Sizing Guide

6 Sizing Guide www.hartandcooley.com
Type B Double-Wall Gas Vents
Condensation
The condensing of water vapor from the products of
combustion of gas fuels can be minimized with the use
of these sizing tables. When the vent system is
designed properly, dilution air, which may enter a draft
hood (if available), reduces the temperature at which
water vapor will condense (dew point). Exceptions that
will cause condensation are as follows:
A. Temporarily (for a few seconds) after burner ignition,
condensation will form on the cold inner liner of the
vent. Before it develops to drop size, the liner will
have been heated above the dew point, and this
condensate will reevaporate. If the vent is located
outdoors and the temperature is very low,
condensation may continue. This is a good reason for
avoiding this type of installation. It is also important
NOT to extend the vent above the roof more than the
rules require.
B. Extremely long vents or long laterals in unheated
spaces can allow the flue products to cool to the
dew point. DO NOT WRAP INSULATION
AROUND B-VENT TRYING TO PREVENT
CONDENSATION. This method is not reliable and
may contribute to other problems.
C. AIR SUPPLY, as covered earlier, is of great
importance to the proper operation of a vent. Again
if sufficient make-up air is not available to replace
that required by the burner and the draft hood, the
system is starved. The first result is that less air
enters the draft hood, and the dew point
temperature rises. In other words, condensation
can occur at a higher temperature. At some point
in the dilution percent, condensation will start in
the vent. Further starving for air can result in
water running out of the vent, and damage will
result to the structure and contents.
Remember—When in doubt for any reason, such as
dimensions being questionable, a borderline chart
selection, or overhead clearance for maximum
connector rise in doubt, ALWAYS USE THE NEXT
LARGER SIZE and prevent problems that might occur.
This does not apply to table minimums.
Vent Caps
Listed vent caps for double-wall Type B vents are
designed to serve two purposes, (1) prevent rain and
debris from entering the vent, and (2) help prevent a
downdraft condition in the vent due to adverse wind
conditions. These Tables apply to vents, vent caps or
roof housing of the same make and style as the vent.
For safe, efficient operation, DO NOT use combination
roof jacks or caps or termination designs fabricated by
other than the vent manufacturer. ALWAYS install an
approved vent cap immediately after installation of the
vent to exclude debris and prevent damage.
Wall Furnace Vents
Wall furnaces (vented recessed heaters) require a
12-foot minimum vent height measured from the floor
to the top of the vent, or, in the case of combined
vents, to the top of the vent connector. Many vented
wall furnaces require connection to oval vents.
Flashing and Top Assembly Using a Cap
Model RHW/RM Metal Cap Termination
Model RHW/RM cap sizes 3" through 12" round are
listed by U.L. for installation on gas vents terminating a
sufficient distance from the roof so that no discharge
opening is less than 2 feet horizontally from the roof
surface. The lowest discharge opening shall be no
closer than the minimum height shown in the Table in
Figure 2. These minimum heights may be used
provided that the vent is NOT less than 8 feet from any
vertical wall. This also means that no installation shall
terminate by piercing a wall with a short pipe and cap.
These requirements satisfy all national codes.
Termination Dimensions
for Type RHW/RM Caps
Flat to 6/12 1.0
Over 6/12 to 7/12 1.25
Over 7/12 to 8/12 1.5
Over 8/12 to 9/12 2.0
Over 9/12 to 10/12 2.5
Over 10/12 to 11/12 3.25
Over 11/12 to 12/12 4.0
Over 12/12 to 14/12 5.0
Over 14/12 to 16/12 6.0
Over 16/12 to 18/12 7.0
Over 18/12 to 20/12 7.5
Over 20/12 to 21/12 8.0
Model RHW/RM caps are listed under the “Draft Loss
and Wind Effect” requirements of UL Standard 441.
MIN. HEIGHT SEE TABLE
Figure 2
Roof Pitch
Minimum Height
from Roof to
Lowest Discharge
Opening - Feet