WATER PIPING DESIGN FOR A RESIDENCE In the packet is a floor plan of a residence with water supply piping shown. Follow the process as a step by step procedure for the proper method of selecting sizes for a water supply system. First, as in any design, start with a floor plan. Indicate the plumbing fixtures that will require water, and indicate the fixture unit demand for each fixture.

Connect all the fixtures with a line that will represent cold water piping, then extend the line to the logical location of a water source. The cold water lines in this illustration will be shown green in color. Indicate dimensions as necessary to determine the longest distance water will have to travel from source to farthest fixture. Show pertinent information for piping such as available pressure, meter, rise, fittings, and valves. Connect the cold water line to the water heater. The entrance water line should be located somewhere near the most dense concentration of fixtures.

The next step is to indicate the requirement for supply in fixture units for EACH SEPARATE SEGMENT OF PIPE from the farthest fixture to the source of water. Then with the fixture unit / gallons per minute conversion chart, determine the required gpm for each segment of pipe. Realize that the water does not have a fixture unit value, hence it does not consume water. It merely takes some of the cold water and heats it for use in fixtures that supply hot water. The fixture unit value for the pipes in and out of the water heater will be the sum of the fixtures that supply hot water.

The next step is to connect all fixtures that will be supplied with hot water with a line to represent hot water piping. Connect the line to the water heater.

Indicate the fixture unit value for each segment of pipe from the farthest fixture to the water heater. Then with the conversion chart, indicate the gpm demand for hot water for each segment of pipe in the hot water service.

In the calculation of fixture units for segments of pipes to fixtures, realize this condition: Imagine a sink. Turn on hot water only. All the water comes out of the hot water pipe. Now turn off hot and turn on cold water fully. All the water comes out of the cold water pipe – the same volume as the hot water. Now turn hot half on, and cold half on. The same volume, except half comes from hot and half comes from cold. This is to illustrate that the water heater does not contribute to fixture unit count. Water to a fixture comes from either a cold pipe or a hot pipe, or a mixture.

The next step is to determine the measured length of pipe from the water source to the fixture farthest away from the source. It is rather apparent that the longest length of travel for water is from the source, into the building to the water heater, then from the water heater to the bath room on the left side of the plan. The dimensions from the source = 60’ + 12’ + 20’ to the water heater, then back 12’ + 35’ to the left side bath room, which totals 139 feet, measured length. Then realize equivalent length for fittings cannot be added because the pipe sizes are not known, so take half the measured length as an allowance for equivalent length of fittings, and add it back to the measured length making; 139’ + 69’ = 208’ equals calculated length, which means in essence, the available pressure must be able to push water through 208 feet of straight pipe with no fittings.

Next, determine the available pressure, setting aside allowances for pressure for meter, for rise, and for fixtures. See that the total amount of water required for the system is 15 gpm. Go to the water meter chart and find at the bottom, 15 gpm. From there move straight up the chart until it intersects the middle meter size, which is ¾”. From that intersection, read directly to the left and see that it takes 5 psi to operate the ¾” meter. Next, the supply pipe rises 10’, so the pressure required to raise water 10’ =.433 x 10 = 4.33 psi. Then allow 15 psi to operate fixtures. So total amount lost to the total pressure equals 60 – 5 – 4.33 – 15 = psi that is available to push the water through the system.

So, at this point remains psi pressure to push the water a distance of 208 feet. From that, convert the amount of pressure per 100 feet of length; [ / 208 ] x 100 = psi per 100 feet [ / 208 ] x 100 = psi per 100 feet Then do not allow water in the system to travel faster than 8 feet per second. On the pipe size chart, find at the bottom, draw a line straight up until it intersects the 8 fps line. Then where each pipe diameter intersects either the VERTICAL line or the 8 FPS line, read to the left and determine the maximum gpm each pipe diameter will supply. Make a small chart.

Pipe dia. GPM ½” 3 ¾” 9 1”18 1 ¼”28 1 ½”44 Use this chart to show diameter for each segment of pipe in the system, based on the required gpm for each. Now verify the assumption of water meter size: See that the total demand for the building is 15 gpm. From the pipe chart above, see that a 1” pipe is required to supply 15 gpm based on the criteria. So a 1” meter is required. From the water meter chart, see that a 1” meter requires less than 2 psi to provide 15 gpm, so the 5 psi allowance was OK.

Next was an assumption for equivalent length of fittings, in which we allowed 69 feet. From the plan begin at the source (meter) and follow along the path of pipe of the longest length and list the fittings and size, since you now know the pipe diameters. Then tally the length: Since the actual equivalent length of fittings amounts to only 21 feet, the allowance of 69 feet in the calculation was sufficient.