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Let’s talk about how you can size a horizontal branch. Bet you can guess that we are going to use a sizing table. Hey, they’re easy, fast, and accurate, so why not use them? Look at Figure 4.16. This table shows you the maximum number of fixture units that may be placed on a single horizontal branch of a given size. If you look closely, you will see, once again, that not more than two toilets can be installed on a single 3-inch pipe that is installed horizontally. It should also be noted that the table does not represent the branches of a building drain and that other restrictions may apply if doing a series of battery venting.
|
Pipe size (in) |
Maximum no. of fixture units |
|
1 Vi |
1 |
|
V/2 |
2 |
|
2 |
6 |
|
3 |
20t |
|
4 |
160 |
|
6 |
620 |
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FIGURE 4.17 ■ Stack-sizing table for Zone Three. (Courtesy of McGraw-Hill)
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Stack sizing requires you to know the number of fixture units that will discharge into the stack from a single branch and the total number of fixture units that will be allowed on the stack. So, let’s say that you have a stack with two branches. There is a bathroom group on each branch, and those two bathroom groups are all that will discharge into the stack. What size pipe is the smallest allowable for use as the stack? To figure this, use the table in Figure 4.17. So that you don’t have refer back to the fixture-rate table, I will tell you that each bathroom group is rated for six fixture units. Well, we have two toilets, so we know the pipe size must be at least three inches in diameter. With 6 fixture units per branch we might get by with a 2-inch pipe if there were no toilets involved. But, toilets are involved and the total load on the stack will be 12 fixture units, so we have to go with a 3-inch pipe. For informational purposes, check out the sizing chart in Figure 4.18. Notice the difference in the number of fixture units allowed on a branch with Figure 4.18 when compared to Figure 4.17. There are two codes at work in these examples, and you can see that the difference for 4-inch pipe on a per-branch basis is 70 additional fixture units with one of the codes.
FIGURE 4.18 ■ Stack-sizing table for Zone Two. (Courtesy of McGraw-Hill)
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FIGURE 4.19 ■ Stack-sizing tall stacks in Zone Two (stacks with more than three branch intervals). (Courtesy of McGraw-Hill)
