92 psi at the Meter and a Third-Floor Shower
At a new three-story house in Ann Arbor the static water pressure measured at the meter outlet is 92 psi. The meter is a 5/8-inch meter, and the longest developed length from the meter to the third-floor shower is 130 feet of 3/4-inch copper. The shower head is 28 feet above the meter. The installer proposes to skip a pressure-reducing valve because 'the city usually runs about 60 psi,' and he plans to install a tankless water heater in the basement with a check valve on the cold inlet. At the design flow of 8 gpm, the meter and service losses total 6 psi and the pipe friction loss on this run is 9 psi. He also wants to run the last 20 feet of 1/2-inch PEX through an unconditioned attic.
1. Is a pressure-reducing valve required, and at what setting? What else does the PRV (or the check valve) make necessary?
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Yes - a PRV is required. The code threshold is 80 psi: where the static pressure on the downstream side of the meter exceeds 80 psi, a pressure-reducing valve must be installed. A measurement of 92 psi triggers the requirement, and 'the city is usually about 60' is not a defense - the system must be protected against the pressure that actually exists, including nighttime surges.
Setting: adjust the PRV to a downstream pressure not exceeding 80 psi; 50 to 60 psi is the practical target. Set it high enough that the third-floor fixtures still get their minimum residual pressure (see next task) and low enough to protect fixtures, the water heater, and appliances. PRVs must be accessible and provided with a strainer, and the installation should include a means to relieve thermal expansion.
Closed system / thermal expansion: a PRV, a check valve, or a backflow preventer on the supply creates a closed system. A tankless water heater with a check valve on the cold inlet certainly does. Once the system is closed, heated water has nowhere to expand, so a listed thermal expansion tank (and often a thermal expansion relief valve) is required on the cold side of the water heater. Without it, pressure spikes on every heating cycle and can blow the T&P valve or damage the tankless unit.
2. With the PRV set at 60 psi, calculate the residual pressure at the third-floor shower. Is it adequate? Show the arithmetic.
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Start from the pressure that is actually available, then subtract each loss.
- Available pressure downstream of the PRV: 60 psi.
- Elevation loss: water column pressure = 0.433 psi per foot of height. 28 ft x 0.433 = 12.1 psi
- Meter and service loss at 8 gpm: 6 psi
- Pipe friction on the 130-foot developed length at 8 gpm: 9 psi
Residual pressure at the shower:
60 - 12.1 - 6 - 9 = 32.9 psi
That is comfortably above the 8 psi minimum residual required at a shower (the fixture minimums for ordinary fixtures generally fall in the 8 psi range; flushometer valves need substantially more, roughly double). Even if the PRV were set at 50 psi, the residual would be about 22.9 psi - still acceptable.
What this tells the installer:
- The elevation penalty is real and often forgotten. At 28 feet the column costs about 12 psi before a drop of water moves.
- The check on a water supply design is always: available pressure minus elevation minus device losses minus friction, compared to the fixture minimum - not static pressure at the meter.
- If the run had been much longer or the flow higher, the 3/4-inch main would be the place to look first; increasing pipe size reduces friction far more effectively than raising the PRV setting.
3. Evaluate the plan to run the last 20 feet of 1/2-inch PEX through the unconditioned attic. What are the code and practical problems, and what would you require?
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Freezing. Michigan winters will freeze a pipe in an unconditioned attic. Water piping in unconditioned or exposed locations must be protected from freezing: either keep the run inside the building's thermal envelope, or insulate and protect it with listed heat trace and insulation on a thermostatically controlled circuit. Simply insulating a pipe in a vented attic does not protect it - insulation slows heat loss, it does not add heat.
Other requirements for the attic run:
- PEX must be protected from UV light and from physical damage; in an attic it needs protection from sunlight at penetrations, and supports at the manufacturer's spacing.
- Insulation and heat trace must not be concealed in a way that prevents inspection or replacement.
- Provide freeze protection on the entire exposed length, not just 20 feet of it - the fixture branch's vulnerable section is the whole exposed portion.
Sizing note: if the 20-foot segment carries the full 8 gpm design flow, 1/2-inch PEX is far too small - velocity and friction loss become excessive and the shower will be noisy and weak. If it is a branch serving only the shower (roughly 2 to 2.5 gpm), 1/2-inch is normally fine. Confirm which load that segment actually carries and size it accordingly; use 3/4 inch if it carries the group load.