may be run in 15mm (hence the 15mm connections on boiler) but the majority of the pipe work must be in 22 to allow the applaince to get enough gas to work efficiently and to its max capabilities
It is needed to allow an adequate gas flow to the boiler.

agree, just because the gas inlet is in 15mm doesn't mean the whole supply can be (unless boiler is 300mm from meter outlet). If the supply to the boiler is undersized then the boiler will not use it's full performance. 22MM is essential in most cases, an 837 may require 28mm gas pipework.
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This is interesting, I don't get it though. If you need a 22mm pipe supply, why have a 15mm inlet on the boiler? If the gas can't flow down a 15mm pipe all the way from the meter, how come it can flow down 1 metre of pipe at the end?
IMO it is simply because it is cheaper to use 15mm gas valve . remember correct pipe sizing is up to the installer. it does not matter what the size valve is on the boiler you still need the correct working pressure up to that valve.

if you had a boiler that was so far away from the gas meter that it need to be run in 35mm pipe should the valve be 35mm on the boiler?
also just add the longer the pipe the greater the loss of pressure over its length and each elbow is worth 0.5 meters of loss. so you need to up size the pipe the longer the run is to minimize pressure loss through the pipeing
It is impossible to say from a distance what size pipe you need as it depends on how far the boiler is from the meter, and also on other appliances fed from that pipe.
15 is unlikely as that would only cover a very short distance. 22 is more likely, but could still be too small and 28 needed for long distance and large cooker coming off same pipe.

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