Ariston Pressure Relief Valve Leaking: Causes
On this page (6 sections)
The Short Answer: A relief valve releasing water from an Ariston tank is usually responding to something real: water pressure that is too high, pressure building as water heats in a closed plumbing system, or water that is too hot. Less often the valve itself is worn or dirty. Never cap or plug it. Find the cause, then replace the valve only once pressure and temperature are normal.
What the valve does
The temperature and pressure (T&P) relief valve opens to release water if the tank’s pressure or temperature goes above the limits stamped on the valve’s tag. It is the last safeguard against a tank overheating or over-pressurizing. A drip from its discharge pipe is a signal to investigate, not a fault to silence.
First, make sure the water is really coming out of the valve’s discharge pipe. Water at the threads where the valve screws into the tank, or at nearby fittings, is a different, usually simpler repair. Water on the front cover is covered in Ariston leaking from the front panel.
Match the pattern to the likely cause
| When it discharges | Likely cause | Next step |
|---|---|---|
| A little after each heating cycle, then stops | Thermal expansion in a closed system | Expansion tank check |
| Steadily, all the time | High house pressure, or a worn or dirty valve seat | Pressure test, then the valve |
| Started right after someone tested the lever | Debris on the seat, or the valve did not reseat | Valve replacement if it continues |
| Hot water or steam, heater very hot | Overheating: thermostat or element fault | Turn off power now |
| At night only | City pressure rises when demand is low | Pressure test |
Why pressure builds
Water expands as it heats. If the home has a pressure-reducing valve, a check valve or a backflow preventer at the meter, that expanding water cannot push back into the street main, so pressure in the house rises each time the heater runs. A. O. Smith’s bulletin on thermal expansion (opens in a new tab) explains the effect. An expansion tank on the cold line absorbs it. If there is no expansion tank, or the existing one has lost its air charge, the relief valve does the job instead and drips. See water heater expansion tank full of water.
High incoming water pressure has the same effect without any heating. Many plumbing codes and manufacturers treat static pressure above about 80 psi as too high for household plumbing; a pressure-reducing valve fixes it.
What you can check safely
- Water pressure. A pressure gauge that threads onto a hose faucet costs little at hardware stores. Read it in the morning and late at night. Readings above about 80 psi point to the supply.
- Expansion tank. If there is one near the heater, tap it. A tank that sounds full of water throughout, rather than hollow at the top, has likely lost its air charge.
- Temperature. Check the water at a tap. Water much hotter than the setting points to a control fault, which is a technician’s job.
- Discharge pipe. Make sure it runs to a safe place and is not blocked or reduced in size.
What the plumber will check
The plumber measures static and working pressure, checks for a pressure-reducing valve or check valve on the main, and tests or installs an expansion tank, setting its air charge to match the house pressure. If pressure and temperature are normal and the valve still drips, they replace it with a valve of the same pressure and temperature rating and connection size. Replacing the valve without fixing high pressure usually means the new one drips too; see relief valve leaking on a new water heater.
If the cause is overheating, a technician tests the thermostat, the high-limit safety and the element; see Ariston water heater reset button.
Frequently Asked Questions
Q: Is a dripping relief valve dangerous? A: A small drip from expansion is not an emergency, but it is a sign the system needs attention. A valve discharging hot water or steam is urgent: turn off power and call for service.
Q: Can I put a cap on the pipe until it is fixed? A: No. Capping the relief valve or its discharge pipe removes the tank’s protection against overpressure and is not allowed by plumbing codes. Put a bucket under the pipe instead.
Q: Does a small under-sink heater need an expansion tank? A: It depends on the plumbing. If the home is a closed system and the relief valve drips after heating cycles, a plumber may recommend a small expansion tank sized for the heater.
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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