Condenser Boiler Pipe Diameter? We are looking to replace our existing Worcester combi boiler (about 10...

We are looking to replace our existing Worcester combi boiler (about 10 years old) with a newer condenser model.

Two of the quotes that we received mentioned that the diameter of our gas pipework may not be sufficient.

So my request for information is: is this correct? Our flat is fairly small (5 radiators) and the existing combi boiler works fine with this pipework, main me to think that this may be a bit of a con. It wasn't mentioned by any of the other plumbers who quoted us, either.

It would be a royal discomfort to change the pipework, as it is a listed building, and the pipework itself can be no more than 12 years antiquated, as this was when the building was converted to residential dwellings.
Thanks,
JP
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Answers:    It is see as good practice to have a combination on a 22mm gas pipe. This is not other necessary but I would recommend it as it can effect boiler performance. British gas used to demur to take people on to a contract that have 15mm gas supply. Obviously the smaller the input of the boiler the more chance that 15mm will be fine, check the manufacturers instructions for the boile, (avail on row and see what they say)
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Decide what boiler u want then check gas inlet to it > All boilers will show that info. on the owners manual save on it.(Name plate)