I can assure you that I am quite capable of carrying out complex calculation, I size gas pipework for commercial installations which I can tell you are much more invoved than domestic calculations, but when it is written by the manufacturer what is the minimum size they require I do not question it

There is no relation between joist spans and gas regulations, especially if you can overide manufacturers instructions for joists, because you cannot overide gas manufacturers instructions

heeelllooo do well my son and wellcome

now i am thoroughly intreaged do you have a special reason for wanting to use 15mm pipe
im not a plumber you understand just intreaged

The reasons you can't use 15mm is that it cannot supply enough gas!!

OK, you may be able to prove that it can (in certain circumstances) but the manufacturer's advice is to use 22mm.

This then covers 'worst case scenarios' and gives a reasonable safety margin.

Life's too short to argue about every rule and regulation, I just accept what manufacturers suggest and assume they have done the maths for me.  Like I said, the price difference is minimal (a few pence per metre) and as for 'wasting' copper, well.......................  

There has to be a cut off point when 15mm is not generally suitable and 22mm is suitable.  To calculate all marginal situations is not cost effective and cost is what we are talking about mostly as professional builders/plumbers/electricians etc.

To spend more than 30 seconds trying to prove that 15mm 'will do', and you start to lose money.  Just use 22mm as specified, do the job and type the invoice.  There can be no 'come-backs' as you've used the correct tube.  Anything less and you're open to claims.

The boiler manufacturer would probably cancel any warranty if the gas pipe was incorrectly sized.

I know what you mean though about being told 'NO' without any associated explanation, but for something like this where the cost difference is a couple of quid, I'd just accept it.

As an example of a more extreme situation, we had to build over an existing single storey extension once where the existing foundations were 'inadequate' according to the Building Inspector and he wanted everything underpinned.  The client then employed a structural engineer to 'prove' that the old foundations were adequate.  About £1,000 later the engineer did prove the foundations were adequate and the Council accepted his calculations.  Underpinning would have cost £13,000!!

So in this case there was big money to be saved by querying an instruction, but in the case of your gas pipe that's not so.

Plumber are you removing my posts? A little unsporting don't you think? I responded to a moderator who was rather rude. Perhaps you need to have words with said moderator?

Thank you for your response regarding flow rate down 15mm pipe. I suggested originally that 15mm was okay for 1-2m though.............

I have removed several posts including some from scruff we are not here to start a slanging match, you asked a question and it was answered correctly by scruff you simply asked can 15mm pipe be used, the answer No, you did not ask can 15mm pipe be used and if not why.

Scruff is a very well respected contributer to this forum and his plumbing knowledge is far better than most.

I do not think he was been rude

I feel your question has been answered and an explanation why, I have carried out the calculation and the volume of gas from 15mm pipe is not sufficient to supply the boiler in question, do you now agree that the question has been answered.

Sounds like Charley Farley has passed on and beed re-incarnated.

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Not trying to cause trouble only make people think about things.
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.................I can assure you that I am not CF. His postings on the 'other' forum are very rude and I would not resort to such personal abuse. Unfortunately, Scruffs personal abuse post towards me on this thread was removed by Plumber and so you can't see what he wrote.