Any install of a gas boiler (or a gas fire, gas cooker etc.) which isn't
on an external wall with an external feed is surely potentially going to
involve a gas pipe through a non-ventilated void?
The gas to our boiler in the airing cupboard which has a flue to the roof
certainly goes under the upstairs floor and that isn't a ventilated void.
Cheers
Dave R
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Windows 8.1 on PCSpecialist box