2.12 Flue
system
Warning
It
is essential
that
the
products
of
combustion
are flued to the
outside
of the building. 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 3m. 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
,,,,hen 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. See figures 9 -
19.
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 65°C.
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
the
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 terminal.
Flue terminals
should
be fitted
on
all flues, the terminal
must
be of the
approved
type,
and
have
outlet grilles
on
all sides giving a total free area of at least
double
that of the flue.
Caution
It
is imperative that the flue
should
be properly sealed
where
it passes
through
the roof, this can
best
be
achieved
by
using
the
approved
method
of roof flashing plate
and
cravat.
Note
It
should
be
noted
that claims
made
under
warranty
and
attributed
to the ingress of
water
may
not
be considered especially if
an
approved
method
of sealing has
not
been used,
or
if
the
design
of
the flue has
not
made
provision for possible condensation problems.
15
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