Second-Floor Repipe and a Shower That Goes Cold
A 2,400 sq ft two-story house has a 1-inch service, 62 psi static pressure at the meter, and no pressure-reducing valve. The master bath is on the second floor, 22 feet above the meter; the developed length from the meter to the master shower valve is 118 feet. The master bath group is a 1.6 gpf water closet, two lavatories, a shower, and a tub, all currently fed by a single 1/2-inch PEX trunk that runs up an interior wall. The owner's complaint: the shower is fine alone, but when the kitchen sink or a toilet runs, the shower goes cold and weak, and it takes about 40 seconds for hot water to arrive. The house is being repiped anyway because of a slab leak, and the contractor wants to reuse the 1/2-inch trunk run 'since it is already in the wall.'
1. Do a pressure budget for the master bath and determine whether the 1/2-inch trunk can serve that group at 118 feet.
Read the worked answer
Start with what is left after gravity.
- Static at the second floor: 62 psi minus (22 ft x 0.433 psi/ft) = 62 - 9.5 = 52.5 psi.
- A pressure-balancing shower valve needs roughly 20 psi flowing to control temperature, so the friction budget is about 32.5 psi over 118 feet, or ~27 psi per 100 ft.
- 1/2-inch PEX (ID ~0.485 in) at 3 gpm runs around 25 psi per 100 ft; at 5 gpm it runs around 65 psi per 100 ft. Over 118 feet that is ~30 psi and ~77 psi respectively.
- Conclusion: the 1/2-inch trunk can pass only about 3 gpm before the entire budget is gone, and the master group needs more than that when the shower, a lavatory, and a flush overlap. The trunk is the cause of the weak flow.
- 3/4-inch PEX (ID ~0.68 in) at 5 gpm is roughly 13 psi per 100 ft, about 15 psi over the run - leaving about 37 psi at the valve. That works.
- Confirm with the manufacturer's PEX table, include fitting equivalent length, and keep velocity reasonable (about 4 to 5 fps here).
2. Will upsizing the trunk alone stop the shower from going cold when a toilet flushes? Explain and specify the fix.
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No. Sizing reduces the size of the pressure dip; it does not eliminate it.
- A water closet flush is a sudden 3 to 4 gpm draw on a shared line. Every fixture downstream sees a transient pressure drop for a second or two regardless of pipe sizing.
- An ordinary two-handle or non-compensating single-handle shower valve responds to that drop by changing the hot/cold mix, and the bather gets a slug of cold or hot water.
- The fixture-level fix is a listed pressure-balancing (ASSE 1016) or thermostatic mixing valve, which holds outlet temperature within the code tolerance when supply pressure varies. That is required equipment, not an upgrade.
- Do the sizing anyway - both are needed, and the undersized trunk makes the transient far worse.
- Also rule out: a failed cartridge allowing hot-to-cold cross-flow at another fixture, a deteriorated dip tube or sediment in an older heater, and an oversized-in-name-only 1/2-inch branch from the trunk to the shower.
3. The contractor proposes adding a hot water recirculation loop to 'fix the cold shower.' Evaluate that proposal against the two complaints.
Read the worked answer
Recirculation addresses the 40-second wait, not the temperature swing - they are different problems.
- The 40-second wait is dead leg delivery time. A dedicated return line with a pump, or a thermostatic bypass valve at the farthest fixture, solves it. Insulate the hot line while the wall is open; that alone can cut the wait substantially.
- A recirculation loop will not stop a shower from going cold when a toilet flushes. It keeps the supply line hot; it does not hold the mix ratio at the valve.
- Watch the trade-offs: a continuously running pump wastes energy and can shorten heater life; a demand-controlled pump on a timer or aquastat is better. On a tankless unit, verify the minimum activation flow and that the recirculation design is listed for that heater.
- Watch the cross-connection: a bypass valve must be listed for the purpose and must not permit hot-to-cold cross-flow in a way that defeats temperature control.
- Deliverable to the owner: pressure-balancing valve at the shower, 3/4-inch trunk sizing, insulated hot lines, and recirculation only if the wait time is still objectionable after the repipe.