AJC5001 wrote:If the new boiler is going in the same place as the existing one and using the same supply pipe, why can't the pressure be measured at the existing boiler?
Adrian
This is a good point that warrants further comment. You are correct PROVIDED the rate of gas consumption of the proposed new boiler is no higher than the input rating of the existing old boiler. If the new boiler is going to be bigger, i.e. capable of consuming gas faster than the old, the pipe sizing calculation should be carried out.
Most installers don't bother though as the result will usually be 22mm and sometimes 28mm, but as 28mm is so much more expensive, they usually kid themselves they made a mistake and 22mm will be fine. Which it always is as far as whether the boiler actually works, and this is all the customer actually cares about as illustrated by many of the posts in this thread. The non-compliance with the gas regulations will be skirted, worked around and generally ignored, probably for the whole life of the new boiler! One point to beware of though is if the manufacturer is called upon to honour their guarantee, they will sometimes seize on an undersized gas supply to wriggle out of their responsibilities under the guarantee, citing incorrect installation.
If I were still installing boilers I think I would be forced by market conditions and the consumer scepticism expressed in the thread to use undersized gas pipes too, and just take it on the chin and install the odd bigger pipe here and there if/when caught out. It is actually one of the reasons for my decision to drop installation works and concentrate on breakdowns and repairs only, about 15 years ago.