Why Your Water Heater Relief Valve Keeps Dripping (It Is Not the Heater)
Homeowners replace water heaters over this. The valve is doing exactly what it was designed to do — the problem is upstream of it.
July 17, 2026 · 6 min read
There is a valve on the side or top of your water heater with a lever and a pipe running down toward the floor. It is the temperature and pressure relief valve, usually called the T&P, and it is a safety device — it opens if the tank ever exceeds safe temperature or pressure.
If yours discharges periodically, releasing a cup or so of water every day or two, that is not a faulty valve. It is a working valve doing its job in response to a condition somewhere else in the system. Replacing the water heater does not fix it, and people do exactly that.
What is actually happening
Water expands when it is heated. In a water heater, a tank full of cold water becoming a tank full of hot water means a measurable increase in volume, and that volume has to go somewhere.
In an open system, it pushes back into the municipal supply line. Slight, harmless, nobody notices.
In a closed system, that path is blocked. Now the expanding water has nowhere to go and pressure inside your plumbing rises sharply every heating cycle. Eventually it reaches the T&P valve's set point, the valve opens, releases a little water, and pressure drops. Then the cycle repeats.
What makes a system closed
This is the part that surprises people, because it is often a change nobody thinks of as significant.
- A pressure reducing valve at the meter. Very common in this area, and many PRVs incorporate a check function that prevents backflow.
- A backflow preventer, sometimes required by the municipality.
- A check valve anywhere on the incoming supply.
- A water meter with a built-in check valve, which some utilities have installed as standard.
The awkward part: any of these can be added years after a house was built. A home that operated as an open system for a decade becomes closed the day a PRV goes in, and the T&P valve starts weeping shortly afterward. The homeowner connects it to nothing, because from their perspective nothing changed.
The fix
An expansion tank. It is a small tank, usually mounted on the cold supply near the water heater, with a rubber diaphragm inside separating water from a pocket of pressurised air. When water expands, the diaphragm flexes and the air compresses, absorbing the volume without a pressure spike.
That is the whole mechanism, and it is why the fix is straightforward once the diagnosis is right. In many jurisdictions an expansion tank is now required on a closed system, and it should be fitted whenever a water heater is replaced on one.
How to tell if yours has already failed
Expansion tanks do not last forever. The diaphragm eventually fails or the air charge leaks away, at which point the tank fills with water and stops absorbing anything. The T&P valve starts discharging again, and the homeowner concludes the tank did not work.
Two quick checks. Tap the tank — it should sound hollow toward the top and solid toward the bottom. A tank that sounds solid all the way up is waterlogged. Second, press the Schrader valve on the end, the same fitting as a bicycle tyre. Air should come out. Water coming out means the diaphragm has failed and the tank needs replacing.
When it is not thermal expansion
Two other causes worth ruling out, because the response is different.
High incoming pressure. If your static pressure is running well above what residential plumbing is designed for, the T&P may be discharging simply because system pressure is too high all the time, expansion or not. A gauge on an exterior hose bib settles it in a minute, and a pressure reducing valve is the fix.
A genuine temperature problem. If the water coming out is scalding, or the tank is making noises it did not used to, the T&P may be responding to overheating — which points at a failed thermostat or gas control. That one is a safety matter and it should not wait.
There is also the simple possibility of a worn valve. T&P valves can fail to reseat after years of service, particularly in hard water where mineral builds on the seat. But that is a conclusion to reach after ruling out expansion and pressure, not before.
Why this matters beyond the drip
It is tempting to treat a small periodic discharge as a nuisance. Two reasons not to.
The pressure spikes causing it are hitting every fixture, valve, appliance and connection in the house, several times a day. That is a genuine factor in the fixture failures and small leaks that homeowners experience as bad luck.
And a T&P valve that opens repeatedly can eventually fail to reseat, at which point it either drips continuously or, worse, seizes. It is the last line of defence on a pressure vessel and it should be a device that never has to operate.
If yours is discharging, call (469) 338-5533. Diagnosing it takes a pressure gauge and a few minutes, and the fix is usually small.
Frequently asked
Can I just cap off the T&P discharge pipe to stop the dripping?
No, absolutely not, and this is genuinely dangerous. The T&P valve is the safety device that prevents a water heater from becoming a pressure vessel failure. Capping or plugging its discharge defeats the one protection standing between an overheating tank and a serious incident. If it is discharging, fix the cause.
How long does an expansion tank last?
Several years typically, though it varies with water pressure, cycling frequency and water quality. They are not permanent and they are inexpensive to replace. Worth checking during any water heater service — tap it and check the Schrader valve. A waterlogged tank is doing nothing at all while everyone assumes the system is protected.
Do I need an expansion tank with a tankless water heater?
Generally the thermal expansion issue is much smaller with tankless, because there is no large volume of stored water being heated as a batch. Some installations still call for one depending on the configuration and local requirements, particularly where there is a recirculation loop or a buffer tank. Worth confirming rather than assuming either way.
Where should the expansion tank be installed?
On the cold water supply line to the water heater, upstream of the heater and downstream of any check valve or PRV. Orientation and support matter — a full expansion tank is heavy, and hanging one off the pipe without proper support strains the connection. Most installations mount it vertically with dedicated support, and that is worth doing properly.