Minimizing the volume of water in the piping between the hot-water source and each fixture is one key to reducing waste in a hot-water system. To find the volume of water contained in piping runs of various diameters, divide the total length of each trunk, branch, or twig by the corresponding ft/cup value. For quick approximations, divide by the “copper rule” values in the bottom row. An efficient layout for copper will perform even better with CPVC or PEX.
Flow rate affects how hot and cold water interact in the piping during hot-water delivery. A flow rate of 3 to 4 gpm creates a “plug flow” (top), which pushes cold water out of the pipe without much mixing, minimizing wasted water and time-to-tap. At low flow rates (bottom), a thin stream of hot water rides up on top of the cold water (or spirals around it) and cools quickly; up to twice the standing volume of water must flow through the pipe to achieve the desired temperature. At flow rates typical for many fixtures (center), hot and cold water mix reasonably well, but up to 1.5 times the standing volume of water in the pipe must flow through before hot water arrives.
Pipe sizing is determined by the flow rate of each fixture in gallons per minute (gpm) and the maximum acceptable velocity (in feet per second) of the pipe used to serve it. Note that velocity increases as pipe diameter decreases. Numbers in red exceed the recommended maximum hot-water velocity of 5 ft/sec for copper, or 10 ft/sec for PEX and CPVC (not shown).