So, at 16.5mb the boiler works fine within the makers spec's. So you would rip apart the house to needlessly install a lager bore gas pipe. After this expensive new larger bore pipe is installed, with much disruption, the boiler's performance is exactly the same.so minimum 18mb at the boiler inlet. So on that basis 13.5 and 16.5mb don't meet the regs, whether or not the boilers would work fine at those pressures.
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