Sizing Guide

Manifold Connectors
Vent connectors from a group of appliances on one
level may enter from below or from the side of the
manifold. In either case, the connector rise should be
measured as the vertical distance from the draft-hood
outlet to the lowest level at which the connector enters
the manifold. Care must be exercised in designing
these systems, especially with connector turns and
lengths, because heat loss is apt to be greater for such
systems causing accompanying capacity reduction.
Sizing of Manifolds
As shown in Figure 12, manifolds may be designed
either as (1) tapered or (2) constant size. Choice is
dictated on the basis of convenience and cost.
Tapered
Use total heat input to each portion of manifold under
construction, using V capacities with a 10% reduction
from Common Vent Table at total vent height.
Constant Size
Determine the required size of the common vent, based
on total input and total vent height, using V capacities
with a 10% reduction from Common Vent Table and
then use this size for the entire manifold.
Table Limitations
When three or more appliances are connected to the
same manifold, the largest CANNOT exceed 7 times the
input of the smallest one. No more than eight appli-
ances of identical input may be connected to the same
manifold, unless the Tabled Capacity is reduced by 10%
or using a connector rise of at least 3 feet. Manifold
lengths shall not exceed 1½ feet for each inch of
manifold diameter. This length limitation also
incorporates any offsets in the vertical common vent.
Multiple-Story Venting
A multiple-story vent system serves gas appliances at
two or more different levels of a building. In designing
multiple-story vent systems, use the Vent Connector
and Multiple-Appliance Vent Tables. When properly
designed, such multiple-story vent systems will
function satisfactorily when combinations of one
appliance to all appliances on the system are operating.
Figures 13 and 14 illustrate the major principles of
multiple-story installation, which are as follows:
• The overall system should be divided into smaller
simple combined vent systems for each level, using a
minimum total vent height for each level as illustrated.
• Each vent connector from the appliance to the
common vent should be designed from the Vent
Connector Table as in multiple-appliance vent systems.
• For sizing of the common vent section, the Common
Vent Table is used. The common vent for each
system must be sized large enough to accommodate
the accumulated total input of all appliances
discharging into it, but should NEVER be smaller in
area than the largest section below it.
• The vent connector from the first floor or the lowest
appliance to the common vent is designed as if
terminating at the first tee or interconnection. The
next lowest appliance is considered to have a
combined vent that terminates at the second
interconnection. The same principle continues on to
the highest connecting appliance, with the top-floor
appliance having a total vent height measured to the
outlet of the common vent. The multiple-story
system has no limit in height, as long as the common
vent is sized to accommodate the total input.
Figure 11
INSUFFICIENT
CONNECTOR
RISE WILL
CAUSE
SPILLAGE
ON NO. 5 AND
POSSIBLY ON
NO.’S 3 AND 4
ALL HEATERS HAVE ADEQUATE
CONNECTOR RISE
CEILING
1
2 3 4
5
1
2 3 4
5
CEILING
INCORRECT
CORRECT
MINIMUM SLOPE OR HORIZONTAL
SUFFICIENT
RISE
Figure 12
TOTAL
HEIGHT
(1) TAPERED MANIFOLD
EACH SECTION SIZES TO HANDLE INPUT OF
COMBINATION OF APPLIANCES ATTACHED
(2) CONSTANT SIZE MANIFOLD
ENTIRE MANIFOLD SUFFICIENTLY LARGE FOR
ALL APPLIANCES ATTACHED—SAME SIZE AS
COMMON VENT
TOTAL
HEIGHT
CAPPED TEE
TOO MUCH SLOPE
1
www.hartandcooley.com Sizing Guide 19