and why there are Accident and Emergency Departments,
Insurers can replace items. They cannot restore life.
If that is true then (a) the boiler is not condensing and (b) the gas being burnt is more pure hydrogen fuel than hydrocarbon fuel
CO is retained in the red blood cells rendering them useless, very low percentages of CO will be fatal if the exposure is long enough that 60% (?) of the red blood cells are affected.
And if, as you seem to be implying, a 10mm diameter hole in a flue pipe is so harmless then why is there so nuch fuss made about inspecting flues for leaky joints or damage,