If the inlet air is ducted to the outside, then the
lowest edge of the inlet air duct must be at
least 500mm above the outside floor or ground
level, it must also be fitted with an access point
(s) to allow for cleaning and servicing to occur.
The openings in the structure through which
the ductwork passes must be of fire resistant
material and constructed to prevent the
likelihood of any fire from spreading.
In ducted applications the ductwork must be
designed so as to give a static pressure within
the limits stated in section 8 of this manual.
It should be noted that if the static pressure is
too high, nuisance shut-down will occur when
the heater goes out on the overheat limit
thermostat, if the static pressure is too low,
then damage can be caused to the fan motor.
Warm air outlets on ducted applications must
be such that they cannot be closed or become
blocked, which again would lead to an
increase in static pressure and nuisance shut-
down.
The outlets must not be sited so that warm air
can be discharged onto combustible materials,
if necessary, guard rails should be used to
ensure that effected areas are kept clear.
Return air intakes must not be located so that
potentially harmful or hazardous contaminated
air can be drawn into the system.
2.11 Flue system
Warning
Each heater must have its own separate flue,
with a flue diameter of not less than is detailed
in section 8 within this manual.
The minimum vertical length of flue must not
be less than 1m.
The flue should rise vertically, and the number
of bends should be kept to a minimum.
It is strongly advised that BS 5854; 1980, and
BS 5440; parts 1 and 2, are used as
consultative documents when considering flue
requirements.
Care should be taken to ensure that the flue
terminal is not situated in a high-pressure area,
the proximity of buildings and other obstacles
which will influence this must be taken into
account, preferably at the design stage.
Provision must be made for the disconnection
of the flue for inspection and service
requirements, and it is strongly advised that
where bends are fitted inspection covers are
included.
The materials from which the flue is
constructed must be non-combustible,
resistant to internal and external corrosion, and
be capable of withstanding the stresses and
loadings associated with normal use.
When designing the flue system the prevention
of the formation and entrapment of
condensation must be a key consideration.
Twin wall or insulated systems are
recommended, as they tend to inhibit the
formation of condensates.
Where condensation is unavoidable traps
should be included to encourage the
condensates to flow freely to a point from
which they may be released, preferably into a
gully.
The condensate pipe from the flue to the
disposal point must be made from corrosion
resistant pipe of not less than 25mm internal
diameter.
If the flue passes through a wall, ceiling, or
roof made from combustible material then it
has to be sleeved so as to provide a minimum
of a 25mm void between the exterior of the flue
and the internal wall of the sleeve.
The maximum permitted temperature of any
adjacent combustible material is 65oC.
The position of the flue and its terminal should
be such that it does not impair the combustion
process.
It should terminate in an exposed position so
as to allow the escape and dissipation of flue
gases without risk of their re-entering the
property through windows, ventilation ports,
etc.
The flue should extend to at least 1m above
the height of any object within 3,5m of the
