Shower Temperature Swings After a Valve Replacement
A homeowner in a 1994 two-story colonial reports that the second-floor shower swings from 95°F to 135°F whenever the first-floor toilet is flushed or the kitchen faucet is opened. The water heater is a 50 gal gas tank set to 140°F, with no mixing valve and no thermal expansion tank. The shower has a new single-handle pressure-balancing valve (ASSE 1016) installed by the homeowner last month; the old valve behaved the same way. Static pressure at the front hose bibb is 82 psi. A 1/2 in copper branch from the 3/4 in main serves the shower, toilet, and vanity, and the shower valve has no integral check stops or access panel. The PRV at the meter is 15 years old.
1. Separate the possible causes and describe the pressure and temperature tests you would run to confirm the root cause.
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Possible causes include the shared 1/2 in branch, which drops pressure at the shower when the toilet or kitchen faucet opens; a pressure-balancing valve that is installed backward, has hot and cold reversed, or is outside its compensating range; a failing 15-year-old PRV; and a 140°F tank with no tempering valve, which explains the 135°F outlet. Tests: attach a pressure gauge at the shower outlet or nearest bibb and record static and dynamic pressure while someone flushes the toilet and opens the kitchen faucet. Measure shower temperature with a thermometer through the swing. Verify valve orientation and check stops, check PRV outlet pressure, and confirm whether an expansion tank exists. Compare the pressure drop at the shower with the pressure drop at the hose bibb.
2. What code and standard requirements apply to the shower valve and the hot-water temperature, and what corrections would you recommend?
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ASSE 1016 requires a pressure-balancing or thermostatic shower valve that limits outlet temperature. The plumbing code requires shower valves to be pressure-equalizing or thermostatic and limits the maximum hot-water temperature at the shower to 120°F; verify any local amendment. A 140°F tank needs a master tempering valve set to 120°F or lower. Recommend installing a thermostatic mixing valve at the water heater, replacing the 15-year-old PRV, adding a thermal expansion tank, and running a dedicated 3/4 in branch to the bathroom group. Provide access to the valve and install it per the manufacturer with check stops.
3. How would you verify the repair and protect against scald if the homeowner refuses a dedicated branch line?
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After repair, run the shower at a stable temperature while someone flushes the toilet and opens the kitchen faucet. Record the temperature swing with a thermometer and the pressure with a gauge. The outlet should remain within a few degrees and must not exceed 120°F. If the homeowner refuses the branch line, install a listed thermostatic mixing valve at the shower or at the water heater and set it to 120°F or lower, then verify with a thermometer. Document the test results and explain the residual risk. A pressure-balancing valve is not a substitute for a temperature-limiting device.