Second-Floor Pressure Collapse on a Long 1/2-Inch PEX Run
A 1970s ranch house with a 1 in. meter and 62 psi static pressure at the meter outlet. The service lateral is 1 in. copper for 40 ft, then a 3/4 in. copper main runs 35 ft to the water heater, and from there a single 1/2 in. PEX line runs 70 ft developed length, including fittings, up to a second-floor bathroom group: shower, lavatory, and water closet. Homeowners complain that shower pressure collapses to a trickle whenever the kitchen faucet is opened or a toilet is flushed, and that hot water takes about 25 seconds to arrive. Measured flowing pressure at the shower head is 20 psi with only the shower running, and 11 psi with the shower plus a toilet flush. Calculated peak demand for the house is about 9 gpm. The contractor proposes adding a 2.5 gpm rain shower head and a body-spray set that together draw 5 gpm.
1. Diagnose the cause of the pressure collapse using the given numbers. Is this a supply-pressure problem or a pipe-sizing problem?
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Start with velocity, not opinion. 1/2 in. PEX has an inside diameter near 0.475 in., so the cross-section is about 0.177 sq in. At 6 gpm, roughly 23 cu in. per second, velocity is about 10 to 11 ft per second. Good design practice and most code velocity limits keep domestic water under about 5 to 8 ft per second; above that, friction loss per foot climbs steeply and noise and erosion of fittings increase.
Friction arithmetic. Over a 70 ft developed run at that velocity the friction loss is on the order of 10 to 15 psi before fitting allowances, using the manufacturer's table. Subtract that from 62 psi static, then subtract the losses in the 3/4 in. main and the service, and a 20 psi flowing reading at the shower with a single fixture open becomes unsurprising. When a toilet flushes, the shared 1/2 in. line carries the additional flow, velocity rises again, loss rises with roughly the square of the flow, and the shower collapses to 11 psi.
Conclusion. The supply is fine at 62 psi static. This is an undersized branch. A 1/2 in. line should never be the sole feed to an entire bathroom group 70 ft from the heater.
Confirm. Verify static pressure at the meter and at a hose bibb near the house to rule out a failed PRV or a partially closed valve, then compare measured flowing pressure to the friction calculation. Do not install the new shower head and sprays until the branch is corrected.
2. What is your recommended correction, and how much does it actually improve the numbers? Include the tradeoff the homeowner will notice.
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Upsize the run. Replace the 70 ft of 1/2 in. PEX with 3/4 in. PEX to a point close to the bathroom group, then reduce to 1/2 in. only for the individual fixture branches, each a few feet long. 3/4 in. PEX has an inside diameter near 0.671 in., about double the cross-section, so at 6 gpm velocity drops to roughly 2.5 to 3 ft per second and friction loss falls by well over half, often by 70 percent or more because loss scales steeply with velocity. That converts 11 psi at the shower to something comfortably in the usable range even with a flush underway, and it lets the 5 gpm rain head and sprays perform.
The tradeoff: wait time. Volume in the existing 1/2 in. line is about 0.64 gallon (0.177 sq in. times 840 in.). At 2.5 gpm that alone is roughly 15 seconds of the measured 25 seconds; the rest is the 3/4 in. main and the distance from the heater. Upsizing to 3/4 in. increases stored volume by roughly 2.3 times for that run, which makes the wait worse, not better.
Address both at once. Since the wall is open anyway, propose a demand-initiated recirculation system with a return line or a crossover valve, insulated hot lines, and a thermostatic mixing valve at the heater if the stored temperature is raised for recirculation. Alternatively, run a dedicated recirculation return in 1/2 in. and set the timer for the family's schedule.
Verify upstream too. Confirm the 3/4 in. main and the 1 in. meter support the new peak demand, and check the water heater's capacity and recovery against the added 5 gpm, not just the piping.
3. The customers ask whether they can add the 5 gpm body-spray set without changing any pipe. Give your answer and the reasoning you would put in writing.
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Answer: no, and say why in numbers. The bathroom group already fails at the current demand. Adding a 2.5 gpm rain head plus sprays drawing 5 gpm means the upstairs branch sees roughly 7 to 9 gpm when someone else is using water. In 1/2 in. PEX that is around 12 to 15 ft per second, with friction loss high enough to drop the shower below the minimum pressure needed to run the sprays properly and to make the toilet refill slowly. The fixture will not perform as advertised and the complaint will come back to you.
Also check the shower valve and the heater. The mixing valve must be listed for the flow and must limit outlet temperature; a body-spray system with a rain head typically needs a valve rated for the combined flow, plus a drain sized for it. A 40 to 50 gallon tank may not keep up with a 7 gpm shower plus normal household use, so confirm first-hour rating before selling the upgrade.
Written response. State the measured pressure readings (20 psi single fixture, 11 psi with a flush), the calculated velocities, and the required correction: upsize the branch to 3/4 in., add recirculation if wait time matters, verify the heater, and then install the fixtures. Offer the corrected scope and price rather than a refusal, and note that installing the fixtures first would have to be undone to open the wall again.