Rheem 97MDV060 Series Installation Instructions Manual page 25

GENERAL VENTING REQUIREMENTS AND
VENT PIPE SIZING AND MAXIMUM VENT LENGTHS
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.
used when constructing a vent system. Figure 10 shows
There are several different types of elbows that can be
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.
FIGURE 10
ELBOWS
STANDARD 90°
2" D = 2 5/16"
3" D = 3 1/16"
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
mulations of snow in winter. Table 2 shows the equivalent
outside of the structure, such as in areas with large accu-
lengths of different types of elbows.
LONG SWEEP 90°
2" D = 3 1/4"
3" D = 4 1/16"
GUIDELINES
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
Examples:
pressure switch operation.
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 =
Qty = 5, 90° long-sweep elbows = 25 equivalent feet
Total =
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 =
Qty = 5, 90° standard elbows =
Total =
This installation is NOT acceptable as it exceeds the 65
foot maximum specified for this model.
31 equivalent feet
56 equivalent feet
31 equivalent feet
50 equivalent feet
81 equivalent feet
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