Water Heater Relief Valve Stuck Open: What to Do
On this page (4 sections)
The Short Answer: A relief valve that runs in a steady stream is either responding to dangerous temperature or pressure, or is stuck open from debris or wear. Either way: turn off the heater’s gas or power, close the cold-water inlet valve, and keep away from the discharge, which may be scalding. Never cap or plug the valve or its pipe. A plumber will find out which it is and replace the valve.
Make it safe, in this order
- Turn off the heat. On a gas heater, turn the gas control knob to OFF. On an electric heater, switch off its breaker.
- Close the cold-water inlet valve on the pipe that feeds the heater. This stops the house supply from pushing more water through the valve.
- Open a hot-water faucet nearby. This relieves pressure in the tank and usually slows the discharge.
- Leave the discharge pipe open. It must stay unobstructed until the valve is replaced.
- If the inlet valve will not close, shut off the main water supply to the house.
On an electric heater, keep the breaker off once the tank may be partly empty. Restoring power to a tank that is not full can burn out the elements; see water heater turned on without water.
Overheating, overpressure, or a failed valve?
| What you notice | Likely cause | Why it matters |
|---|---|---|
| Steam, or water at the taps far hotter than normal | Overheating from a thermostat or gas control that did not shut off | The valve is doing its job; the heater must not be used until repaired |
| Stream slows sharply once the cold inlet is closed | Pressure from the house supply: high street pressure or thermal expansion in a closed system | Pressure must be corrected or the new valve will discharge too |
| Valve began running right after its lever was lifted | Debris on the seat or a damaged valve | See relief valve leaking after testing |
| Lever stays in the raised position | Seized or broken lever | Valve replacement |
| Old valve with heavy mineral crust | Worn or fouled seat | Valve replacement |
A. O. Smith’s thermal-expansion bulletin (opens in a new tab) explains how heating water in a closed system raises pressure, a common reason relief valves open.
What the plumber will check
A plumber will confirm whether the tank was overheating by checking the thermostat or gas control and the water temperature, measure house water pressure, and check the pressure-reducing valve and expansion tank. They then replace the relief valve with one that matches the heater’s rating plate and check that the discharge pipe is the full size of the valve outlet, runs to an approved location, and has no valve or cap on it. If the cause was overheating, the faulty control is replaced before the heater is put back in service.
Frequently Asked Questions
Q: Can I cap the relief valve until the plumber arrives? A: No. Capping removes the tank’s protection against overpressure and overheating. Turn off the heat and close the cold inlet instead; that stops the flow safely.
Q: Does a stuck-open relief valve mean I need a new water heater? A: Usually not. The relief valve is a replaceable part. If overheating caused it, the thermostat or gas control is also repaired. The tank itself only needs replacing if it is leaking.
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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