Slant/Fin Gas Steam - Manual (Page 4)

4
VENTING A GAS-FIRED SYSTEM
A boiler venting system provides an escape path for the products
of combustion. There are three major components for venting a
gas-fired Caravan: a draft hood for each module, a breeching
manifold and a chimney.
Draft Hood
The draft hood is part of each boiler module, although shipped in
a separate carton. It compensates for excessive draft. The dimen-
sional relationships between the draft hood, the boiler and sur-
face on which the boiler is mounted, are critical. These factors
are all interdependent in allowing the draft hood to function with-
out moving parts. Attempts to alter these dimensions will result in
unsafe operating conditions.
Breeching
Breeching is a term used to describe a manifold(s) that connects
individual boiler modules to a chimney. Breeching is usually con-
structed of sheet metal having a smooth interior surface with all
joints made tight against leakage. Pitch breeching up toward
chimney. Horizontal breeching size should be maintained for the
total length. Connections from the modules into the breeching are
best made at 45˚ angles in the direction of the chimney.
See figure 3.
GXHT600Z 2 GXH300 12” 10” 10” 9”
GXHT900Z 3 GXH300 14” 12” 12” 12”
GXHT1200Z 4 GXH300 16” 14” 14” 12”
GXHT1500Z 5 GXH300 16” 16” 16” 14”
Model No.
No. of
Modules
Module
No.
20
Feet
Chimney Height*
Table 2: Breeching dimensions: gas fired systems
25
Feet
30
Feet
50
Feet
Consist of
Based on single row of modules with no elbows.
• Height measured from the top of draft hood to top of chimney
Note: For conditions other than those shown, consult factory, or National
fuel gas code NFPA 54.
1,800,000 18” 18” 18” 16”
2,100,000 20” 18” 18” 16”
2,400,000 20” 20” 20” 18”
2,700,000 22” 22” 20” 18’
3,000,000 24” 22” 22” 20”
3,300,000 24” 24” 22” 20”
3,600,000 26” 24” 24” 20”
3,900,000 26” 26” 24” 22”
4,200,000 26” 26” 24” 22”
4,500,000 28” 26” 26” 24”
BTU Input
20
Feet
Chimney Height*
Table 2A: Chimney and breeching sizing for multiple banks **
25
Feet
30
Feet
50
Feet
•• Based upon system total BTU input
The bottom portion of the breeching should be installed at least 3
feet above the top of modules’ draft hoods. If this height cannot be
obtained, install the breeching as high as possible. See Page 5,
Table 3 for heights less than 3 feet. The portion of chimney height
rising directly above each draft hood is most effective in preventing
flue gas spillage. When the boiler room is in an unheated space,
insulate vent pipe and breeching to maintain flue gas temperature
and good chimney operation. Do not lower draft hoods’ skirt
height. For some combinations of low chimneys, low ceilings and
long breeching, a draft inducer may be needed. Inducers, when
required, are locally specified and warrantied. The breeching should
extend into but not beyond the chimney liner.
Chimney
Caravan gas-fired modular boilers operate efficiently with masonry
or prefabricated chimneys as well as with “Type B” or double-walled
metal vent pipe. The latter type of chimney construction is generally
the least expensive. Chimney height is usually governed by the
building height and the size of the boilers (See Table 1). However,
the chimney should be high enough to minimize the effects of turbu-
lent winds and pressure common near rooftop obstructions. A vent
cap should be used where permitted for additional protection
against adverse wind conditions and precipitation. The National
Board of Fire Underwriters recommends that the chimney should
extend at least 3 feet above the roof and be 2 feet higher than any
obstruction within 10 feet.
Sizing horizontal breeching connectors and chimneys for
gas-fired systems
Horizontal breeching connectors shall be constant sized. The chim-
ney and the horizontal breeching connector are sized using Table 2.
When there are multiple banks of boilers, the horizontal breeching
connector for each bank is sized using Table 2. To size the common
horizontal breeching connector, add up the total input and refer to
Table 2A.
The minimum chimney opening will be equal to the size of the
largest horizontal breeching section connected to it.
Figure 3: Suggested venting system constructions.