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EQUIVALENT VENT LENGTHS
The concept of equivalent vent lengths is frequently used
in piping systems to account for pressure drop of fittings,
such as elbows. The equivalent length of a fitting is the
length of a straight section of pipe that has an equivalent
pressure drop in the application as the fitting used. With
the equivalent length vent concept, a vent system can use
up to the maximum number of elbows and vent length of
straight pipe as long as the maximum equivalent vent
length is not exceeded.
There are several different types of elbows that can be
used when constructing a vent system.
Figure 10
shows
the standard dimensions for standard and long-sweep 90°
(1/4 turn) elbows as specified by ASTM 3311, Standard
Specification for Drain, Waste and Vent (DWV) Plastic Fit-
tings Patterns.
A long-sweep (AKA Long-Radius) 90° (1/4 turn) elbow
has an equivalent vent length of 5 feet of straight pipe for
either 2 inch or 3 inch plastic pipe. A standard 90° elbow
has an equivalent vent length of 10 feet of straight pipe.
This equivalent length can be used in circumstances
where it might be necessary to lengthen the vent at the
outside of the structure, such as in areas with large accu-
mulations of snow in winter.
Table 2
shows the equivalent
lengths of different types of elbows.
Table 3
specifies the equivalent maximum vent lengths
specified by the manufacturer for each furnace. Listed
table maximums have been qualified by the manufacturer.
Dependant on individual installation specifics, installations
beyond the table recommendations may cause erratic
pressure switch operation.
Examples:
1. A 60KBTU direct-vent installation needs a 31 foot long
vent run with qty=5, 90° long-sweep elbows and 2
inch pipe.
31 feet of 2 inch pipe =
31 equivalent feet
Qty = 5, 90° long-sweep elbows = 25 equivalent feet
Total =
56 equivalent feet
Since the maximum equivalent vent length specified for a
60KBTU furnace is 65 feet, this installation is acceptable.
2. If the installation from Example 1 were installed with
standard elbows instead of long-sweep elbows, the
calculation would be as follows:
31 feet of 2 inch pipe =
31 equivalent feet
Qty = 5, 90° standard elbows =
50 equivalent feet
Total =
81 equivalent feet
This installation is NOT acceptable as it exceeds the 65
foot maximum specified for this model.
VENT PIPE SIZING AND MAXIMUM VENT LENGTHS
GENERAL VENTING REQUIREMENTS AND
GUIDELINES
FIGURE 10
ELBOWS
STANDARD 90°
2” D = 2 5/16”
3” D = 3 1/16”
LONG SWEEP 90°
2” D = 3 1/4”
3” D = 4 1/16”
