Water distribution systems are designed with a certain velocity, pressure and volume at every fixture and device, even at peak demand. It is our job to make sure that this is accomplished with our codes and standards. We must also assure not only to our customers, but the water purveyors that we are preventing cross-connections via backflow conditions. 

Here in Wisconsin, the first thing that we need to accomplish is a water calculation worksheet. This worksheet, once completed, will determine minimum pipe sizes to serve all the fixtures and devices that need a supply of water. 

With the first part of the water calculation worksheet, there is information that is required to obtain before any water distribution piping can be installed. Lines 1 and 1a you can get from looking at the plan and counting up the demand from each fixture. For Lines 2-5, you may need to reach out to the water purveyor in that area to obtain the rest of the information. 

The second part of the worksheet is finding out the pressure loss from the water service. This can vary so much from one building to the next based on water demand, material, elevation and sizes. 

We do have a minimum water service size of 3/4 inch. If the building has fire sprinklers, it will drastically change the size, but other than that, we can pretty much use whatever size and material we would like, within reason of course. 

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Now that we have found out all the pressure losses through the water service, we have to find out the pressure demands and losses in the water distribution system. 

Line C takes in the pressure loss through the water meter. This one in Wisconsin is more confusing than it needs to be. We have a graph in our code book that shows how much loss is through each size meter and certain GPMs. The confusing part is, that not every meter has the exact same pressure loss, and that particular graph is from a meter made in the 1960s that is no longer used. This goes along with the first part of the water calculation worksheet where you have to contact the water purveyor and get a proper make, model and size to accurately determine the pressure loss for the meter.  

Line D wants to find out which fixture in your building requires the most pressure to work properly. This fixture can also vary drastically from one building, to the next or even inside the same building! Line E wants to know the elevational difference of that particular fixture. If the building is a standard residential house the fixture that requires the most pressure to perform properly would be the pressure balanced shower valve, and if there is a valve on the second floor, it would be that fixture compared to if the pressure balanced shower valve is on the first floor. Now if we are in a larger commercial building, we would have to determine which fixture requires the most pressure and not only at an elevational difference but a developed length difference. For example, if we have a flushometer water closet, which demands 15 psi, on the second floor and has a developed length of 25 feet, it may not require as much “starting pressure” as an outside hose bib which has a demand of 8 psi on the first floor, but has a developed length of 500 feet. 

Since we have to find out how much pressure is available at the most demanding fixture, we have to find out everything that will lose pressure between the main building control valve and that controlling fixture. There are several things that have a pressure loss. The most common device that we install in Wisconsin is a water softener, due to having very hard water. Just like the water meter, each water softener has a different pressure loss for different demands and sizes.  

The really nice thing about Line F is that back in the early 2000’s, we had a couple companies do an experiment with demand flows in “average” houses. This was accomplished to make sure that we do not oversize our water distribution piping and also take an appropriate pressure loss for the water softeners and filters being installed. 

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This table gives you a reduced gpm demand for each fixture unit in a residential building, compared to a public building. 

Line G is for an instantaneous or tankless water heater. There are many great parts of instantaneous or tankless water heaters, but I feel the one major drawback would be the pressure loss through it. If your controlling fixture is also required to have a hot water demand, we have to take into account the loss through that water heater.

As you can see in this graph is that it doesn’t take a large gpm demand to lose a fair amount of pressure. 

Line H is for the place where we put in the developed length to the controlling fixture, which requires the most pressure. We don’t need to count every single fitting from the main building control valve to the controlling fixture and take the loss for each fitting. We just take the developed length and multiply it by 1.5 to take in consideration for fitting loss. 

We complete this water calculation worksheet to get a pressure available for uniform loss. Then we take that final number and apply it to our water distribution tables. There is a different table for each piping material, and that will determine the maximum number of fixture units we can put on any certain size pipe, without exceeding 8 fps flow velocity.  

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There are other aspects we may need to address to accurately get a pressure available for uniform loss. Do we even have enough pressure to supply our entire water distribution system or do we need to install a booster pump? Do we have too much pressure at the main and need to install a pressure reducing valve to make sure we don’t exceed what our materials can handle? Water hammer arrestors can also play a vital role in protecting our water distribution system. Hot water circulation systems and expansion tanks may also need to be addressed to accurately size our water distribution system.  

Source: https://www.plumbermag.com/how-to-articles/distribution-water-system-plumbing/understanding-water-distribution-systems-and-how-to-figure-them-out?ref=loggedin