recently i have updated a conventional boiler with a vaillant 831, the gas pipe is under floor and about 10m long, it looks ok so i did not replace the gas pipe, however after replacing the boiler i found that the inlet pressure is only 17mb, meter working pressure is 21mb, aothough the minimum requirement is only 16mb for vaillant and the boiler should work safely(flow rate is ok), the pressure drop is aoubt 4mb, must i replace the gas pipe under the floor boards? thanks all!