Fresh Air
7
Combustion Air Supply
(Continued)
Combustion air opening (gas-fired installations)
Install a combustion air opening. Ensure that all code requirements are met.
Minimum round duct diameters are as follows:
Boiler Model
Model No.
VB2-18
VB2-22
VB2-33
VB2-40
VB2-50
VB2-63
Round duct
for combustion air supply
*
Ø”
4
4
5
6
7
7
*
These figures are based on a
minimum equivalent duct length of
20 ft/6 m. For equivalent duct lengths
in excess of 20 ft/6 m, up to and
including 50 ft/15 m, increase round
duct diameter by one size.
1.
Size combustion air opening
according to chart above.
2.
Corrosion resistant screens must not
have individual grids less than
¼” x ¼” / 6 mm x 6 mm. Screens or
louvers must not reduce the free area
required for combustion air. Increase
duct diameter if louvres are used.
3.
A combustion air opening from the
outdoors shall be located not less
than 12 inches / 30 cm above the
outside grade.
4.
The outside inlet of the combustion
air duct must not permit the entry of
rain or wind and must not reduce the
free area of the air opening. It must
not be blocked by snow.
5.
Manually operated combustion air
dampers are
not
allowed. The air
opening must not be of a type that
could lead to situations where the air
supply may be closed off while the
burners are in operation.
6.
If the combined MBH input exceeds
400 MBH for all gas burning
appliances, the following excerpt (to
the right) from the CAN/CGA B-149
Code may be used.
The heating contractor must check with
local authorities (municipal building
department, gas utility) for combustion
air requirements particular to the area.
Combustion air supply shall have a
cross sectional area of not less than
100 inches
2
/ 650 cm
2
plus 1 inch
2
/
6½ cm
2
for each 14 MBH in excess
of 400 MBH. This/these opening(s)
shall be either located at, or ducted
to, a point neither more than 18
inches / 46 cm nor less than 6 inches
/ 16 cm above the floor level.
Ventilation air opening (gas-fired installations)
In addition to the combustion air
opening, a ventilation air opening should
be installed at the highest practical
point communicating with the outdoors
with a minimum area of 10% of the
combustion air opening or 10 inches
2
/
65 cm
2
, whichever is greater.
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