Second-Floor Shower Drops to a Trickle When the New Basement Sink Runs
You are called to a 1995 two-story, 2.5-bath home after a remodel added a basement bathroom and a laundry. Static pressure at the hose bibb is 62 psi. The homeowner complains the second-floor shower drops to a trickle when the new basement bathroom sink is used, and the upstairs shower valve has been replaced twice with no change. The service is 3/4-inch copper from the main. The new basement fixtures were tied off the 1/2-inch line feeding the second-floor bathroom group. Developed length from the meter to the far second-floor fixture is about 90 feet with roughly 20 fittings. Measured flow at the second-floor tub with all fixtures closed is 3.2 gpm; with the basement sink running it is 1.4 gpm.
1. Is this a pressure problem or a volume problem? Interpret the numbers and name the likely cause.
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It is a volume (friction/velocity) problem, not a static-pressure problem.
- 62 psi static is within normal design range (roughly 40-80 psi).
- The second floor sits about 20 ft above the gauge: 20 x 0.433 = 8.7 psi of elevation loss, leaving about 53 psi static upstairs. Still adequate.
- The flow loss appears only when two fixtures share the branch (3.2 gpm to 1.4 gpm). Friction loss rises with roughly the square of velocity, so a shared run that is fine for one fixture chokes when it carries two.
- A 1/2-inch branch was never sized for the added basement group. The two replaced shower valves confirm the valve is not the cause.
Conclusion: the common 1/2-inch section is undersized for the connected load and is being asked to serve two floors.
2. What measurements confirm the diagnosis, and what findings would change your mind?
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Measure residual (flowing) pressure, not static, with a gauge on the second-floor hose bibb or a tee at the shower valve:
- Everything closed (static, for reference).
- Basement sink only.
- Second-floor tub only.
- Both together.
Also gauge the water heater outlet and the meter to separate service/meter loss from branch loss. A sharp drop only in case 4 pins the common branch.
Change your mind if:
- Static pressure upstairs is below about 40 psi - then look at the PRV setting, an undersized meter, or a partly closed valve.
- The drop occurs with a single fixture running - then look for a clogged cartridge or aerator, a failing galvanized nipple, or scale in the 3/4-inch service.
- Pressure at the meter also collapses during flow - the problem is upstream of the branch and re-piping inside will not fix it.
3. Recommend the repair and describe how you prove it worked before you leave.
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Repair: give the basement group its own supply run from the main distribution point rather than teeing it off the 1/2-inch line that feeds the second floor. If the shared section cannot be removed, upsize the common segment to 3/4-inch. Size each branch to its fixture load; do not simply extend 1/2-inch to more fixtures.
Verification: run the second-floor tub and shower while the basement sink and laundry are open. Target residual pressure of about 20 psi or better at the second floor with all fixtures flowing, and confirm that shower flow does not visibly change when the basement sink opens. Record gauge readings before and after on the invoice.
If swing remains after the re-pipe, note that a pressure-balance or thermostatic shower valve protects against scald - but it does not fix low volume, so it is a supplement, not the repair.