Sizing Guide

8 Sizing Guide www.hartandcooley.com
Type B Double-Wall Gas Vents
FAN MAX refers to the maximum input rating of a
Category I fan-assisted appliance attached to the vent.
NAT MAX refers to the maximum input rating of a
Category I draft-hood-equipped appliance attached to
the vent. There are no minimum appliance input
ratings for draft-hood-equipped appliances.
FAN+FAN refers to the maximum combined input
rating of two or more fan-assisted appliances attached
to the common vent.
FAN+NAT refers to the maximum combined input
rating of one or more fan-assisted appliances and one
or more draft-hood-equipped appliances attached to the
common vent.
NAT+NAT refers to the maximum combined input
rating of two or more draft-hood-equipped appliances
attached to the common vent.
NA means not allowed due to physical or geometric
constraints.
DRAFT HOOD is a device built into an appliance, or
made a part of the vent connector from an appliance,
which is designed to (1) provide for the ready escape of
the flue gases from the appliance in the event of no
draft, backdraft, or stoppage beyond the draft hood;
(2) prevent a backdraft from entering the appliance;
and (3) neutralize the effect of stack action of the
chimney or gas vent upon the operation of the
appliance.
VENT is a passageway used to convey flue gases from
gas utilization equipment, or its vent connector, to the
outside atmosphere.
VENT CONNECTOR is the pipe or duct that connects a
fuel gas-burning appliance to a vent or chimney.
FLUE COLLAR is that portion of an appliance designed
for the attachment of a draft hood, appliance adapter,
vent connector or venting system.
FORCED DRAFT indicates that the combustion air fan
or blower is located ahead of the burner compartment.
INDUCED DRAFT indicates that the combustion air fan
or blower is located at or after the exit of flue products
from the heat exchanger.
CONDENSING APPLIANCE is one which by reason of
having sufficient heat removed from its products of
combustion, water vapors will condense in its heat
exchanger and continue to condense in the venting
system.
Definition of ANSI Categories of Appliances*
Gas Appliances Categories. Vented gas appliances are
classified for venting purposes into four categories as
follows:
Category I An appliance that operates with a negative
vent static pressure and with a vent gas
temperature that avoids excessive
condensate production in the vent.
Category II An appliance that operates with a negative
vent static pressure and with a vent gas
temperature that may cause excessive
condensate production in the vent.
Category III An appliance that operates with a positive
vent static pressure and with a vent gas
temperature that usually avoids excessive
condensate production in the vent.
Category IV An appliance that operates with a positive
vent static pressure and with a vent gas
temperature that causes excessive
condensate production in the vent.
*Remember that these definitions apply to the
appliance and do not necessarily reflect the
performance of the connected vent system.
Single-Appliance Vent Systems
General Rules for Venting Single Appliances.
Normally, a vent equal to the size of the draft hood
outlet can be considered satisfactory for venting a
single appliance. It is important to note that this rule
may NOT apply to cases where an extra high vent is
required, and it may be desirable to calculate the
system to determine whether is it possible to reduce
the size of the vent.