2021 IRC Chapter 29
Water supply pressure and sizing is the part of the code that decides whether the water that makes it to a fixture actually behaves the way an occupant expects: enough pressure to fill a tub in a reasonable time, not so much that a valve or fitting fails early, no hammer when a washing machine solenoid snaps shut, and pipe sized so a full house of fixtures running at once still delivers water where it is needed. None of this is about the fixtures themselves, that is Lesson 2, it is about the pressure and volume the distribution system has to deliver to reach them. This lesson covers the minimum and maximum pressure rules, when a pressure-reducing valve is required, water hammer arrestors, the minimum size of the water service pipe, and water supply fixture units, the method the code uses to size pipe for a house full of fixtures that are never all running at once.
The code does not set one fixed minimum psi that applies at every fixture in every house. Instead, P2903.1 says the water service and distribution systems are designed and sized for peak demand using Table P2903.1, which lists a required flow rate and flow pressure at the outlet of each fixture type individually: a lavatory needs 0.8 gpm at 8 psi, for example, while a shower or a bathtub with a balanced-pressure or thermostatic valve needs 20 psi (2.5 gpm for the shower, 4 gpm for the tub). Where the pressure from the public main or a private supply is not enough to meet those minimums, P2903.3 requires the pressure to be raised with an elevated tank, a hydropneumatic booster system, or a booster pump. This matters in practice because a common claim, that code requires exactly 20 psi or exactly 40 psi at every fixture, is not accurate; the real requirement is fixture-specific, and an inspector checking pressure complaints has to look at what a specific fixture needs rather than a single number for the whole house.
Unlike the minimum, the maximum is a single hard number. P2903.3.2 sets the static water pressure at not greater than 80 psi. Where the pressure in the main exceeds 80 psi, an approved pressure-reducing valve (PRV), conforming to ASSE 1003 or CSA B356, has to be installed on the domestic water branch main or riser at the connection to the water service pipe. High incoming pressure is common on properties at the low end of a municipal system or fed by a booster station, and without a PRV that pressure reaches every fixture, appliance, and joint in the house continuously, not just occasionally, which is why premature valve and supply-line failures are a frequent finding in older houses on high-pressure mains.
P2903.5 requires a water hammer arrestor wherever quick-closing valves are used, the solenoid valves in clothes washers, dishwashers, and ice makers being the common residential case. A quick-closing valve stops a moving column of water almost instantly, and the pressure spike from that sudden stop, water hammer, travels back through the piping as a bang that can loosen fittings and stress joints over time. Arrestors have to be listed to ASSE 1010 and installed according to the manufacturer's instructions, which in practice cover branch size, distance from the valve, and orientation; an improvised air chamber, a capped vertical stub of pipe that old-house plumbing sometimes relied on, does not meet this standard for new work because it slowly fills with water and loses its cushioning air.
P2903.7 sets an absolute floor: the water service pipe, the pipe running from the utility main or well to the building, may not be smaller than 3/4 inch in diameter under any circumstance. Beyond that floor, the actual size for a given house is a design calculation, not a fixed number, based on the total water supply fixture unit demand the house will place on it, the available pressure at the meter, the friction loss the meter itself introduces, and the developed length of the pipe run including the equivalent length of every fitting and valve along the way. Two houses with an identical fixture count can need different service sizes if one has a long run from the meter and the other is close to it, because developed length is one of the four inputs to the calculation, not incidental to it.
P2903.9 lays out three separate shutoff requirements that inspectors check at three different points in the system. Every dwelling unit needs an accessible, full-open main shutoff valve near where the water service enters the building (P2903.9.1), so the whole house can be isolated at once. Each water heater needs its own readily accessible full-open valve in the cold supply (P2903.9.2), covered in Lesson 3, so the tank can be serviced without draining the house. And shutoff valves are required on each fixture supply pipe to each plumbing appliance and each plumbing fixture (P2903.9.3), with one specific exception: bathtubs and showers are not required to have individual fixture shutoffs, a distinction inspectors sometimes get backwards when checking a rehab bathroom.
