Gas supply to Combi Boiler

1 Dec 2005
Reaction score
United Kingdom
Hello Chaps

I have a quick question regarding the diameter of the gas supply pipe to combi boilers that I'm sure will be obvious to a pro' but not to me.

I'm in the process of choosing a new combi to replace an ageing boiler which serves a conventional system. The existing gas feed is taken from the meter by means of 15mm pipe which runs under a suspended floor and then under a screed floor in an extension.

I would prefer not to incur the additional inconvenience and expense of taking a new 22mm supply from the gas meter but as far as I can tell most Combis today on the market have a 22mm gas inlet. I've found four suitable models that have a 15mm gas connections. Does this therefore imply that these particular boilers will cope with a gas feed through 15mm pipe or am I missing something here.

Thanks in advance for your advice.

No hat.
Sponsored Links
You don't have a hope of running any combi boiler through more than a couple of metres of 15mm gas pipe.

The size of the inlet connection makes no difference - except that the small ones are a pain because of their resistance. Combis are 23kW to 40kW, whereas a conventional boiler for a 3 bed semi may only be 14kW, and bigger boilers need disproportionately bigger gas supplies.
(1kW = 3412 Btu/hr)

The distance you can go with a given pipe size depends on the boiler demand, the number of bends, and what else shares the supply. It is extremely common to find that pipes are too small. The pressure drops predicted from published tables are plain wrong - the measured drops are always higher.
For high performance combis it is common to use 28mm pipe for the bulk of the run. Sometimes a larger size is needed.

There are several issues here:

  • 1) Corgi have requirements about pressure drops in pipes. Any instalation is liable to be inspected and the installer is in trouble if the pressure drop in the supply pipe at full load conditions, is too high.

    2) Many old combis would actually run with the lower pressures caused by restricted supplies, but a lot of modern ones will not.

    3) Even if the boiler seems to run ok, it could be restricted by the supply. If this is the case not only will the hot water performance be limited, but the burner may not be burning properly, with entirely bad consequences!

    4) You're liable to find that your guarantee on the boiler is invalid.
Providing a new big gas pipe can be expensive in terms of damage done to the building. Don't forget that they don't freeze so can go in lofts or outside.
Many thanks Chris, I'm not entirely surprised by this. There goes the floor!

Thanks again.

interestingly enough vaillant were quite happy with15mm going to the combi.
Put in 22mm anyway.
I have no faith in vaillant right now, their cust service is dire, or the communication within it.
Sponsored Links
I don't care what Valiant are happy with , so long as it receives the correct gas pressure.
It is important to us though as RGI's, because of boiler performance, the manufactures and keeping the dog happy :LOL:

I'm am sure lots of customers think we are trying to hike up the price, when informed of the cost of having to upgrade the supply.
I didn't say you couldn't use 15mm!

Not impressed with Vaillant at the mo either. Discovered that their combis are the only ones I came across which do not always modulate on HW. So if you get a bit of sludge or scale your shower goes hot, cold, hot, cold...

And their diverter valves (latest type, and they've tried about 5) on the 824's are still packing up early. The ones on the Ecotec are another similar Giannoni, but with a placcy lower bit - oh dear.
Hey gas man chill!
I enquired with vaillant as the specs say 15mm for my eco tec "pro"!.
My RGI wanted to put 22mm in and I needed to know whetheer it was unnecessary work (floor boards - nice ones as well), however I listened to my rgi and now have a lovely supply of gas.

As for Vaillant - they fitted 4 "aqua sensors" in as there is a small fan which rattles around like a madman when the hot water is on.
All 4 were bad. They say they've never heard of this problem before.
Thing is I paid best part of £800 for what I thought was quality. Well not so impressed. The timer which was supplied has started to go yellow like old plastic. I sent them an email asking when I would recieve my new one. Cheeky blinders only told me to ring 0870........ I told them to ring me, ever heard of customer service.
To top it all;
The diverter valve is making noises as well, the pump is loud and vibrates,

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local