Anybody here knows about gas/boilers

To make a long story short, I got nPower to do me a replacement boiler. One of the reasons is they give a fixed quote, and I knew the gas pipe wasn’t right, so they’re supposed to do all the work they need to within the quote.
Also, I know nothing about gas, boilers or plumbing, and don’t need to pay until it’s all done properly apparently.

Anyhow. I’ve got a 15mm pipe coming in the old boiler. The new boiler is a Glow Worm combi (CX30 or something), and should have a 22mm. I’m told all new boilers should.
The engineer who’s here now has a poke around and figured there’s a 22mm pipe just above the false ceiling that then goes into a 15 mm down to the boiler. He says “he’s ok to use up to a meter of 15mm pipe”.
Sounds a bit odd to me. As I said, don’t know about gas, but my understanding of physics is that the gas will only go as fast as the smallest pipe.

So. Am I being bullsh**ed or is it ok? Does anybody know?

Ask Mark (TDJ), an expert in this field :smiley:

I’d be suspicious of this, too.

I’m sure there’s a reason that new boilers have 22mm inlets .

I suppose the answer on whether you could use 15mm pipe depends on what other gas appliances you have in the house, distance from the supply, bends in the pipe etc.

Definitely needs an expert, this :slight_smile:

JohnP might have an idea

It’s all about gas flow and pressure that the 22mm pipe can deliver to the boiler. From momory of talking to my dad (was corgi registered fitter for years) whilst doing our swap it’s 1mbar of pressure needed by most new boliers. Under pressure screws the combustions rate/CO emmissions and possibly the bolier long term.


I have just replaced my old boiler with a new one and had almost exactly the same issues.

I fretted for ages that I had only 15mm supplying the boiler and did not have a clue how far back down the pipes I would have to dig to get to a 22mm supply.

My plumber - also Corgi registered - said that it is OK to use about 1m of 15mm from a 22mm supply (made me scratch my head too, but if two plumbers are saying it independently . . . )!

I’ve forgotten by physics on this but it is a straightforward equation to calculate the flow/pressure difference a bit of 15mm will make.

Weirdly though my situation turned out fine as I had 22mm just below the boiler anyway but, amazingly, my new Vaillant boiler has a 15mm inlet and does not need a 22mm supply - checked with Vaillant.

I’m not sure how far along you have taken things but I did a lot of research on boiler reliability and Glo-worm were about the worse out there (I was getting rid of a Glo-Worm as it was a dog!). If you can afford to pay a bit more a Vaillant or Worcester-Bosch one will be much better.

Good luck



I couldn’t leave this one alone . . .

The size of the pipe required to correctly supply your boiler is dependent on the size of the boiler and the amount of gas it will demand to work correctly.

The smaller its capacity (measured in Kw) the less gas it will need and the longer length of 15mm pipe that will be acceptable. Bends and joins in the supply pipe affect it’s carrying capacity as will other appliances on the same circuit. Therefore a big boiler will need 22mm supply all the way but a small one might be OK with a run of 15mm (as long as there are not too many turns etc or other appliances (hob etc).

Your fitter needs to do a proper calculation that compares the boilers capacity with the feeding supply. If you are in any doubt ask him to show you how he has worked it out . . plumbers have tables and calculators for such things - if yours doesn’t have these or can’t ‘show you his working out’ check to see if he has a holster and big hat!

I’m assuming he is GasSafe registered?

From my basic memory of A level physics wouldn’t the same volume of gas running through a narrower pipe be at a higher pressure than a wider pipe?

Or was that a lower pressure but higher velocity? (from my knowledge of polymer dynamics)

Yes, but the pressure is fixed so the volume of gas available is what’s in question depending on the pipe used.

Thanks for the replies guys.
I’m checking with Glow Worm, and as I’m using the 6 month interest free scheme NPower is doing, the credit company won’t pay nPower until I sign an agreement I’m happy the work is completed (one reason I went with nPower), so I can make sure it’s right.

I did some research on the boilers, and was tempted to go for a Vaillant instead, but because space issues, the Glow Worm works better (more compact, low ceilings in small kitchen). From what I read as well, the opinions are all over the place. It’s generally agreed they used to be crap boilers but since they were bought by Vailant they make better boilers apparently.
We’ll see how it pans out. Can’t be worse than the ruddy Ariston that was their before.

Couple of bits.

Yes, the length of 15 mm pipe will limit the gas flow, it’s about frictional loss which is quite critical on low pressure fluids systems, and the number of bends and albows add to problems quite remarkably.

But then I have about 8 mtrs of 15 mm pipe with 4 or 5 bends and the boiler works quite well, thank you. It shouldn’t but does and I cant be a*sed to change it even if I was allowed to.

Glow Worm boilers are pretty good now. (They were cwap.)

If it’s a condensing boiler, expect problems at three years old and budget for that

Aristons. What can I say? Changing that was a wise choice. Changing it for a wind up timer, a Bunsen burner and kettle would almost be a wise choice.

I reckon your installer knows what he’s doing.

Thanks Oldguy.
As always an excellent reply from a wise man :slight_smile:

That was a very interesting and uninformed read.:slight_smile:

PM sent.

Sounds like a gas explosion might be in the pipeline.

I’ve got a GlowWorm combi boiler, and a 22mm gas supply pipe. Near the boiler (some last 50cm) the pipe is 15mm. When I saw the guy doing this I called BritishGas (as an independent company) and asked whether they would do this as well. They said that up to 1m of 15mm pipe is OK.

It seems to me that, by using the 22mm main supply, enough pressure is being built to pass a required volume of gas through up to 1m of 15mm pipe. I’m probably wrong here…

No. Your right.

FYI Vaillant own Glowworm :slight_smile: the regs require that there is no more than 1 mb of pressure drop across the length of the gas pipe, gas pipe size is worked out using tables and is dependent on the input load in cubic metres of gas.the engineer has to work out the overall length of the supply pipe, including an allowance for each fitting, which is given in the tables.the tables will say how much volume of gas can be carried over what distance by each diameter pipe.the manufacturers can supply the final connection in what ever size they want, Glowworm and some others use 22mm to encourage the use of 22mm pipe up to the boiler inlet,other companies (inc Vaillant) use 15mm. as long as the supply pipe under full load has less than 1mb drop in pressure over its length, and the minimum inlet pressure required by the manufacturer is obtained, then there is no need to go straight to the boiler in 22mm, 1m of 15mm pipe will not normally effect pressure on this size of appliance, assuming that there are no bends in that 1m, if there are then you can quickly go beyound the capacity of the pipe. simple innit :slight_smile: a simple pressure test will prove whether your supply is correctly fitted.

I could not have put that more eloquently myself.

Yup. What he said, but put more simply, keep the 15mm pipe as short and staight as possible.

(Still think your fitter knows what he’s doing.)

Don’t mind me asking but how much is it?

Was wondering how much it would be to get a new system fitted in, so a Valliant boiler new pipes and radiators, 5 radiators, 1 long radiator, 1 wall mounted for bathroom, so 7 radiators in total.