APPENDIX A: BREECHING/CHIMNEY SIZING
DRAFT REQUIREMENT
:
For MG heating plants, the
chimney-breeching system should produce -06" to -08"
W.C. (as measured with draft gauge) in vent connector
and -01" to -03" W.C. in combustion chamber.
BREECHING LENGTH
Maximum breeching length (from the furthest module to
the chimney) for each bank of boilers is limited by the
chimney height. As a general rule, maximum breeching
length should not exceed 75% of chimney height; this can
be superseded by specially engineered venting systems.
For power gas heating plants, chimney height is mea-
sured from base on which the module sits. Therefore, it is
important to locate the heating plant as close to the chim-
ney as possible. To minimize breeching length and corre-
sponding chimney height requirement only 6" required
between chimney and first module.
Example:
Let’s assume a MG-1925A heating plant has
been specified and the chimney height is limited to 21-
feet due to building constraints (see Figure A1). The
breeching length for the heating plant should not exceed
15.75 feet. Using the chart in Figure A2, an MG-1925A
heating plant requires approximately 9-feet in length plus
a 6" minimum clearance between the first module and the
chimney; thus, breeching length is compatible with the
allowable chimney height.
Breeching Length Formula:
For your installation.
1. _________ ft. x 75% = _____________ft.
(Chimney Ht.) (Max. Breech Lg.)
2. _______ft. + __________ft. = _____________ft.
(Plant Lg.) (From 1st Mod (Req. Breech Lg.)
To Chimney)
3. If required length is equal to or less than maximum
length, application is O.K. If required length exceeds
maximum length, then you would have to consider ap-
propriately dividing the heating plant or bank of boilers.
BREECHING DIAMETER
1. Determine if one full size breeching, which is preferred,
or a tapered transition to a smaller diameter will be used
in the breeching run to serve the particular heating plant
or bank of boilers.
2. Determine the number of vent connectors being ac-
commodated by each section of the breeching (one vent
connector per module).
3. Treating each section separately, determine the correct
breeching size from sizing table in Figure A2. (Size is
based on heating plant operating at sea level; first module
6" from chimney; breeching full size and insulated from top
of draft regulator to chimney entrance; and 1-ft minimum
vent connector height. The recommendations in Figure A3
are not to be used for any other configuration; for non-stan-
dard configurations, consult Hydrotherm factory directly.)
Example:
Let’s assume a MG-2310A heating plant.
1. Breeching will be circular with a tapered transition,
identified as Section A and Section B (see Figure A3).
2. The number of vent connectors feeding into Section A
is 6 (all of the vent connectors in the heating plant). The
number of vent connectors feeding into Section B is 3.
3. For Section A size, under the heading “No. of Modules”
in Figure A2, go down to 6 and read across to the col-
umn, “Breeching Sizing,” where you’ll find that Section A
MG-1925 HEATING PLANT
SHOULD NOT EXCEED
(75% x 21 FT) = 15.75 FT
21 FEET
9 FEET
6"
FIGURE A1
18
Boiler
Model
Length
3' 7"
5' 4"
7' 1"
8' 10"
10' 7"
MG-770
MG-1155
MG-1540
MG-1925
MG-2310
No. of
Modules
Breeching Sizing (Minimum)
Circular
Dia. In. I.D.
Rectangular
Sq. In. I.D.
2
3
4
5
6
10
12
13
14
15
86
124
146
169
194
FIGURE A2
SECTION A
6 CONNECTORS
(15" DIAMETER)
SECTION B
3 CONNECTORS
(12" DIAMETER)
MG-2310 HEATING PLANT
FIGURE A3