Valor Ultimate Turbochim BR517S Installer's Manual page 14

6.12 Drill the flue hole through the wall. The hole centre should be
on the flue duct centre line. For buildings constructed of brick, stone
etc use a 3½in. (89mm) core drill.
For timber framed buildings a sleeved 25mm gap is required between
the flue duct and the combustible leaf. For horizontal installations,
elbow ducts will be required to keep the gap clear of the wall on
which the appliance is mounted (see section 2.5.1). Drill a 5in
(127mm) dia. hole. The interior surface of the wall where the hole is
exposed must be finished with non-combustible materials.
6.13 If it is intended to recess the fan box into the outer wall, make a
rectangular opening as shown in figure 16. The depth of the recess is
145mm leaving the 50mm minimum projection required outside the
wall.
Note that the fan box is not designed to be recessed into a timber framed
Figure 16
building.
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