Section 3 — Stack and Intake Vent Sizing and Installation
3-8 Part No. 750-363
These installations utilize the boiler-mounted blower to draw
combustion air from outside and vent combustion products to the
outside.
To prevent accumulation of condensation in the vent, it is required
to install the horizontal portion of vent with a slight slope of at least
1/4” per foot of horizontal run, pitched either back to the boiler or
to a low point equipped with suitable condensate trap and drain.
3.4 VENTING FOR MULTIPLE UNITS
Cleaver-Brooks recommends that each model CFLC in a multiple
boiler installation be vented individually for safe, reliable, and
optimum combustion performance. If common flue venting is the
only feasible solution, ensure an unrestricted flow of flue gas from
each boiler and a draft within the allowable tolerances.
To help maintain balanced draft conditions in common flue vented
CFLC boiler systems, isolation dampers (flue or combustion air)
with “prove open” switches are required. The damper prove must be
wired in the boiler's safety interlock circuit.
When there is insufficient (or excessive) vertical flue stack to ensure
neutral draft, active draft controls are recommended for common
flue vented CFLC boilers.
Ductwork sizing, layout, and connections require special attention
in common flue venting systems. Good design practice should be
followed such as the use of 45 degree wye connections (not 'tees')
for boiler branch connections into common flue duct. The number
of elbows should be kept to a minimum.
Common flue venting should always be larger than the boiler branch
connection joining it. As a rule-of-thumb, the cross-sectional area of
any ductwork downstream of a wye branch connection should be
equal to or greater than the combined area of the incoming vent
sections.
See Examples 1-3 below for suggested venting arrangements.
When multiple CFLC boiler are connected in a Falcon Lead-Lag
network, a 'Fan rate during off cycle' feature is available. When
enabled, a boiler that goes off line and completes its post purge will
continue to operate the combustion air blower at a user-selectable
rate. This feature provides a further measure when needed to
prevent flue gases from flowing back into the boiler and room.
To ensure safe and reliable operation of multiple CFLC boilers in a
common flue venting system, please contact the Cleaver-Brooks
authorized representative. Cleaver-Brooks offers proven flue
systems for all Cleaver-Brooks models through its Exhaust Solutions
group.
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