Appliances recommended as needing a flue are:

• instantaneous water heaters, supplying a bath or shower
• instantaneous water heaters installed in confined spaces
• instantaneous water heaters which serve more than one hot water outlet
• solid fuel appliances
• any appliance which is fitted with a flue spigot.

Defective or inappropriate flues can cause a build up of combustion byproducts,
which may lead to carbon monoxide poisoning. To help prevent this
from happening and to ensure the continued safe operation of your appliance
it's recommended that flues and draught diverters fitted to it are of a type
approved by the manufacturer and properly fitted and maintained. In any case,
flues must be made of suitable material, be maintained in good condition and
be effectively insulated to prevent nearby combustible materials from being
damaged or set alight.

A flue spillage test to check the effectiveness of flues will be carried out as part
of the examination. Information about the test can be found in BS 5482-3.
Insufficient gas pressure can cause the burner flame on an appliance to go out,
causing gas to leak into your boat, which in turn can ignite and start a fire or
explosion. To minimise the risk of this happening you must make sure that
sufficient gas pressure is available to all the appliances. This can be assessed by
ensuring that a satisfactory flame is present at all burners when all burners are lit
and fully open. The examiner will do a flame pattern assesment to test this. [8.2]