Tankless Retrofit, a New PRV, and a Two-Inch-of-Water Problem on the Top Floor
A three-story single-family home has a 1 in. type L copper water service with 95 ft developed length and 78 psi static street pressure. The existing 3/4 in. meter drops about 9 psi at 20 gpm, and the highest fixture is 24 ft above the meter. The owner replaces a 40-gallon tank with a tankless heater rated 11 gpm and adds a recirculation return. A dual-check backflow preventer is going in at the meter, and a pressure-reducing valve set at 50 psi is going downstream of it. The top-floor shower valve needs 20 psi flowing pressure at the valve to deliver its rated 2.5 gpm. The plumber intends to keep the existing 1/2 in. branch to that shower and to skip a thermal expansion tank because 'the PRV takes care of the pressure.'
1. Working from the numbers given, is 50 psi at the PRV likely to leave 20 psi at the top-floor shower valve? Show the reasoning.
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Start with the static loss and work down; the answer is 'probably not - and certainly not by inspection.'
Useful constants: 1 ft of elevation ≈ 0.433 psi.
- Elevation: 24 ft × 0.433 ≈ 10.4 psi lost just climbing to the top floor.
- Available static at the top floor: 50 − 10.4 ≈ 39.6 psi before any friction.
- Friction still to be subtracted: the meter (about 9 psi at 20 gpm), the dual-check backflow preventer (typically several psi at design flow), the PRV's own friction loss, plus the pipe and fittings along a 95 ft developed length and the 1/2 in. branch. A 1/2 in. branch feeding a 2.5 gpm shower is already at a velocity where losses climb quickly; at simultaneous demand elsewhere the branch flow will exceed 2.5 gpm.
The arithmetic makes it clear that the margin between 39.6 psi and the 20 psi required is not large enough to absorb everything above. The correct approach is a flow test, not a guess: flow the system at peak demand (shower plus a couple of fixtures) and read residual pressure with a gauge at the highest fixture. If residual falls below what the fixture needs to deliver its rated flow, the fix is either a higher PRV setpoint (if the appliances and code allow it), upsizing the 1/2 in. branch, or a booster. Also verify that raising the setpoint stays within the code's limit for requiring a PRV in the first place - at 78 psi static, the PRV is not optional.
2. What is wrong with the decision to omit a thermal expansion tank, and what has to be installed with the PRV and backflow preventer?
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The dual-check backflow preventer plus the PRV converts the system to a closed system, and a closed system needs a place for expanded water to go.
Why the reasoning fails:
- The PRV controls pressure coming in; it does nothing about water heated in the tankless heater expanding against a closed check valve.
- During a recovery cycle, pressure can rise sharply with nowhere to relieve. The result is nuisance discharge from the water heater's temperature-and-pressure relief valve, or worse, a failed tank, fitting, or connector.
Correct installation:
- A listed thermal expansion tank on the cold side, at the water heater or on the cold line between the backflow device and the heater, sized for the system's stored volume and the supply pressure.
- A T&P relief valve piped to a safe discharge point - not capped, not plugged, not reduced, and terminating where scalding discharge cannot injure anyone.
- A backflow preventer with a relief port or a means of relieving thermal expansion is sometimes used, but it is not a substitute for a properly sized expansion tank.
- Confirm the PRV is installed with the required strainer and that the setpoint is adjustable and locked where required.
The judgment being tested: the plumber is introducing two devices that create the closed condition, so the plumber owns the expansion control - it is not 'the homeowner's problem later.'
3. What sizing and commissioning issues does the 11 gpm tankless heater create beyond the pressure question?
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Four separate issues, all of which the installer has to resolve before calling the job complete:
- Capacity is not the same as delivery. An 11 gpm rating exists at a specific temperature rise. In a Massachusetts winter, cold incoming water can be near 40°F. At a realistic 70°F rise, output is roughly half of the headline number. Check the manufacturer's flow-versus-rise chart for the actual ground water temperature before promising simultaneous showers.
- The branch, not the heater, may be the bottleneck. A 1/2 in. branch cannot deliver the heater's capacity to a fixture, and a tankless unit firing against a restricted branch can short-cycle or fail to modulate. The 1/2 in. shower branch should be evaluated as part of the same calculation as the service.
- Minimum activation flow and recirculation. A recirculation return with a tankless heater requires a manufacturer-approved recirculation arrangement (buffer tank, dedicated recirculation model, or timed control) or the unit will cycle continuously and scale prematurely. Confirm the return is not introducing cold water into the hot outlet.
- Commissioning is a test, not a signature. Verify gas input and manifold pressure (a tankless unit typically needs far more Btu input than the tank it replaced), measure flow at the fixtures, measure hot water temperature at the furthest fixture, and record residual pressure. If gas piping has to be resized for the larger input, that is a separate permit scope on the gas side.