269441-UIM-A-0407
Unitary Products Group
15
COMBUSTION AIR/VENT PIPE SIZING
The size of pipe required will be determined by the furnace model, the
total length of pipe required and the number of elbows required.
Table 8 lists the maximum equivalent length of pipe allowed for each
model of furnace. The equivalent length of elbows is shown in Table 9.
The equivalent length of the vent system is the total length of straight
pipe PLUS the equivalent length of all of the elbows.
The following rules must also be followed:
1.
Long radius (sweep) elbows are recommended. Standard elbows
may be used, but since they have a longer equivalent length, they
will reduce the total length of pipe that will be allowed. Short radius
(plumbing vent) elbows are not allowed. The standard dimensions
of the acceptable elbows are shown below.
2.
The maximum equivalent length listed in Table 8 is for the vent pip-
ing and the air intake piping separately. For example, if the table
allows 65 equivalent feet for a particular model, then the vent can
have 65 equivalent feet of pipe, AND the combustion air intake can
have another 65 equivalent feet of pipe.
3.
Three vent terminal elbows (two for the vent and one for the com-
bustion air intake) are already accounted for and need not be
included in the equivalent length calculation.
4.
All combustion air and vent pipe must conform to American
National Standards Institute (ANSI) and American Society for Test-
ing and Materials (ASTM) standards D1785 (Schedule 40 PVC),
D2665 (PVC-DWV), F891 (PVC-DWV Cellular Core), D2261
(ABS-DWV) or F628 (Schedule 40 ABS). Pipe cement and primer
must conform to ASTM Standard D2546 (PVC) or D2235 (ABS). If
ABS pipe is to be used, any joint where ABS pipe is joined to PVC
pipe must be glued with cement that is approved for use with
BOTH materials. Metallic materials must not be used for venting or
air intake.
5.
If a flexible connector is used in the vent system, it must be made
of a material that is resistant to acidic exposure and to at least
225° F temperature. Flexible connectors are also allowed in the
combustion air pipe.
6.
All models are supplied with 2" vent connections. When the pipe
must be increased to 3" diameter, the transition from 2" to 3" must
be done as close to the furnace as possible. For upflow models,
the transition from 2" to 3" should be done immediately above the
furnace. For downflow or horizontal models, the transition from 2"
to 3" pipe should be done immediately after the drain tee or drain
elbow.
7.
In Canada, vents shall be certified to ULC S636, Standard for Type
BH Gas Venting Systems. IPEX System 636 PVC is certified to
this standard.
8.
In Canada, the first three feet (900 mm) of the vent must be readily
accessible for inspection.
Example:
An 80,000 BTUH furnace requires 32 feet of pipe and four 90º elbows.
Using 2" pipe and standard elbows, the total equivalent length will be:
This exceeds the 65 foot maximum equivalent length of 2" pipe allowed
for that model and is thus not acceptable.
By using sweep elbows, the total equivalent length will be:
This is less than the 65 foot maximum equivalent length of 2" pipe
allowed for that model and is thus acceptable.
Alternatively, using 3" pipe and standard elbows, the total equivalent
length will be:
This is less than the 90 foot maximum equivalent length of 3" pipe
allowed for that model and is thus acceptable.
This furnace may not be common vented with any other appliance,
since it requires separate, properly sized air intake and vent lines.
The furnace shall not be connected to any type of B, BW or L vent
or vent connector, and not connected to any portion of a factory-
built or masonry chimney
The furnace shall not be connected to a chimney flue serving a sep-
arate appliance designed to burn solid fuel.
When combustion air pipe is installed above a suspended ceiling or
when it passes through a warm and humid space, the pipe must be
insulated with 1/2” Armaflex or other heat resistant type insulation.
Vent piping must be insulated with 1/2” insulation if it will be sub-
jected to freezing temperatures such as routing through unheated
areas or through an unused chimney.
TABLE 8:
Maximum Equivalent Pipe Length
Model Input
BTUH (kW)
Pipe Size
Inches (cm)
Maximum
Equivalent
length feet (m)
40,000
(11.7)
2
(5.1)
65
(19.8)
40,000
(11.7)
3
(7.6)
90
(27.4)
60,000
(17.6)
2
(5.1)
65
(19.8)
60,000
(17.6)
3
(7.6)
90
(27.4)
80,000
(23.4)
2
(5.1)
65
(19.8)
80,000
(23.4)
3
(7.6)
90
(27.4)
100,000
(29.3)
2
(5.1)
30
(9.2)
100,000
(29.3)
3
(7.6)
85
(25.9)
120,000
(29.3)
3
(7.6)
60
(18.3)
TABLE 9:
Equivalent Length of Fittings
Fitting
Equivalent Length
2" 90º sweep elbow
5 feet of 2" pipe
2" 45º sweep elbow
2-1/2 feet of 2" pipe
2" 90º standard elbow
10 feet of 2" pipe
2" 45º standard elbow
5 feet of 2" pipe
3" 90º sweep elbow
5 feet of 3" pipe
3" 45º sweep elbow
2-1/2 feet of 3" pipe
3" 90º standard elbow
10 feet of 3" pipe
3" 45º standard elbow
5 feet of 3" pipe
2" corrugated connector
10 feet of 2" pipe
3" corrugated connector
10 feet of 3" pipe
32 feet of 2" pipe =
32 equivalent feet
4 - 90º standard 2" elbows = (4x10) =
40 equivalent feet
Total =
72 equivalent feet of 2" pipe
32 feet of 2" pipe =
32 equivalent feet
4 - 90º standard 2" elbows = (4x5) =
20 equivalent feet
Total =
52 equivalent feet of 2" pipe
32 feet of 3" pipe =
32 equivalent feet
4 - 90º standard 2" elbows = (4x5) =
40 equivalent feet
Total =
72 equivalent feet of 3" pipe
