Why Does My Water Heater Overflow Keep Discharging?
On this page (6 sections)
The Short Answer: The “overflow” pipe on a tank water heater comes from the temperature and pressure (T&P) relief valve. It discharges when tank pressure or temperature exceeds the valve’s rating, most often from high house water pressure, thermal expansion with nowhere to go, or overheating. A recently tested or old valve may also fail to reseat. Find the cause; never cap or plug the pipe.
Is it really the relief valve?
Make sure the water is coming from the discharge pipe and not from somewhere else nearby:
| Where the water is | What it is |
|---|---|
| Dripping from the end of the pipe that runs down from a brass valve near the top of the tank | T&P relief valve discharging |
| Wet threads where the valve screws into the tank, discharge pipe dry | Leak at the valve fitting |
| Drain valve near the bottom | Drain valve leak; see plastic drain valve dripping |
| Tank body, all fittings dry | Tank leak; see leaking from the bottom |
The usual causes
Thermal expansion. Water expands as it heats. In many homes, a pressure-reducing valve, a backflow preventer, or a check valve at the water meter stops that extra volume from flowing back to the street. Without a working expansion tank, pressure climbs during every heating cycle until the relief valve opens. A. O. Smith’s bulletin on thermal expansion (opens in a new tab) explains how this works. The tell: dripping mainly while the heater is heating, often after a long period with no hot water use.
High house water pressure. Plumbing codes generally call for a pressure-reducing valve when supply pressure is over 80 psi. A failed reducing valve can let street pressure into the house and push the relief valve open.
Overheating. If the thermostat or gas control fails and the water gets too hot, the valve opens on temperature. The discharge will be very hot or steaming, and taps may be scalding. Turn the heater off; see suddenly too hot.
A worn or fouled valve. After the test lever has been lifted, or after years of service, debris or mineral deposits can keep the valve from closing fully. See leaking after testing.
What you can check safely
- Read your water pressure. An inexpensive gauge on an outdoor hose bib shows static pressure. Check it at different times of day. Also note whether pressure climbs when the water heater is heating and no water is in use; that points to thermal expansion.
- Look for an expansion tank on the cold-water pipe near the heater. If there is one, gently tap it: a tank that sounds and feels full of water from top to bottom may be waterlogged. Check whether it is several years old.
- Feel the discharge water carefully from a distance, or note whether it steams. Hot discharge points to overheating.
- If it is a heavy, steady flow, turn the heater off (gas control to OFF, or the breaker) and close the cold-water inlet valve.
What the plumber will check
They will measure static pressure and watch how it changes during a heating cycle, check the expansion tank’s air charge against house pressure, test the pressure-reducing valve, check whether the heater overshoots its setting, and replace the relief valve if it is worn. The replacement must match the pressure and temperature rating on the heater’s label. They will also confirm the discharge pipe is the full size of the valve outlet, runs downhill to a safe spot, and has no valve or cap.
Repair or replace
All of these are repairs: an expansion tank, a pressure-reducing valve, a new relief valve, or a thermostat or gas control. A discharging relief valve alone does not mean the tank is bad. Replacing only the relief valve without addressing pressure or expansion usually means it starts dripping again.
Frequently Asked Questions
Q: Is a small drip from the relief valve normal? A: No. An occasional drip during heating is a sign of thermal expansion or high pressure that should be corrected, even if it seems minor.
Q: Can I just replace the relief valve? A: If the valve is old or was recently tested and won’t reseat, a new valve may fix it. If pressure or expansion is the cause, a new valve will discharge just like the old one.
Q: Why did this start after my water meter was replaced? A: Utilities sometimes install a check valve or backflow device with a new meter. That closes the plumbing system so expansion has nowhere to go, and an expansion tank is then needed.
Manuals and supporting references
These links distinguish general safety information from model-specific documentation. A manual directory helps you locate your unit’s instructions; it does not establish that every model uses the same parts, codes, or test values.
- A. O. Smith technical bulletin 45: thermal expansion (opens in a new tab)Background on pressure rise in a closed water system. Pressure testing and repairs are professional work.
- A. O. Smith technical bulletin 51: puddles around a heater (opens in a new tab)Common water-leak sources and the distinction between fittings and a failed tank.
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