The Water Heater That Started Dripping After the PRV Went In
A service call: 50-gallon natural gas water heater, 40,000 BTU/hr input, 0.80 recovery efficiency, set to 130 degrees F, incoming water 58 degrees F, family of four. It sits in a garage on a 16-inch platform with no drain pan and no bollard. The T&P relief valve has a 3/4-inch outlet bushed down to 1/2-inch copper that runs 4 feet and stops 18 inches above the floor; the end is threaded with a cap screwed on 'to keep dust out.' The owner says the T&P began dripping about a month ago, shortly after the city replaced the service line and a plumber installed a 1-inch pressure-reducing valve set at 60 psi because street pressure measured 105 psi. They also mention the second shower sometimes runs cold, especially after someone does dishes.
1. Why did the T&P valve start discharging after the PRV was installed, and what is the correct remedy?
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The system became closed, and the heater is now absorbing thermal expansion.
- The PRV acts as a check valve. Once it was installed, water heated in the tank had no path back to the main, so pressure climbed until the T&P opened. The valve is doing its job, not failing.
- Size the effect: water at 58 F to 130 F expands roughly 1.3 percent, so a 50-gallon tank pushes out on the order of 0.6 to 0.7 gallon per heat-up, plus the piping volume.
- Remedy: install a listed thermal expansion tank on the cold side, precharged to the PRV setting (60 psi), with acceptance volume greater than the calculated expansion - a 2-gallon tank is the usual minimum here. Verify the PRV is actually delivering about 50 to 60 psi, not passing 105 psi through.
- Confirm the T&P is the correct 150 psi / 210 F rated device and that it is not being kept open by debris.
- Do not solve this by removing the PRV, raising the T&P setting, or capping the discharge. All three are failures that can turn the tank into a bomb.
2. List every deficiency in the T&P discharge piping and the installation, and state the correct condition for each.
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Walk the discharge line first.
- Reduced size - the discharge must be the full size of the valve outlet (3/4 inch here). The bushing down to 1/2 inch is a violation.
- Threaded end - the discharge terminates with threads and a cap. It must be unthreaded, with no valve, no plug, and no cap; a capped relief line is a pressure vessel waiting to fail.
- Termination height - it must terminate within 6 inches of the floor, or to the exterior, and discharge where it can be seen. Eighteen inches is too high to be a safe, observable air gap.
- Material and routing - copper is fine, but it must be rated for 210 F, must run by gravity with no traps or low spots, and must not be concealed where a discharge would go unnoticed.
- Pan - a pan with a drain is required where leakage would cause damage; here the more pressing garage items are:
- No bollard - protect the heater from vehicle impact as required in a garage.
- Ignition source - an FVIR heater with the ignition source elevated is required in a garage; confirm the unit is listed for garage installation.
- Correct all of this before restarting the heater.
3. Is the heater undersized for this family, and does that explain the second shower going cold? Show the math.
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Run the recovery and first-hour numbers before condemning the tank capacity.
- Recovery: (40,000 BTU/hr x 0.80) / (8.33 lb/gal x (130 - 58 F)) = 32,000 / 600 = 53.4 gallons per hour.
- First hour rating: (0.7 x 50) + 53.4 = 35 + 53.4 = about 88 gallons.
- Demand: a 10-minute shower at 2.5 gpm is 25 gallons at roughly 105 F. Hot fraction = (105 - 58) / (130 - 58) = 0.65, so about 16 gallons of hot water per shower. Three back-to-back showers draw only about 49 gallons of hot water, well inside 88 gallons even before recovery is counted.
- So capacity is not the primary explanation. Look instead at:
- A shower valve with no pressure-balancing or thermostatic element.
- Cross-flow through a failing cartridge at another fixture.
- A deteriorated dip tube or heavy sediment shortening effective capacity.
- Simultaneous fixture use on an oversized branch or a long, uninsulated run to the master bath.
- The temperature complaint is a control and distribution problem, not a storage problem; do not sell a bigger tank as the fix.