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Water Heaters · Safety ValvesWater Heater Relief Valve (T&P) Replacement
Water heater relief valve (T&P) replacement stops a dripping valve only when the valve itself is worn; if the drip comes from real over-pressure in the plumbing, a new valve will drip as well. So the visit starts with a pressure gauge on the system. The valve swap, a repair inside the $150–$800 band, comes after the cause is known.
The temperature-and-pressure relief valve is the brass valve with a small lever on the top or upper side of the tank. It opens if tank pressure reaches 150 psi or the water nears boiling, about 99 °C, and it is the last thing standing between a failed thermostat and a ruptured tank. A probe on the back of the valve sits in the hottest water in the tank. It is a safety device first and a plumbing part second.
A drip has two possible causes. The first is a worn valve: scale on the seat, a tired spring, or grit lodged after the lever was last lifted. The second is thermal expansion. Water grows by a little under two per cent in volume as it heats, and where a check valve, backflow preventer or pressure-reducing valve stops it from pushing back into the street main, pressure in the house climbs until the relief valve opens, exactly as designed.
A gauge tells the two apart. A pressure gauge with a tell-tale needle goes on the drain valve or a laundry tap and stays there through a full heating cycle. Steady pressure with a dripping valve means the valve is worn and gets replaced. Pressure that climbs toward 150 psi as the tank heats means the system needs an expansion tank, or the one it has is waterlogged, and resting pressure above 80 psi points to the pressure-reducing valve.
The discharge pipe is checked on every one of these visits, because it is where the dangerous shortcuts show up. It has to be the full size of the valve outlet with no reduction and no shut-off, run downward all the way, and end in the open near the floor or over a drain where a discharge can be seen. The end is left plain: never threaded, never capped, never plugged. A capped relief line turns a nuisance drip into a tank with no safety valve.
Replace the valve, or look further, when
- The discharge pipe drips steadily whether the tank is heating or not: a worn valve
- It drips only while the tank reheats and stops when a tap is opened: thermal expansion
- The lever was lifted for a test and the valve has wept ever since: debris on the seat
- The discharge pipe is capped, plugged, reduced or missing: have it corrected today
- The valve releases hot water in bursts or the taps run scalding: shut off the fuel and call
How it works
Static pressure is read cold, then the tank heats with the gauge attached and the peak is recorded. The thermostat setting and the water temperature are checked at the same time.
Fuel and water off, the tank is drained below the valve, and a new valve with the same pressure setting and a BTU rating at or above the heater's input goes in.
The discharge pipe is rebuilt if it is undersized, uphill or capped, the tank is refilled and reheated, and the gauge confirms the result. Arrange a visit.
Common questions
Is a dripping relief valve an emergency?
A slow drip is a this-week job, not a middle-of-the-night one, provided the discharge pipe is open and the water at the taps is a normal temperature. A steady stream, steam, or scalding water at the taps is different: turn off the gas control or the breaker and call. Either way, why a relief valve drips explains what you are looking at.
Can I cap the pipe to stop the drip?
Never. The relief valve is the only thing that keeps a tank from becoming a sealed pressure vessel if a thermostat sticks, and a cap or plug defeats it completely. Put a bucket under the pipe instead. The same goes for threading a hose onto the end and running it uphill, or outdoors where it can freeze shut.
Should I test the lever myself?
Manufacturers ask for a periodic lift of the lever, and it is a fair test on a newer valve. On an old valve that has never been operated, expect that it may not reseat, because scale breaks loose and lands on the seat. Do it when a replacement could be arranged without drama, not on a Sunday night, or leave it for the annual maintenance visit.
We replaced the valve last year and it is dripping again. Why?
Because the valve was probably never the problem. Repeat drips usually trace to thermal expansion in a closed system, often after the water meter or a backflow device was changed, or to an expansion tank that has lost its air charge. A gauge reading through one heating cycle settles it, and do I need an expansion tank covers the fix.
Does the relief valve get replaced with a new water heater?
Yes. A new tank ships with its own valve matched to its rating, so the old one is never transferred. What does carry over is the cause: if the old tank's valve dripped from expansion, the new one will too until an expansion tank goes in. We check pressure during every water heater replacement for that reason.