Water Service, Distribution, and Backflow Protection
The water side of the exam tests whether you can deliver enough water at enough pressure to the highest and farthest fixture, while making sure nothing from the building can ever flow backward into the potable supply. That means sizing by water supply fixture units, doing pressure budget arithmetic in psi and feet of head, choosing the correct backflow device for the hazard, and knowing your approved materials and support spacing. The numbers in this chapter, especially 0.433 psi per foot and the 80 psi pressure-reducing valve threshold, appear on nearly every version of the test.
Sizing by Water Supply Fixture Units
A water supply fixture unit, or WSFU, is a demand value that accounts for both how much water a fixture uses and how often it is used. A private lavatory is about 1 WSFU, a bathtub about 4, a residential water closet with a flush tank about 2.2 to 3, a flushometer closet as much as 6 to 10, and a hose bibb about 2.5. Cold and hot demand for a fixture are each less than its total demand, which is why the tables list separate cold, hot, and total columns. Once you have the total WSFU on a section, you convert it to a probable demand in gallons per minute using the Hunter curve chart, then choose a pipe size that will carry that flow without exceeding the velocity limit or the pressure you have available. The relationship is not linear: fifty fixture units do not demand five times what ten fixture units demand, because the more fixtures on a system the less likely they are to run at once. Sizing is done section by section from the meter outward, and every section carries only the fixture units downstream of it. Water service, meaning the pipe from the main to the building, is sized on the same total but includes the meter loss, and no water service in most codes may be smaller than 3/4 in.
The Pressure Budget
Every sizing problem is really a subtraction problem. Start with the static pressure available at the main, subtract the pressure lost to elevation, subtract the loss through the meter and through any backflow assembly, softener, or filter, subtract the friction loss in the pipe and fittings, and whatever is left must still meet the minimum residual pressure at the highest and farthest fixture. Elevation costs 0.433 psi for every foot of rise, or stated the other way, 1 psi lifts water 2.31 ft. A fixture 40 ft above the meter has already lost about 17.3 psi before any pipe friction is counted. Most fixtures need at least 8 psi residual, showers and some valves need 15 to 20, and flushometer valves need about 20 to 25. Friction loss is computed over the developed length, which is the measured pipe length plus an allowance for fittings, commonly taken as 50 percent of the measured length as a rule of thumb. On the high side, static pressure over 80 psi requires an approved pressure-reducing valve, because high pressure wastes water, stresses fixture connections, and causes leaks. When a pressure-reducing valve or a check valve or a backflow assembly closes the system, thermal expansion has nowhere to go and an expansion tank becomes necessary.
Cross-Connection Control and Backflow Prevention
A cross-connection is any point where potable water can meet a non-potable substance. Backflow happens two ways: back-siphonage, caused by negative pressure in the supply such as a main break or heavy fire flow, and back-pressure, caused by a downstream system pushed above supply pressure by a pump, elevated tank, or boiler. The device you choose depends on the hazard level and on whether the connection is under continuous pressure. An air gap is the only absolute protection and must be at least twice the effective opening diameter and never less than 1 in. A reduced pressure principle assembly, or RP, is the highest-rated mechanical device and protects against both back-siphonage and back-pressure at a high hazard connection under continuous pressure. A double check valve assembly, or DC, is for low hazard connections under continuous pressure. A pressure vacuum breaker, or PVB, protects against back-siphonage only but may be under continuous pressure, and it is installed at least 12 in above the highest downstream outlet. An atmospheric vacuum breaker, or AVB, is the cheapest and most limited: back-siphonage only, never under continuous pressure, no valve downstream, and installed at least 6 in above the flood level rim of the fixture. Hose bibbs need a vacuum breaker because a hose in a bucket is a textbook cross-connection.
Approved Materials, Joints, and Support
Copper tube is classified by wall thickness: Type K is the heaviest and is used for underground and water service, Type L is the general interior standard and is required under slabs in many jurisdictions, and Type M is the thinnest and is limited or prohibited for some uses. CPVC and PEX are both approved for hot and cold distribution, with PEX requiring protection from ultraviolet light and from direct contact with certain materials, and neither being used for the direct connection to a water heater without an approved metallic transition where the manufacturer requires it. Any solder or flux used in potable water piping must be lead free, meaning no more than 0.2 percent lead, and wetted surfaces of pipe, fittings, and fixtures must not exceed a 0.25 percent weighted average lead content. Where copper meets steel, a dielectric union or approved fitting prevents galvanic corrosion. Support spacing is a memorization item: horizontal copper 1-1/2 in and smaller every 6 ft, copper 2 in and larger every 10 ft, CPVC every 3 ft, PEX every 32 in horizontally, and cast iron every 5 ft with additional support at joints. Vertical piping is supported at each floor level, and hangers must be of a material compatible with the pipe so they do not cause corrosion.
Water Hammer, Valves, and Shutoffs
Water hammer is the shock wave that travels back up a pipe when a fast-closing valve stops moving water. Solenoid valves on clothes washers and dishwashers, and flushometers, are the usual culprits, and the fix required by code is an approved water hammer arrestor installed near the quick-closing valve. An old-style capped air chamber is not an accepted permanent solution because it waterlogs; a manufactured arrestor with a sealed piston or bellows keeps its air charge. Arrestors are sized and located per the manufacturer's listing, which the code adopts by reference. Beyond arrestors, valve placement is a code requirement in its own right. There must be a shutoff on the water service, a shutoff for each dwelling unit, a shutoff on the cold inlet of every water heater, and a stop at each fixture supply so one repair does not require draining a building. Valves must be accessible, which means reachable without removing permanent construction. Full-port ball valves and gate valves are used where free flow matters, while globe valves are used only where throttling is intended because they impose a much higher pressure drop.
Last updated: July 2026