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3.2.3 To a precast concrete or
clay flue block system
conforming to BS1289 with
dimensions as in figure 3. The
current versions of BS1289 and
BS EN 1806 recommend that
there should be an air space or
insulation between the flue
blocks and the plaster because
heat transfer may cause
cracking on directly plastered
flues. However, generally this
appliance is suitable for
installations under all
circumstances unless there is a
history of cracking problems.
Remember that faults such as
cracking may be caused by
poorly built and restrictive
flues, e.g. mortar extrusions, too
many bends, flue heights below three metres, restrictive terminations etc.
3.2.4 If the fireplace opening is greater than the acceptable dimensions given in this
guide, do not use the back of a fire surround or marble to reduce the opening. This may
cause cracking of the surround back or marble.
3.3
Suitable flues and minimum flue sizes are as follows: -
It should be noted that, as with many appliances, sharp bends or horizontal runs in
metal flues at the top of the system can be a cause of problems in these types of
installation.
225mm x 225mm conventional brick flue.
!
175mm diameter lined brick or stone flue.
!
200mm diameter factory made insulated flue manufactured to BS4543.
!
175mm diameter flue pipe. See BS6461 Part 1 for suitable materials.
!
Single wall, twin wall or flexible flue liner with a minimum diameter of 125mm.
!
The materials to be used are stainless steel or aluminium as specified in BS715. The
liner must be sealed to the surrounding area above the fireplace opening and to the top
of the chimney. An approved terminal must be fitted.
A properly constructed precast concrete or clay flue system conforming to BS1289
!
or BS EN 1806. This system is only suitable if the conditions stated in section 3.2.3 are
met.
INSTALLER GUIDE
Figure 2. Metal flue box dimensions
Page 7
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