Muddymike wrote on 03/03/2021 :
When our old gas boiler was replaced at our last house the engineer said he
would have to take up the parquet flooring right through the length of our
hall to install a new pipe. We objected telling him to find another route.

He then agreed to test the gas flow through the old pipe and guess what?
Yes it was more than adequate to feed the new boiler.

We had the very same situation, except the guy wanted to charge us to
run an ugly pipe all the way around the outside of the house. I did the
calcs for him and proved him wrong/ unecessary.