Water Supply and Distribution, Sizing and Materials
Water Supply and Distribution
Pressure is the whole story
Distribution questions reduce to delivering adequate flow at adequate pressure to every fixture. Know static, residual and flow pressure, and the minimum residual required at the most remote fixture (commonly 8 psi for flush-tank water closets and 15 psi for flushometer valves). Above roughly 80 psi a pressure-reducing valve is required; where supply is inadequate, a booster pump and tank are the answer, not bigger pipe.
Layout and sizing
Service line, meter, main, risers, fixture branches. Size by converting fixture units to demand in gpm (for flush-tank systems, 1 WSFU is roughly 1 gpm), then select pipe size for the developed length and allowable pressure loss. Keep velocities reasonable - around 8 fps cold and 5 fps hot - to limit noise and erosion. Provide valves: a main shutoff, fixture stops, and drain valves at low points. On a closed system (check valve, PRV, or backflow preventer at the meter), add a thermal expansion tank.
Traps
- Adding WSFU from a table without converting to gpm before sizing.
- Forgetting developed length and using plan distance.
- Omitting thermal expansion control after installing a PRV or backflow device.
- Ignoring water hammer; arrestors belong near quick-closing valves.
- Sizing hot water the same as cold when the velocity limit is lower.
Worked example
A branch serves two bathrooms, each with a tank-type water closet (2.5 WSFU), lavatory (1) and tub/shower (2), totaling 11 WSFU. At roughly 1 gpm per WSFU, design demand is about 11 gpm; use that with developed length and a pressure-loss table - not the fixture-unit total - to pick the pipe size.
Sizing and Materials
Know materials and their limits
Copper (types K, L, M - K thickest, M thinnest, generally interior above-ground), PEX, CPVC, galvanized steel, and PVC/ABS for drainage only. PVC and ABS are not for hot-water distribution. Cast iron, PVC and ABS are the usual DWV choices. Joining methods matter: solder and press for copper, solvent cement for PVC/CPVC/ABS, crimp or expansion for PEX, threaded or grooved for steel. Potable fittings and solder must be lead-free.
Practices that get inspected
Support spacing varies with pipe size and material - larger pipe and plastic need closer supports. Protect pipe from freezing, abrasion at penetrations, and physical damage where exposed. Use dielectric unions or approved isolation between copper and steel to stop galvanic corrosion. Isolate plastic from heat sources and use listed fire-rated assemblies at rated penetrations. Burial depth and bedding are specified, and thrust blocks restrain buried mains at changes of direction.
Traps
- Using DWV-pattern fittings on a pressure line, or pressure fittings in drainage where a sweep is required.
- Connecting copper directly to steel and creating a corrosion cell.
- Choosing PVC for hot water or for a temperature and pressure relief discharge.
- Forgetting that a listed transition fitting is normally required between PEX and other materials.
- Ignoring the difference between schedule 40 and schedule 80 fittings.
Rule of thumb
Match the material to the service first (potable vs. DWV, hot vs. cold, above vs. below ground), then to the joining method, then to support spacing. If a question shows PVC on a hot-water line, the answer is almost always "wrong material."
Example: a 3/4 in. copper line must connect to a steel nipple at a water heater. Specify a dielectric union or brass-lined connection, not a direct copper-to-steel coupling.