If you’ve ever taken a shower and heard a toilet flush, you begin to understand that water system design is tricky. Do you know what happened when you got scalded in the shower? The toilet flush demanded a water flow of cold water only (unless you flush your toilet with hot water) which dropped the pressure in the cold water part of the water distribution system. As the pressure drops, the flow drops. On the hot water side, though, no flush, no pressure drop, no change in flow. So in the shower you get the same amount of hot water and less cold water…and that’s not good.
A well designed water system reduces this effect. When the pipes are sized correctly for the likely flows, the pressure drops of another use has a smaller effect. Of course, most plumbing Codes further protect the person in the shower by now requiring a temperature actuated mixing valve at the shower that prevents scalding.
Water system design, then, must account for water flows (in gallons per minute), pressure drop from friction in pipes and from elevation changes (in psi) and water velocity (in feet per minute). These varying factors, along with never knowing which combination of plumbing fixtures will require water at the same time makes the water system design complicated.
Similar to sewer design discussed above, the concept of the water service fixture unit (WSFU) helps simplify the process. The Code lists WSFUs for cold water, hot water and total water for each type of plumbing fixture. Then the design water flow in gpm can be found for any section of piping in the system. A water flow chart then shows the pipe size and friction loss options. This section explains the theory, but doesn’t give enough information to actually do the design. Go to your Plumbing Code if you want to understand more.
Another basic concept you should understand about water systems is the pressure loss with height. I recommend you memorize that one psi equals about 2 feet of height gain. So a 50′ raise from the ground floor to the 4th floor will drop the water pressure by 25psi. If your public water system provides 45 psi at street level, you will have 20 psi at the 4th floor. Since 20 psi is about the minimum water pressure often recommended, this would work. If the building proposed is an 8 story, what happens? We have a 100′ rise, which gives about a 50psi pressure drop. Since we are only starting with 45 psi, we won’t be getting any water up to the 8th floor. Thus we’d need a booster pump.
The US Army Field Manual 3-34.471 shows a worked example of water system design and several helpful graphs.
