184
Horizontally Vented Duct Furnaces (Category III)
— Pressures
in Category III venting systems are positive and therefore care
must be taken to avoid flue products from entering the heated
space. Use only vent materials and components that are UL listed
and approved for Category III venting systems.
All vent pipe joints must be sealed to prevent leakage into the
heated space. Follow instructions provided with approved venting
materials used. The proper vent pipe diameter must be used, to en
-
sure proper venting of combustion products. See the section Verti
-
cally Vented Duct Furnaces on page 183 for vent pipe diameters.
The total equivalent length of vent pipe must not exceed 50 ft
(15.25 m). Equivalent length is the total length of straight sections,
plus 5 ft (1.52 m) for each 90 degree elbow and 2.5 ft (0.76 m) for
each 45 degree elbow. The vent system must also be installed to
prevent collection of condensate. Pitch horizontal pipe runs down
-
ward
1
/
4
in. per ft (21 mm/m) toward the outlet to permit conden
-
sate drainage. See Fig. 147.
Insulate vent pipe exposed to cold air or routed through unheated
areas. Also insulate vent pipe runs longer than 10 ft (3 m).
Insulation should be a minimum of
1
/
2
in. (12.7 mm) thick, foil-
faced material suitable for temperatures up to 500°F (260°C).
Maintain 6 in. (152 mm) clearance between vent pipe and
combustible materials.
A vent cap
listed for horizontal venting
must be provided. Vent
cap inlet diameter must be same as the required vent pipe diame
-
ter. The vent terminal must be at least 12 in. (305 mm) from the
exterior wall that it passes through to prevent degradation of build
-
ing material by flue gases.
The vent terminal must be located at least 1 ft (305 mm) above
grade, or in snow areas, at least 3 ft (1 m) above snow line to pre
-
vent blockage. Additionally, the vent terminal must be installed
with a minimum horizontal clearance of 4 ft (1.2 m) from electric
meters, gas meters, regulators or relief equipment.
Each duct furnace must have its own individual vent pipe and
terminal.
Do not connect vent system from horizontally vented
units to other vent systems or a chimney.
GAS SUPPLY, PIPING, AND CONNECTIONS
Installation of piping must conform with local building codes and
ordinances, or in the absence of local codes with ANSI Z223.1,
the National Fuel Gas Code. In Canada, installation must be in ac
-
cordance with CAN/CGA-B149.1 for natural gas and B149.2 for
propane units.
Use a pipe sealant resistant to LP gases on gas supply connections
to heater. Gas piping must be sized for the total Btu input of all
furnaces or units serviced by a single supply.
Be sure that gas regulators servicing more than one heater have the
proper pipe and internal orifice size for the total input of all heat
-
ing units serviced by the regulator. See Fig. 148.
Individual duct furnaces require a minimum inlet gas pressure as
shown in Table 41.
Table 41 —
Inlet Gas Pressures
5 FT. MIN.
(1.52 M)
CONDENSATE
DRAIN
LISTED VENT TERMINAL
USE INSULATED VENT OUTDOORS
ROOF
THIMBLE
SEE VENT PIPE
DIAMETER TABLE FOR
VENT DIAMETER
2 FT. MIN.
(0.61 M)
1/4 IN. (6 mm) PITCH PER
1 FT. (305 mm) HORIZONTAL
RUN
10 FT (3.05 M)
MAX (NOT TO
EXCEED 75%
VERTICAL FLUE
HEIGHT)
CONDENSATE
DRAIN
AIR-HANDLING UNIT
OPTIONAL
VENT KIT
TOTAL VERTICAL
FLUE HEIGHT
Fig. 146 —
Indoor Vertical Venting
NOTE: See the section “Vertically Vented Duct
Furnaces” on page
183 for vent pipe diameters.
a39-4396
Category I Vertical Furnace Venting (through Roof)
(Manifold furnace vents acceptable)
IMPORTANT: Through-the-wall vents shall not terminate over
public walkways, or over an area where condensate or vapor
could create a nuisance or hazard.
GAS
SUPPLY
MINIMUM
(IN. WG):
50,000 - 400,000
BTUH MODELS
MINIMUM (IN. WG):
401,000 AND
HIGHER BTUH
MODELS
MAXIMUM
(IN. WG)
NATURAL
GAS
5.0
6.0
13.5
PROPANE
GAS
11.0
12.0
13.5
