Second-Floor Shower Starved: Is It Pressure, Pipe Size, or the Valve?
A 1970s two-story, 2,300 sq ft house has a 5/8 x 3/4 in. meter, a 3/4 in. Type L copper building supply with 80 ft of developed length (fitting allowance included), and a 1/2 in. copper branch 30 ft long serving one second-floor bathroom group. Static pressure measured at the meter outlet is 48 psi; the second-floor showerhead is 24 ft above the meter, and static pressure at the second-floor lavatory is 38 psi. The homeowner replaced the showerheads with two 2.5 gpm models, added a body spray, and now reports a weak shower whenever the washer or a hose bib is running. The washer and hose bibs share the same 3/4 in. trunk. The homeowner's proposed fix is "put in a bigger pipe."
1. Compute the residual pressure at the second-floor shower with the washer running. Is this a code violation, a performance failure, or neither?
Read the worked answer
Elevation loss is fixed and cannot be piped away: 24 ft x 0.433 psi/ft = 10.4 psi. That checks the 38 psi static reading at the lavatory (48 - 10.4 = 37.6 psi).
Flowing losses at a realistic simultaneous demand of about 14 gpm (washer 3.5, shower 2.5, body spray 2.5, lavatory 2.2, plus allowance):
- 5/8 in. meter at 12-14 gpm: roughly 6 psi
- 3/4 in. copper, 80 ft developed, at 14 gpm: about 0.18-0.20 psi/ft = 15 psi
- 1/2 in. branch, 30 ft, at about 7 gpm: about 0.15 psi/ft = 4.5 psi
Residual = 48 - 6 - 10.4 - 15 - 4.5 = about 12 psi.
The adopted code minimum is 8 psi at the fixture (confirm the exact wording in the Oklahoma-adopted text), so this is not a violation - it is a performance failure. A 2.5 gpm shower plus a body spray wants roughly 20 psi to feel right. The critical point: the 38 psi static reading tells you nothing about the flowing condition, which is where the complaint lives. Recheck the friction numbers against the actual tables rather than these approximations before you quote a remedy.
2. What do you do first on site, and which of the options actually buys pressure?
Read the worked answer
First: measure flowing pressure, not static. Screw a gauge on the washer bib, open the shower, body spray, and washer, and read the gauge. If flowing pressure collapses further than the calculation predicts, look for a partially closed stop, a clogged aerator screen, or a meter screen - not pipe size.
Then rank the fixes by psi gained per dollar:
- Reduce demand (remove the body spray, 1.8 gpm heads): recovers 2.5-5 psi instantly and is free.
- Upsize the 1/2 in. branch to 3/4 in.: about 4 psi.
- Upsize the building supply to 1 in.: about 8-10 psi at 14 gpm - the biggest single gain.
- The meter may be the real bottleneck. A 5/8 in. meter at 14 gpm eats 6 psi or more. Ask the water utility about a 3/4 in. or 1 in. meter before you repipe the house.
A booster pump is a last resort requiring utility/AHJ acceptance. Also check whether the pressure ever exceeds 80 psi, in which case a regulator is required anyway. Elevation loss caps what any repipe can achieve - do not promise a 40 psi shower on a 24 ft rise.
3. The customer adds that the shower runs cold whenever the washer fills. Does that change your answer?
Read the worked answer
Mostly no, and you should separate the two complaints or you will repipe a house and still get a call-back.
- Weak flow is the supply/sizing issue analyzed above.
- Temperature swing is a balancing problem. A single-handle valve sees the cold-side pressure drop when the washer valve opens, and the diaphragm/cartridge shifts toward hot. The fix is a pressure-balancing or thermostatic mixing valve at the shower (and anti-scald protection is required by code in most jurisdictions).
Upsizing pipe reduces the magnitude of the drop but never eliminates the swing. Also verify the water heater can carry the simultaneous load and that the dip tube/heat trap is intact - a cold shower complaint is often a hot-side delivery problem rather than a cold-side pressure problem. Document the flowing-pressure test results; they justify whichever remedy you sell.