Baxi 746 Installer's Manual page 7

INSTALLER GUIDE
the fireplace opening.
3.2.2 To a fireplace incorporating a metal flue
box conforming to BS715 with a minimum
internal depth of 150mm. Incombustible
mineral wool insulation of not less than
50mm thickness must be applied to the top
surface of the firebox (See figure 2) and it
must stand on a non-combustible hearth. See
figure 1.
3.2.3 To a fireplace that has a precast
concrete or clay flue block system
conforming to BS1289 or BS EN 1806. The
appliance is suitable for installations
conforming to older versions of BS1289 as
well as the current edition. The flue blocks
must have a minimum width not less than
63mm and a cross-sectional area not less than
13,000mm 2 . Older editions of BS1289
required a cross-sectional area of 13,000mm 2 .
Figure 2. Metal flue box
The current revision of the standard requires
16,500mm 2 . This appliance is suitable in both cases. The total depth of the opening
measured from the finished front of the fireplace (Including plaster, surround etc.) must
be not less than 120mm (See 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 The following flues are suitable:
225mm x 225mm conventional brick flue.
!
If a flue liner is used, it must be a minimum of 125mm diameter. 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.
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