Vent Pipe Sizing And Maximum Lengths; Equivalent Vent Length - Rheem R95P Installation Instructions Manual

GENERAL VENTING REQUIREMENTS AND
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GUIDELINES
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VENT PIPE SIZING AND MAXIMUM VENT LENGTHS
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EQUIVALENT VENT LENGTHS
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The concept of equivalent vent lengths is frequently used
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in piping systems to account for pressure drop of fittings,
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such as elbows and valves. The equivalent length of a fit-
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ting is the length of a straight section of pipe that has an
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equivalent pressure drop in the application as the fitting
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used. With the equivalent length vent concept, a vent sys-
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tem can use up to the maximum number of elbows and
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vent length of straight pipe as long as the maximum equiv-
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alent vent length is not exceeded.
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There are several different types of elbows that can be
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used when constructing a vent system. Figure 12 shows ! !
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the standard dimensions for standard and long-sweep 90°
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(1/4 turn) elbows as specified by ASTM 3311, Standard
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Specification for Drain, Waste and Vent (DWV) Plastic Fit-
tings Patterns.
FIGURE 12
ELBOWS
STANDARD 90°
2" D = 2 5/16"
3" D = 3 1/16"
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A long-sweep (AKA Long-Radius) 90° (1/4 turn) elbow has
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an equivalent vent length of 5 feet of straight pipe for either
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2 inch or 3 inch plastic pipe. A standard 90° elbow has an
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equivalent vent length of 10 feet of straight pipe. This
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equivalent length can be used in circumstances where it
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might be necessary to lengthen the vent at the outside of
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the structure, such as in areas with large accumulations of
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snow in winter. Table 3 shows the equivalent lengths of dif-
ferent types of elbows.
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LONG SWEEP 90°
2" D = 3 1/4"
3" D = 4 1/16"
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Table 4 specifies the equivalent maximum vent lengths
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specified by the manufacturer for each furnace. Listed ) ! !
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table maximums have been qualified by the manufacturer. * !
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Dependant on individual installation specifics, installations
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beyond the table recommendations may cause erratic
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pressure switch operation.
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Examples:
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1. An R95PA0401317MSA direct-vent installation needs a !! GP!'%%3!&'!O!*./)!!"
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31 foot long vent run with qty=5, 90° long-sweep el-
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bows and 2 inch pipe.
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31 feet of 2 inch pipe =
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Qty = 5, 90° long-sweep elbows = 25 equivalent feet
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Total =
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Since the maximum equivalent vent length specified for an
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R95PA0401317MSA furnace is 65 feet, this installation is
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acceptable.
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2. If the installation from Example 1 were installed with
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standard elbows instead of long-sweep elbows, the cal-
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culation would be as follows:
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31 feet of 2 inch pipe =
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Qty = 5, 90° standard elbows =
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Total =
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This installation is NOT acceptable as it exceeds the 65
foot maximum specified for this model.
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31 equivalent feet
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56 equivalent feet
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31 equivalent feet
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50 equivalent feet
81 equivalent feet
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