Adding Up Fixture Loads to Determine Pipe Sizing

Assigning the water supply fixtures to the various branches is the first step in determining the minimum pipe sizes.

Adding Up Fixture Loads to Determine Pipe Sizing

Table 1

In my last column “Totally Loaded” in January, we examined plumbing codes and determined how to properly assign the “total” load of the water distribution system. This month, we’ll take the next step in the process and assign the Water Supply Fixture Units (WSFU) to the branches and begin to determine minimum pipe sizes.

As I’ve mentioned in the past, various plumbing codes have their own unique methods of assigning loads to the water distribution system and, of course, sizing it.

For the remainder of my articles, I will focus on using only one code to keep this from becoming too complex and wordy by jumping from code to code. Always remember to use the plumbing code in your area.

Let’s do a quick review of what we accomplished in the last column.

By using Table 1 (left), we assigned the total WSFU to Figure 2. Our total WSFU for Figure 2 was 15.

The next step is to assign the remaining hot loads back to the water heater and then the remaining cold loads back to the water meter. Let’s start with the hot loads. First, let’s go back to Table 1 and determine the hot loads which will be assigned to each fixture. The first column, labeled “hot” indicates the hot load for each of our fixtures requiring hot water.

Figure 3 shows the hot loads assigned to each fixture.

Next, we’ll add the hot loads back to the water heater. See Figure 4. Special note: Never exceed a hot load. Notice that bathroom groups also have hot, cold, and total loads. When each of the groups’ hot water fixture branches connect together, you must verify that you have not exceeded the hot load for the group.

Also remember that whatever the hot load is out of the water heater, is the cold load into the water heater.

Next, we’ll assign the cold loads to each individual fixture branch and add them back to what I will refer to as the cold main. The cold main is the pipe that runs from the water meter to the tee — which feeds the cold to the water heater. I have highlighted the cold main in green.

Again, we go back to Table 1 and determine the colds for each of the fixtures in our design (the second column on the table). Using Table 1 for the cold loads, always remember to never exceed a cold load.

Notice that bathroom groups also have hot, cold and total loads. When each of the groups’ cold fixture branches connect together, you must verify that you have not exceeded the cold load for the group.

When you add up the cold loads individually for a lav (0.5), tub/shower (1.5) and water closet (2.0) it equals 4.0 wsfu. However, when we look at the cold load for the bathroom group (using this code) it equals 3.5 wsfu. This discrepancy is due to the probability of simultaneous use. In other words, this codes table has accounted for the unlikelihood that all three fixtures will be using cold at the same time and given a .5 WSFU credit to the fixtures where they tie their branches together.

At this point you may be asking yourself why it is so important that we load the water piping with the WSFU. The answer is that without the loads assigned, we cannot assign a pipe size which is our end goal. I will add that while having the WSFU assigned to each segment of pipe aids in sizing, the same thing can be accomplished if the gallons per minute were determined along with other variables we’ll discuss later.

We are almost to the end of assigning the loads to the piping. The next step is critical in making sure the loads add up to the total load of the building which is 15 WSFU.

For this step, we will focus on the segment of cold main piping immediately before the tee to the water heater. Here we take the total load downstream and add it to the hot load upstream to determine the WSFU.

Here, you really need to focus on your water sizing vocabulary. Thus far we have assigned total loads (an assigned value representing both hot and cold), hot loads and cold loads to various parts of the system.

Another thing that needs to be pointed out is “downstream and upstream” as it relates to the tee for the water heater. A downstream fixture is any fixture served with cold and/or hot water after the water heater tee. In this case the kitchen sink (1.5), dishwasher (1.0) and the hose bibb (3.0) are fed with cold/hot water and located downstream of the tee serving the water heater cold water.

The total loads from Table 1 add up to 5.5 wsfu. The hot loads upstream (5.0 wsfu) would be the hot wsfu from any fixture that was served before the cold supply to the water heater tee. When added together, the load on the pipe immediately before the water heater tee is 10.5. This section of pipe serves/supplies both the hot and cold water (or total) loads to the fixture downstream of the tee to the heater and then turns around and serves/supplies the hot water to the fixtures upstream that were already fed with cold water.

If you can conquer this concept in assigning loads to the water distribution system, the remaining loads should be a breeze to assign. If not, practice makes perfect. Review, review, review!

From this point back to the water meter, our focus will be on the cold branches that connect to the “cold main” and the ”total” loads of those fixtures minus their ”hot loads.“ The difference of these loads will be assigned back to the ”cold main.”

Let’s start with the first branch, which connects to the cold main upstream of the tee to the water heater. Take the total load (1.5 wsfu) of the clothes washer and subtract its hot load (1.0 wsfu) served through the water heater. 1.5 wsfu – 1.0 wsfu = .5 wsfu. Add the difference to the cold main (0.5 wsfu) to 10.5 wsfu on the previous segment of pipe immediately before the water and our new load is 11.0 wsfu. See Figure 9.

Continue this process for the next branch to the bathroom group that connects to the cold main. The total load for the bathroom group is 4.0 wsfu. Subtract the hot load for the group (2.0 wsfu). 4.0 – 2.0 = 2.0. The difference is 2.0. Add the difference to the cold main and the new load is 13.0 wsfu. See Figure 10.

We only have one branch left. If the difference added back to the cold main = 15 wsfu, which was our building load when we originally started, we have successfully loaded all the pipe. See Figure 11.

We did it! Our loads equaled out and all the water distribution piping is properly loaded. In my next article we’ll get deeper into sizing but for now, you can see that the things are adding up.  

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Randy Lorge is a third-generation plumber and the director of workforce training and development for the International Association of Plumbing and Mechanical Officials (IAPMO). Lorge is also a member of the planning team for the International Water, Sanitation and Hygiene Foundation (IWSH). This 501(c) (3) foundation has completed water and sanitation projects for those less fortunate in India, South Africa, Indonesia and, more recently, the United States. He enjoys time with his family and spending as much time as possible in his deer stand. To contact Lorge, email editor@plumbermag.com.



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