Baxi Fires Division 740FS Installer's Manual page 10

BS1289 and BS EN 1858
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 BS EN 1856
Part 1. The liner must be sealed to the surrounding area above the fireplace opening and
©
Baxi Heating U.K. Limited 2007.
INSTALLER GUIDE
Figure 3.Precast or clay flue block system
Page 10