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SlimFit
Series 2
®
DIRECT VENT — Boiler room air openings
(Category IV)
Combustion/ventilation air provision
For direct vent installations, combustion air must be ducted
directly from outside to the SlimFit boiler air intake fitting. Two
options are available: sidewall or vertical direct vent. Though
combustion air is ducted to the SlimFit boiler, the boiler room will
still require air openings, either for ventilation or for combustion
air for other appliances plus ventilation.
Sizing air openings
Air openings provide for ventilation (as well as combustion air) to
prevent overheating of the boiler controls and boiler space. Air
is also needed for other appliances located in the same space.
Use Figure 32, page 37, selecting the appropriate installation
conditions.
Air openings must be sized to handle all appli-
ances and air movers (exhaust fans, etc.) using
the air supply.
The sizing given in Figure 32, page 37 is based on the National
Fuel Gas Code - ANSI Z223.1/NFPA54 – latest edition, allow-
ing adequate air opening for gravity-vented gas appliances
(Category I) when located in the same space as the SlimFit
boiler.
When outside air is ducted as directed in this manual, the SlimFit
boiler is rated as direct vent (pressurized vent), and has varying
requirements for combustion and ventilation air, reflected by the
special sizing instructions given in Figure 32, page 37.
The air openings recommended in Figure 32, page 37 will al-
low adequate ventilation and combustion air provided the boiler
room is not subjected to negative pressure due to exhaust fans
or other mechanical ventilation devices. Refer to the National
Fuel Gas Code for dealing with other conditions.
Louver allowance
The free area of openings means the area after reduction
for any installed louvers or grilles. Be sure to consider this
reduction when sizing the air openings.
Special considerations
Tight construction
ANSI Z223.1/NFPA54 defines unusually tight construc-
tion where:
1. Walls and ceilings exposed to the outside atmosphere
have a continuous water vapor retarder with a rating
of 1 perm or less with openings gasketed, and . . .
2. Weather-stripping has been added on openable win-
dows and doors, and . . .
3. Caulking or sealants are applied to areas such as
joints around windows and door frames, between sole
plates and floors, between wall-ceiling joints, between
wall panels, at penetrations for plumbing, electrical,
and gas lines, and in other openings.
For buildings with such construction, provide air openings
into the building from outside, sized per the appropriate
case in Figure 32, page 37 if appliances are to use inside
air for combustion and ventilation.
36
commercial condensing gas
— Boiler Manual
-
fired water boiler
Exhaust fans and air movers
The appliance space must never be under a negative pressure
unless all appliances are installed as direct vent. Always provide
air openings sized not only to the dimensions required for the firing
rate of all appliances, but also to handle the air movement rate of
the exhaust fans or air movers using air from the building or space.
Motorized air dampers
See "Motorized air dampers" on page 30 for recommenda-
tions on wiring motorized air dampers.
Manifolded combustion air option
1. Multiple SlimFit boilers can use a common combustion air
manifold.
a. Size the duct to provide at least 1 square inch per
1,000 Btuh total input of connected boilers.
b. Provide minimum clearance to adjacent vents and
grade/snow line as shown in Figure 31.
c. Provide minimum free area in duct (adjusted for
louver restriction) of
total boiler input, or divide the total MBH (1000's
Btuh) by 2.
d. Example: For an MBS system with total input of
3,000,000 Btuh, or 3,000 MBH: Divide 3,000 by 2 =
1500 square inches minimum for a combined air duct.
e. If combustion air damper is used, wire to boilers to
prevent operation except after damper has opened
(see Motorized air dampers, above).
ONLY air piping can be combined DO NOT use
combined Category IV vent piping Flue gas leak-
age and boiler component damage can occur
Failure to comply could result in severe personal
injury, death or substantial property damage
Figure 31
Manifolded combustion air option
1 square inch per 2,000 Btuh
Part number 550-100-561/0224
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