Flue Sizing
It is important to match the internal diameter of the flue with the outlet on the appliance. It should never be
less than the outlet diameter of the appliance. The appliance manufacturer’s chimney sizing recommendations
should always be followed. For free-standing stoves, boilers and cookers up to a maximum of 20KW, that are
not in a fireplace recess, the minimum flue size is 125mm round or square if the appliance has passed the
DEFRA requirements for smokless zones and 150mm if this is not the case.
For open fires with a standard fire opening up to 500mm wide by 550mm high the minimum required flue diameter
is 200mm round or 175mm square. For larger open fires, such as inglenooks, dog grate installations or special
appliances and stoves designed to operate with a fire opening greater than 500mm x 550mm, the flue size should
be at least 15% of the free unobstructed area of the fire opening (including sides if open). Many Decorative Fuel
Effect gas fires (DFE's) that imitate a coal or log burning open fire require the same chimney arrangement as for
solid fuel open fires and must be installed in accordance with British Standard BS5871:Part 3:2005
It is worth noting that in SAP the assumed standing air loss for a flue with an internal diameter of 200mm and
above is 40 cubic meters per hour. With a flue diameter of less than 200mm or a chimney system that incorporates
a damper the ventilation rate is halved to 20 cubic meters per hour. (The damper must only be closed when the
appliance is not in use). This can have a significant impact on the energy rating of a house.
SAP is the Standard Assessment Procedure used to calculate the overall energy efficiency and carbon output
of a house.
Chimney Position
The ideal location for a chimney is on the inside of the building.
That way it can benefit from being kept warm. Chimneys situated
outside the building can be affected by cold weather causing
poor up draught and condensation, particularly if they are
un-insulated. It is therefore important that a cavity wall is
continued around a lined masonry chimney or a factory made
insulated chimney system is used for external applications.
Insulation is Key
One of the keys to a well performing chimney is consistent insulation along the complete length of the flue.
Clay/ceramic, concrete and pumice liners require insulation to be prepared and added on site. Stainless steel
and ceramic system chimneys are supplied with effective insulation. Double wall pumice chimney systems
have an air gap between the walls of the inner and outer block. The air gap combined with the natural insulating
properties of pumice provide effective insulation along the length of the chimney.
SAP is the Standard Assessment Procedure used to calculate the overall
energy efficiency and carbon output of a house.