Defective or inappropriate flue ductwork serving room-sealed appliances could
cause a build-up of noxious gases, e.g. carbon monoxide. To minimise this risk
flue components, including ductwork and terminals on room sealed appliances,
must be installed according to the manufacturer's instructions. Some flues are
fabricated from corrugated and relatively thin material which is susceptible to
damage if it isn't properly installed.

To minimise the risk of flue gases re-entering a boat it's recommended that flue
terminals and air inlets are not positioned within 500mm (20ins) of ventilators,
opening ports, hatches, or windows. There is also a risk that flammable vapours
could be drawn down into an appliance and for this reason the same separation
of flue terminals/air inlets from re-fuelling fittings or fuel tank vent outlets is
recommended.

It's also recommended that flue terminals are outside areas that could be
enclosed by your boat's canopies. This will help ensure that combustion gases
are safely released from the boat. Making sure that terminals are sufficiently
robust to prevent accidental damage to them and located where the risk of
accidental damage is low will also help prevent combustion gases building up
inside your boat. [8.8]