DEFINITIONS
for the following symbols used in the venting calculations and examples are:
>- greater than
>- equal to or greater than < - less than
< - equal to or less than
Ht - total of both horizontal vent lengths (Hr) and offsets (Ho) in feet
Hr - combined horizontal vent lengths in feet
Ho - offset factor: .03(total degrees of offset - 90°*) in feet
Vt - combined vertical vent lengths in feet
ELBOW VENT LENGTH VALUES
| feet | inches |
1° | 0.03 | 0.5 |
15° | 0.45 | 6.0 |
30° | 0.9 | .0 |
45° | .35 | 6.0 |
90°* | .7 | 32.0 |
*the first 90° offset has a zero value and is shown in the formula as
TOP EXIT / horizontal termination
figure 4
when (Ht) < (Vt)
Simple venting configuration (only one 90° elbow)
vertical rise in
feet
VT
horizontal vent run PLUS OFFSET in feet HT
The shaded area within the lines represents acceptable values for HT and VT .
See graph to determine the required vertical rise VT for the required horizontal run HT.
For vent configurations requiring more than one 90° elbow, the following formulas apply:
Formula 1: HT < VT |
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Formula 2: HT | + VT < 40 feet |
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| 90° | H2 | |
figure 5 |
| V2 |
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| 90° |
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| 90° | H1 |
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Example 1: |
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V1 | = 3 ft |
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V2 | = 8 ft |
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VT | = V1 + V2 = 3 + 8 = 11 ft |
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H1 | = 2.5 ft |
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H2 | = 2 ft |
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HR | = H1 + H2 = 2.5 + 2 = 4.5 ft |
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HO | = .03(three 90° elbows - 90°) = .03(270° - 90°) = 5.4 ft | |||
HT | = HR + HO = 4.5 + 5.4 = 9.9 ft |
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HT + VT = 9.9 + 11 = 20.9 ft
Formula 1: HT < VT 9.9 < 11
Formula 2: HT + VT < 40 feet 0.9 < 40
Since both formulas are met, this vent configuration is acceptable.
