Closed-System Thermal Expansion and Third-Floor Temperature Swings in a Motel
A 40-unit motel has a 75-gallon commercial gas-fired water heater set at 140°F, a 1-inch copper hot-water supply, and a thermostatically controlled recirculation pump on a return loop. Last month the water utility installed a new 1-inch pressure-reducing valve at the service and a check valve at the meter; the incoming pressure was 105 psi and is now 55 psi. There is no expansion tank. The heater's T&P relief valve, rated 150 psi and 210°F, discharges about a cup of water each day, usually after the morning peak. Guests on the third floor complain that shower temperature swings from cold to scalding when other rooms use water. The mixing valve at the heater is set to 120°F. The owner wants to cap the T&P discharge to stop the drip and raise the mixing valve to 130°F to improve the showers.
1. Diagnose the T&P discharge and explain why the owner's proposed fix is unacceptable.
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The new check valve and pressure-reducing valve have created a closed system. When the 75-gallon heater raises the stored water from incoming temperature to 140°F, the water expands and has nowhere to go because the check valve prevents backflow to the main. Pressure rises until the T&P relief valve opens, which is exactly what it is designed to do. The discharge is a symptom, not a nuisance. Capping the T&P discharge is illegal and dangerous because it defeats the final overpressure and overtemperature protection; the heater could rupture or the relief valve could fail. Raising the mixing valve to 130°F does not address expansion and increases scald risk. The correct fix is to install a properly sized thermal expansion tank on the cold-water side between the check valve and the heater, precharged to the reduced line pressure, or to use another approved expansion-control method. I would also verify the PRV is holding 55 psi and that the T&P rating matches the system.
2. What would you check to explain the third-floor hot/cold swing, and how would you test the recirculation system?
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A swing from cold to scalding at upper floors usually points to pressure fluctuation, a failing thermostatic mixing valve, or an unbalanced recirculation loop. I would measure temperature and pressure at the heater outlet, at the mixed outlet, at the recirculation return, and at a third-floor shower while other fixtures are opened. I would verify the recirculation pump is running, the return balance valve is set, and the check valve at the pump is not stuck. I would also check for a failed mixing-valve cartridge, a dip tube problem, or a cross-connection between hot and cold at a single-handle faucet. A simple test is to close the recirculation pump and observe whether the swing changes, then open several fixtures and watch the mixed-water temperature. If the mixing valve cannot hold within a few degrees, it should be rebuilt or replaced with a listed thermostatic mixing valve, not simply set hotter.
3. State the requirements for the T&P discharge piping and the maximum safe hot-water temperature at fixtures.
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The T&P discharge pipe must be the full size of the valve outlet, made of material rated for the temperature and pressure, installed with gravity slope, and terminated without a trap, thread, cap, or plug. It must discharge to a safe place such as 6 inches above a floor drain or outside above grade, where scalding water cannot injure anyone. It must not be reduced, connected to the drainage system as a trap arm, or run uphill. For fixture temperature, the adopted code and ASSE 1016 limits govern; public showers are typically limited to about 120°F, and many codes require lower temperatures at lavatories and bathing fixtures. Raising the mixing valve to 130°F is not an acceptable solution and would increase the scald hazard; the expansion problem must be solved separately.