In the field
A homeowner calls GCHS about weak pressure at the upstairs shower right after a water heater replacement. Before assuming a plumbing defect, GCHS checks the obvious things first: is the shutoff at the heater fully open rather than partly closed from the swap, is there a clogged aerator or a partially closed angle stop at the fixture itself, and only after ruling those out does the inspector look at whether the branch serving that fixture was ever sized for the load it now carries. Most "low pressure after a repair" calls trace back to a valve someone forgot to fully reopen, not to a supply pipe that was undersized all along.
A house is never running every fixture at once, so sizing distribution pipe for the sum of every fixture's actual flow rate would grossly oversize the larger pipes in the house. Instead, P2903.6 assigns each fixture a water supply fixture unit (WSFU) value, a small number that reflects both its flow rate and how often it is actually in use at the same time as everything else, and the code sizes pipe from the total WSFU load on a given branch or riser rather than from the sum of individual flow rates. The values in Table P2903.6 are small: a tank-type water closet is 2.2 WSFU, a lavatory 0.7, a bathtub or a shower stall 1.4 each, a kitchen sink 1.4, a clothes washer 1.4, and a hose bibb 2.5 (combined hot and cold values; the table also gives group values, such as a full bath group at 3.6). Once a branch's total WSFU is known, Table P2903.6(1) converts that total into an estimated design flow in gallons per minute, and because the method already accounts for fixtures not running simultaneously, the estimated flow grows much more slowly than the sum of every fixture's individual flow rate: 5 WSFU is about 9.4 gpm, 10 WSFU about 14.6 gpm, and 50 WSFU only about 29.1 gpm for a flush-tank system.
A worked example: a small rehab house has one full bathroom (water closet, lavatory, bathtub/shower) and a kitchen sink on the branch serving the second floor. Using the individual values above, the water closet contributes 2.2 WSFU, the lavatory 0.7, the bathtub/shower 1.4, and the kitchen sink 1.4, for a running total of 5.7 WSFU on that branch. In Table P2903.6(1), a flush-tank system converts 5 WSFU to 9.4 gpm and 6 WSFU to 10.7 gpm, so 5.7 WSFU lands at roughly 10 gpm of estimated design demand. The precise gpm figure for any real branch has to come from the adopted demand table itself, and the pipe size then depends on that demand, the available pressure, and the length of the run; the point of this example is the method, not a number to memorize.
1. Under P2903.1 and P2903.3, the minimum water pressure required at a fixture is:
2. Under P2903.3.2, static water pressure in a building's supply system may not exceed:
3. A water meter reads 105 psi static pressure on the incoming main. Under P2903.3.2, the correct response is:
4. A required PRV under P2903.3.2 must conform to which standard?
5. Under P2903.5, a water hammer arrestor is required wherever:
6. An inspector finds a capped vertical air chamber, rather than a listed arrestor, serving a new dishwasher installation. This is:
7. Under P2903.7, the water service pipe supplying a dwelling may not be smaller than:
8. Two houses have an identical fixture count, but one has a much longer run from the meter to the building than the other. Under P2903.7, will they necessarily need the same service pipe size?
9. Under P2903.9, which fixture is the specific exception to the requirement for an individual fixture supply shutoff valve?
10. A water supply fixture unit (WSFU) value is best described as:
11. For typical private, residential fixtures in Table P2903.6, which pairing of combined WSFU values is correct?
12. A second-floor branch serves one water closet (2.2 WSFU), one lavatory (0.7), one bathtub/shower (1.4), and one kitchen sink (1.4). What is the approximate total WSFU load on that branch?
13. Why does a branch's total WSFU convert to a design flow well below the sum of every fixture's individual maximum flow rate?
14. A homeowner reports weak pressure at an upstairs shower right after a water heater replacement. Before suspecting an undersized supply branch, what should be checked first?