"bilge" wrote in message
...
I have been told that the 15 mm gas supply pipe to my 8 year old Vaillant
TurboMax vuw 242E should be 20 mm .

Is this correct?

The installation manual only states a minimum of 15 mm.

tia

It depends on the maximum gas requirements of all your appliances and the
diameter, lengths and bends of all your pipework.
There is info on how to calculate it here - (Adobe PDF Document)
http://www.ukcopperboard.co.uk/downl...&%20sizing.pdf
It is important to get it right because if there is not enough gas pressure
available - then an appliance may produce deadly carbon monoxide fumes.
I think it is a good idea to upgrade the pipework to 22mm anyway.

Dave