Closed Water System Pressure Spikes After Water Heater Recovery
At a 4,200 sq ft two-story home, the municipal supply enters through a 1-inch copper service. A 3/4-inch pressure-reducing valve is set to 60 psi, and a double-check backflow preventer is installed downstream of the meter. A 50-gallon gas-fired water heater serves two full baths and a laundry. The homeowner reports that the T&P relief valve drips only after long hot-water draws and that a pressure gauge on the laundry hose bibb reads 105 psi at night, then falls to 62 psi in the morning. A plumber replaced the T&P valve once; the dripping returned in a week. The expansion tank is a 2-gallon bladder type, installed on the cold inlet, with its Schrader valve accessible. There is no recirculation pump. The water heater thermostat is set at 140 degrees F.
1. Diagnose the pressure spikes. Explain why replacing the T&P valve did not solve the problem, and distinguish a failed PRV from thermal expansion.
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The system is closed by the PRV and double-check assembly, so heated water has no place to expand. A 50-gallon heater raising water from about 50 degrees F to 140 degrees F expands roughly 1 to 1.5 gallons; a small or waterlogged 2-gallon tank cannot absorb that volume, so pressure rises until the T&P relieves. A failed PRV can also cause high pressure, but the pattern is the clue: pressure is normal in the morning and spikes after hot-water draws and recovery, which points to thermal expansion. Replacing the T&P only replaces the relief point; it does not remove the cause. Check whether the expansion tank is waterlogged, undersized, or isolated by a closed valve.
2. Describe a step-by-step test of the expansion tank and PRV, including expected gauge readings.
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With the heater off and a downstream faucet open to relieve pressure, measure the static inlet and downstream pressures. Close the faucet and observe the downstream gauge with no flow. A PRV set at 60 psi should hold about 60 psi; if it creeps toward 100 psi or more, the PRV is passing and needs repair or replacement. For the tank, isolate and drain it, then check precharge at the Schrader valve. With no water pressure on the system side, precharge should match the PRV setting, about 60 psi. If water comes out the Schrader, the bladder has failed. If precharge is low, recharge it; if the tank is full of water and will not hold precharge, replace it. Recheck the system after reinstalling.
3. Recommend the correction and the safety checks, including T&P discharge piping and expansion-tank sizing.
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Replace the expansion tank with one sized for the system volume and temperature range, set its precharge to the PRV setting, and install it on the cold supply without an isolation valve that can trap it unless a bypass is provided. If the PRV is passing, rebuild or replace it and verify the seat. Do not cap or plug the T&P relief valve. Verify the T&P rating, usually 150 psi, and confirm the discharge pipe is full-size, terminates safely, and is not threaded. Check for any check valve or backflow assembly that closes the system. After repair, draw hot water and confirm the pressure stays below the relief setting.