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Combustion
Air Supply
Exhaust
Configuration
Heater Venting
Category
Certified
Materials
Combustion Air
Inlet Material
Vertical Natural
Draft Venting
I “B” Vent
From Inside Building
(Non-Direct Venting)
Horizontal Through-
the-Wall Venting
III
Stainless Steel
(Gas Tight)
Vertical Natural
Draft Venting
I “B” Vent
From Outside Building
(Direct Venting)
Horizontal Through-
the-Wall Venting
III
Stainless Steel
(Gas Tight)
Galvanized Steel
PVC
ABS
CPVC
Category III – A heater which operates with a positive
vent pressure and with a vent gas temperature that
avoids excessive condensate production in the vent.
Category IV – A heater which operates with a positive
vent pressure and with a vent gas temperature that
may cause excessive condensate production in the
vent.
See Table K for appliance category requirements.
Support of Vent Stack
The weight of the vent stack or chimney must not rest
on the heater vent connection. Support must be pro-
vided in compliance with applicable codes. The vent
should also be installed to maintain proper clearances
from combustible materials.
Use insulated vent pipe spacers where the vent pass-
es through combustible roofs and walls.
Vent Terminal Location
1. Condensate can freeze on the vent cap. Frozen
condensate on the vent cap can result in a blocked
flue condition.
2. Give special attention to the location of the vent
termination to avoid possibility of property dam-
age or personal injury.
3. Gases may form a white vapor plume in winter.
The plume could obstruct a window view if the ter-
mination is installed near windows.
4. Prevailing winds, in combination with below-freez-
ing temperatures, can cause freezing of conden-
sate and water/ice build-up on buildings, plants or
roofs.
5. The bottom of the vent terminal and the air intake
shall be located at least 12 in. above grade, includ-
ing normal snow line.
6. Un-insulated single-wall metal vent pipe shall not
be used outdoors in cold climates for venting gas-
fired equipment.
7. Through-the-wall vents for Category II and IV
appliances and non-categorized condensing appli-
ances shall not terminate over public walkways or
over an area where condensate or vapor could
create a nuisance or hazard or could be detrimen-
tal to the operation of regulators, relief valves, or
other equipment. Where local experience indi-
cates that condensate is a problem with Category
I and III appliances, this provision shall also apply.
NOTE: For additional information on appliance cat-
egorization, see appropriate ANSI Z21 Standard and
the NFGC (U.S.), or B149.1 (Canada), or applicable
provisions of local building codes.
CAUTION: When condensate traps are installed,
condensate must be routed to an appropriate con-
tainer for neutralization before disposal, as required
by local codes.
WARNING: Contact the manufacturer of the vent
material if there is any question about the appliance
categorization and suitability of a vent material for
application on a Category III or IV vent system.
Using improper venting materials can result in per-
sonal injury, death or property damage.
NOTE: During winter months check the vent cap
and make sure no blockage occurs from build-up of
snow or ice.
Table K: Venting Category Requirements