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 - 135°*) in feet
VT - combined vertical vent lengths in feet
11
ELBOW VENT LENGTH VALUES
| feet | inches |
1° | 0.03 | 0.5 |
15° | 0.45 | 6.0 |
30° | 0.9 | 11.0 |
45°* | 1.35 | 16.0 |
90°* | 2.7 | 32.0 |
*the first 45º and 90° offset has a zero value and is shown in the formula as
HORIZONTAL TERMINATION
when (HT) < (VT)
90°
FIGURE 5
For vent configurations requiring more than one 45º and 90° elbow, the following formulas apply:
Formula 1: HT < VT
Formula 2: HT + VT < 40 feet
Example 1:
90° H2
FIGURE 6 | V1 |
45º 
45º
Simple venting configuration (only one 45º and 90° elbow)
H1
90°
REQUIRED VERTICAL R I S E I N FEET (VT)
3940
30
20
10
0 | 2.5 | 5 | 7.5 | 10 | 12.5 | 15 | 17.5 | 20 |
V1 | = | 8 ft |
VT | = V1 = 8 ft | |
H1 | = | 2.5 ft |
H2 | = | 2 ft |
HR | = H1 + H2 = 2.5 + 2 = 4.5 ft | |
HO | = .03(one 45º elbow + two 90º elbows - 135º) | |
HT | = | HR + HO = 4.5 + 2.7 = 7.2 ft |
CALCULATED HORIZONTAL VENT RUN PLUS OFFSETS IN FEET (HT)
The shaded area within the lines represents acceptable values for HT and VT .
HT + VT = 7.2 +8 =15.2ft
Formula 1: HT < VT 7.2 < 8
Formula 2: HT + VT < 40 feet 15.2 < 40
Since both formulas are met, this vent configuration is acceptable.
