Thread: Calculating DFUs

This sounds like a silly question, but when calulating the DFUs from Table 3004.1, when do you use the "multiple bath" section of the table? The reason I ask is because throughout all the ICC exam questions, they go back and forth from using the "mulitple bath" and individual bath sections for computing the dfus for more than one bathroom:

Example: You have two full bath groups and a half bathroom. Compute the DFUs.

Several practice questions use the individual part of the table to compute the DFUs,

One bath group = 5 dfu, two baths = 5* 2 = 10 dfu
1/2 bathroom (1.6 gall per flush) = 4 dfus

Total dfu = 14

Other practice questions use the multiple bath part of the table:

2 1/2 baths = 9 dfu

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I don't know, but this is my "guess".

You use one when the bathrooms are 'grouped' on a stack, and the other when they are not 'grouped' on a stack.

Originally Posted by Wendell Swedberg

This sounds like a silly question, but when calculating the DFUs from Table 3004.1, when do you use the "multiple bath" section of the table? The reason I ask is because throughout all the ICC exam questions, they go back and forth from using the "multiple bath" and individual bath sections for computing the dfus for more than one bathroom:

Example: You have two full bath groups and a half bathroom. Compute the DFUs.

Several practice questions use the individual part of the table to compute the DFUs,

One bath group = 5 dfu, two baths = 5* 2 = 10 dfu
1/2 bathroom (1.6 gall per flush) = 4 dfus

Total dfu = 14

Other practice questions use the multiple bath part of the table:

2 1/2 baths = 9 dfu

DFUs are used to size the drains that serve plumbing fixtures. Since the drains from every fixture in the house eventually combine into one drain before they leave the house, the value you use in the table depends upon which piece of the drainage system you want to size and the plumbing fixtures that are feeding it. During normal and typical use (even pushed to extremes) all of the plumbing fixtures in the house will not be draining at the same time. Thus, to avoid having hugely over-sized drains downstream in the system, the DFU system allows some combinations of fixtures (groups) to have a DFU that is less than the sum of its parts.

From the table:

lavatory (sink) = 1 dfu
water closet, 1.6 gpf (toilet) = 3 dfu
bathtub with or w/o showerhead = 2 dfu

half-bath group, 1.6 gpf = lavatory + water closet = 4 dfu. No shaving off DFUs here because odds of both draining at same time are high. Group value is still 4 dfu.

full-bath group, 1.6 gpf = lavatory + water closet + bath = 6 dfu total, but the line draining all if them combined together needs to be sized for 5 dfu per the group value in the table.

Looking at 2 1/2 bath groups individually, you have a total of 5 + 5 + 4 = 14 dfu, but the drains from the point where all three of those bath groups join until they leave the home only need to be sized at 9 dfu by using the 2.5 bath multiple-bath group value in the table. The reason for the table having 9 instead of 14 is that it would be very unlikely to have three sinks, two toilets, and two bath tubs draining all at the same time.

Originally Posted by Brandon Chew

Looking at 2 1/2 bath groups individually, you have a total of 5 + 5 + 4 = 14 dfu, but the drains from the point where all three of those bath groups join until they leave the home only need to be sized at 9 dfu by using the 2.5 bath multiple-bath group value in the table. The reason for the table having 9 instead of 14 is that it would be very unlikely to have three sinks, two toilets, and two bath tubs draining all at the same time.

Brandon,

Would you clarify/explain that further?

From the above reasoning, it would be possible to have larger pipe sizes connecting into smaller pipe sizes, and that would not work (not well anyway).

It's always smaller pipe sizes connecting into larger pipe sizes.

It's always smaller pipe sizes connecting into larger pipe sizes

.Absolutely!

Let's toss the kitchen sink at 2 dfu and a clothes washer standpipe at 2 dfu into our 2.5 bath plumbing system to make it interesting. Assume there are no more plumbing fixtures in the home.

The drain serving 1 full bath group and only that full bath group is sized at 5 dfu. The drain serving the half bath and only the half bath is sized at 4 dfu. Likewise, the drain serving the kitchen sink and only the kitchen sink is sized for 2 dfu, and the one for the standpipe and only the standpipe is sized for 2 dfu.

In our house we have 1 kitchen sink, 1 standpipe, 3 lavatories, three water closets, and 2 bathtubs. If we were to add up all of the DFUs for the individual fixtures, the piece of the drain system that served all of those fixtures from the point they joined until they left the home would need to be sized for 20 dfu. But because all of those fixtures are not going to drain at the same time, that's not the way it is done.

The first thing you do is draw a schematic of the house drain system and indicate the location & type of plumbing fixtures. Then you start at the top floor or most remote fixture and work your way through the drain system the way the water flows, till you get to the building drain. I'll use a layout that is very common around here: two full baths on the second floor, kitchen sink and half bath on the first floor, and standpipe in the basement.

The branch drain that leaves each one of the full bath groups is sized at 5 dfu until they join together. From the point where those two baths join until the next fixture comes in, you do not use 5 + 5 = 10, you use the "2 baths" = 8 dfu from the multiple-bath group portion of the table. So the drain from where the baths join until something else comes in is sized at 8 dfu.

Let's say the next thing we pick up is the kitchen sink. From this point until we pick up the next fixture, we use 8 + 2 = 10 dfu. After the kitchen sink, we pick up the half bath. We don't use the value for a half bath alone (4 dfu) and add it to 10 to get 14. We go back to the table and adjust for 2.5 baths as a group to 9 dfu then add the kitchen sink at 2 dfu, and size the branch drain from the half bath to the next fixture at 11 dfu.

Down in the basement we pick up the standpipe, 11 + 2 = 13 dfu. The drain from that standpipe until it leaves the home is sized for 13 dfu. (note that when we added all the individual fixtures in the house we got 20 dfu)

Table P3005.4.1 allows you to determine the minimum pipe size required based on the DFUs. You still need to meet P3005.4 which says: "The size of drainage piping shall not be reduced in size in the direction of flow." So, for example, once you are at a point in the system where your dfu calculations cause you to bump the drain size from 2 inches to 2.5 inches, you can never drop back to 2 inches even if using a group dfu value might allow for a smaller size in Table P3005.4.1. What that group value does is hold back a bit on when you might be forced to bump up from 2.5 inches to 3 inches (until the DFUs in Table P3005.4.1 call for 3 inch pipe).

Brandon,

That I follow and know, but I could not get there through your first post.

Thanks for explaining it further so I could follow it.

(Now, if only I was that good at explaining things on an electrical thread ... maybe my last post did explain it well enough?)