48
Venting
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 and valves. The equivalent length of a fit-
ting 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 sys-
tem can use up to the maximum number of elbows and
vent length of straight pipe as long as the maximum equiv-
alent vent length is not exceeded.
There are several different types of elbows that can be
used when constructing a vent system. Figure 12 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 accumulations of
snow in winter. Table 3 shows the equivalent lengths of dif-
ferent types of elbows.
Table 4 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. An R95PA0401317MSA direct-vent installation needs a
31 foot long vent run with qty=5, 90° long-sweep el-
bows 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 an
R95PA0401317MSA 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 cal-
culation 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 12
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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”
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