New Water Heater Weeping at the T&P Valve
A 50-gallon electric water heater was replaced in an 1,800 sq ft house served by a municipal main. The service has a 3/4-inch pressure-reducing valve set at 60 psi and a double-check backflow preventer at the meter. The old 2-gallon expansion tank was left connected, but the installer did not check its air charge. Within a week the T&P valve, rated 150 psi and 210 F, discharges roughly a cup of water each time the elements recover from a 30-gallon draw. A hose-bibb gauge reads 62 psi cold and peaks at 148 psi during recovery. The T&P discharge is 3/4-inch copper that ends 4 inches above the garage floor with a threaded nipple closed by a hose cap. The heater sits in a closet with a floor drain 8 ft away.
1. Diagnose the recurring T&P discharge. Why is the heater behaving this way, and what evidence points to your answer?
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This is thermal expansion in a closed system, not a failed T&P valve. The pressure-reducing valve and the double-check at the meter both act as one-way devices, so heated water has nowhere to expand. A 50-gallon heater drawing from about 60 F to 140 F grows roughly half a gallon in volume; pushed into a closed piping system, that volume drives pressure up sharply.
The gauge tells the story: 62 psi cold, 148 psi at the peak of recovery. Pressure rises only when the elements are on and falls back when they are off, which is the signature of expansion. A bad T&P valve would weep at random or leak continuously, and a failed PRV would show high static pressure, not a cold reading of 62 psi.
The old 2-gallon tank is probably waterlogged. A bladder tank with a lost air charge has no acceptance volume, so the expanding water compresses nothing and the T&P becomes the relief path. That is a safety device functioning as designed, not the defect.
2. What is your repair sequence, including how you would check and size the expansion tank?
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Isolate and drain the heater, then check the tank. Press the Schrader valve: water means a ruptured bladder and the tank must be replaced; air means check the charge with a tire gauge. The air charge should match the supply pressure, about 60 psi here. An undercharged tank accepts water on the air side and is useless.
Size by acceptance volume, not by tank shell size. The expansion of 50 gallons from about 60 F to 140 F is on the order of half a gallon, but the tank must accept that volume between the precharge pressure and the relief setting, and the heater will see higher temperatures and higher PRV settings on other jobs. A 2-gallon tank with a correct 60 psi precharge is marginal here; a 4.5-gallon tank gives margin and is the practical choice.
Set the precharge with the system depressurized, install the tank on the cold inlet, then restore pressure and verify the peak at the bibb drops well below the 150 psi relief setting. Do not solve this by installing a higher-rated T&P valve or by plugging it. Finally, correct the discharge piping and retest.
3. Evaluate the T&P discharge piping as described. What is wrong and what does the code require?
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Three problems. The discharge terminates only 4 inches above the floor, below the typical minimum of 6 inches, and it ends in a threaded nipple closed with a hose cap. A T&P discharge must be open and unrestricted, with no thread at the outlet and no cap, plug, or valve, because someone will eventually close it and defeat the relief. The pipe must be the full size of the valve outlet with no reduction, must run in a material rated for the temperature, and must terminate in a safe place where scalding water cannot strike anyone.
The right fix is to remove the cap and threaded nipple, replace that end with a plain-end pipe terminating not less than 6 inches and not more than 24 inches above the floor or a receptor, and reroute the run so it discharges over the floor drain or another safe point rather than into a walkway. Also confirm no other valve or fitting is installed between the T&P and the discharge outlet and that the discharge is not trapped where water can stand. Note clearly on the invoice that plugging a T&P is a life-safety issue, so the customer understands why the cap cannot go back on.