[SV650rules]

Modern condensing boilers are noisier because they have a fan inside because instead of the flame going from bottom of heat exchanger to the top it is blown from top to the bottom where the coolest ( return flow) part of heat exchanger lives so that moisture in combustion gases can condense and recover heat, and under that is the condensation sump. Also what the don't tell you is that unless return flow is <55 deg C ( but the lower the better, which is why condensing boilers are better for underfloor heating) they do not condense and efficiency is little better than old boiler it replaced. They now recommend oversize radiators and run them cooler. Problem is that domestic hot water needs to be about 62deg C to prevent legionella living in the hot water tank, so boiler needs lower temp for radiators than for water..