What is a TPR valve?
Updated August 2026
A TPR valve is the temperature and pressure relief valve on a water heater, a brass safety valve that dumps water automatically if the tank ever exceeds roughly 210°F or 150 psi. Both numbers are stamped on its body. It is the one part standing between a stuck thermostat and a tank of superheated water that flashes to steam the instant the shell gives way, which is why code has required one on every storage heater for decades.
Where it is
Check the top of the water heater, or high on one side, for a small brass fitting with a spring-loaded lever you can flip up with a finger. Threaded into it is the discharge pipe, and that pipe follows rules: full 3/4-inch with no reduction, sloping downhill the whole way, ending 6 inches or less above the floor or at a drain, with no threads, no cap, and no shutoff valve anywhere along the run. A pipe that stops 3 feet off the ground, points upward, or wears a cap is both a failed inspection item and a scald hazard. Tankless units carry an equivalent relief valve on the hot outlet.
What it does
Water inside a sealed tank cannot boil at 212 degrees. It stays liquid under pressure and holds an enormous amount of stored energy. If a thermostat sticks closed and the burner keeps firing, the relief valve opens first and sends that energy out the discharge pipe. With no working valve, the tank shell becomes the weak point, and a rupture is violent enough to launch a water heater through a roof. Those events stay rare precisely because these valves exist. Two failure modes matter. The valve crusts shut with mineral scale and gives no warning whatsoever, which is the entire argument for an annual test. Or it weeps steadily, which nine times out of ten is not a bad valve but thermal expansion in a closed system behind a pressure reducing valve or a check valve.
Maintenance
Once a year, set a bucket under the discharge pipe, lift the lever for about 5 seconds, and let it snap closed. You want a hard gush and a clean reseat. Nothing coming out means the valve is scaled shut and needs replacing that day (TPR testing intervals). Fold the test into the same visit as the annual flush and the anode rod check, and while you are down there set the thermostat to 120 degrees (temperature settings), which cuts scald risk and slows scale. One caution: on a heater past 8 years that has never been tested, the valve may not reseat, so buy a $20 spare first and keep it on the bench. Plan the tank swap around year 10 (replacement timing).
What it costs to replace
A replacement TPR valve runs $15–30 as a DIY part, or $150–250 installed by a plumber.
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Frequently asked questions
Why is my TPR valve dripping?
Suspect pressure before you suspect the valve. If the home has a pressure reducing valve or a check valve at the meter, the plumbing is a closed system, and the half gallon of expansion from heating 50 gallons has nowhere to go, so pressure spikes past 150 psi on every cycle and the relief valve does its job. A thermal expansion tank solves it for $40–90 in parts or $150–300 installed. Second suspect: a thermostat set above 140 degrees. Only after ruling out both is the valve itself the answer.
Can I just cap the discharge pipe?
No, and it ranks as the most dangerous thing an owner can do to a water heater. A cap converts a safety valve into a sealed tank and hands the pressure back to the shell. The same goes for plugging the valve, adding a shutoff in the discharge line, or reducing that pipe below 3/4-inch. If the dripping bothers you, fix the cause instead: measure pressure at a hose bibb, add an expansion tank, or replace the valve for $15–30.
How do I know the valve needs replacing?
Three tells. It produces no water at all when you lift the lever. It keeps dribbling after a test even with house pressure under 80 psi. Or the body and lever wear a crust of white mineral deposits and rust streaks. Any one of those and it comes out. Swapping it takes 20 minutes with a pipe wrench and thread tape on a drained tank, or $150–250 for a plumber, and it pairs naturally with the annual flush so the tank gets drained only once.