Water heater relief valve is dripping or discharging: what it means

On the side or top of every tank water heater is a brass valve with a small lever and a pipe running down from it. Most people never notice it until there is a puddle at the end of that pipe. It is the temperature and pressure relief valve, usually called the T&P valve, and it is the only device on the heater whose job is to keep the tank from bursting. A drip from it is a message, not a nuisance, and what the message says depends on how the water is coming out.

What the relief valve does

A water heater is a sealed steel tank full of hot water under pressure. If the thermostat fails and the elements or burner keep running, the water passes boiling, and a tank of superheated water that finally ruptures releases it all as steam. The relief valve prevents that. It opens at a set pressure, commonly 150 psi, or a set temperature, commonly 210 degrees, and dumps water through the discharge line until the tank is back within limits. That is why it must never be capped, plugged, or piped in a way that can block it. A capped relief valve turns a safety device into nothing.

Drip or discharge: reading the symptom

• A few drops after a heating cycle, then dry. Pressure, not temperature. The tank heats, the water expands, the pressure rises to the valve’s limit, and the valve lets a little out. This is the most common case and it points at the house, not the valve.
• A slow, steady drip that never quite stops. Usually a worn or debris-fouled valve that no longer seats fully. Common on heaters past eight or ten years, and after someone has tested a valve that had never been exercised.
• A steady stream, or hot water and steam. The valve is doing its job against a real over-temperature or over-pressure event. This is the one that needs you to act now.

The most common cause: thermal expansion

Water expands as it heats. In a house with a backflow preventer, a pressure regulator, or a check valve at the meter, the expanded water cannot push back into the street, so the pressure climbs with every heating cycle until the relief valve lets it out. Most Southwest Florida homes on city water now have one of those devices, and a heater that starts dripping right after a replacement, or after the utility changed the meter, is almost always this.

The fix is an expansion tank on the cold supply, sized and pre-charged for the house. A new relief valve without one drips again within weeks. If the house also has high street pressure, a regulator or an adjustment to the existing one is part of the fix.

Two other causes: a worn valve or an overheating tank

A relief valve is a spring holding a disc against a seat. After years of heat, mineral scale on the seat, and no exercise, the disc no longer seals and the valve weeps. Sediment in a tank that has never been flushed makes it worse. Replacing the valve is the answer, once expansion has been ruled out.

The serious cause is a heater that is overheating. A stuck thermostat, a failed high-limit switch on an electric heater, or a faulty gas control lets the tank climb past its setting. The signs are hot water at the taps that is hotter than it has ever been, rumbling or boiling sounds from the tank, and the relief valve releasing hot water or steam. Do not wait to see if it settles.

Testing the valve, and why you never cap it

Manufacturers recommend lifting the lever briefly about once a year to make sure the valve moves and reseats. Before you do, know the cautions. The water that comes out is scalding and under pressure, so make sure the discharge line is connected and ends where it should, and keep hands and face clear of the outlet. Lift the lever partway for a second and let it snap back. A valve that has sat untouched for years may not reseat, and then it drips until it is replaced; that is the trade-off, and it is why many plumbers prefer to test it during a service visit rather than have a homeowner start a leak they cannot stop.

Whatever you find, do not cap it, do not put a bucket under it and forget it, and do not add a valve or a plug to the discharge line. The line itself has rules: full size from the valve, no reduction, no shutoff, running downhill by gravity to a visible point a short distance above the floor or pan, or outside where a discharge would be seen and would not scald anyone. A line that runs uphill, ends inside a wall, or has a threaded end that invites a cap is a violation and a hazard.

When to shut the heater off and call

If the valve is releasing a stream of hot water or steam, or the water at the taps is far hotter than normal, treat it as an overheating tank. Turn off the breaker for an electric heater or turn the gas control to Off, then close the cold water valve on top of the tank, and stay away from the discharge outlet while it cools. Do not open the drain valve on a hot tank. Then call.

A drip after heating cycles is not an emergency, but it is not something to live with either, because every cycle is stressing every fitting in the house. A worn valve on an older tank is worth pairing with a look at the heater’s age. Send us a photo of the valve, the discharge line, and the pipes above the heater, and tell us whether it drips, runs, or steams. We will tell you which cause it is and what it takes to fix.