I need to replace my boiler. When he came to fit it, he said that it has a 15mm inlet pipe and the new Valiant needs a 28mm pipe. Is this correct?

the boiler needs a supply of gas as dictated by the the gas supply. it is all about pipe sizing for the property and the need of all appliances

i have never known any domestic boiler ever to have a 28mm gas conection at the boiler!!!22mm or 15mm yes.tom clydebank plumbing&heating

he may mean 28mm from the gas meter end depending how far the distance is from the meter to the boiler

Yes, I agree. He is talking about upgrading the size of the pipe from the gas meter to the boiler as the 15mm pipe may not deliver enough gas to your new boiler. This is very common these days.

This does sound very normal to upgrade your domestic gas inlet supply to 22mm. Depending on which boiler you replace the existing with, not model 'Vailaint' as you mentioned but style. For example a 'combi boiler' would draw off more gas in a shorter period of time as the hot water is instantaneously heated, as opposed to a 'heat only' or 'system boiler' where they would heat a HW cylinder over a period up to an hour. Little behold to upsize a 15mm gas pipe to 22mm is not a difficult job. Hope this helps. DC Plumbing & Heating.

MAKES NO DIFFERENCE about boiler and pipe sizing... its about the volume of gas and how many meters the gas box is away and valiant do show clearly in there book (off the top of my head) that it requires 19 mb so it is completely common to need 28mm gas pipe!