Why the temperature and pressure valve drips

A temperature-and-pressure relief valve drips because the water is too hot, the pressure is too high, or the valve can no longer close. Those causes are not interchangeable. Cooling the set point does nothing for a closed plumbing system that has nowhere to put the expanded water.

Adding an expansion tank does nothing for a valve whose seat was damaged when someone lifted the test lever. Capping the drip fixes the puddle and removes the only device that can save the tank from a failed thermostat.

This page is the diagnosis. The lever test, including manufacturers who would rather you replaced the valve than exercised it, is how to test a relief valve. The discharge-pipe rules are summarized in permits, pans, and straps.

Stop. Do not plug, cap, or valve off a dripping relief valve. Do not reduce the drip by partially closing the cold-water inlet.

If steam or a large volume of very hot water is coming out, shut the heater off — gas control to off, or the electric breaker off from a dry place — and call.

A valve that is fully open is either saving you from a runaway heater or has itself failed open. Either way the heater is not a shower supply tonight.

What the valve is allowed to do

On a storage heater the valve opens if temperature reaches its rating, commonly not above 210°F, or if pressure reaches its rating, commonly 150 psi, and in any case not above the tank’s working pressure. The code language in IRC P2804, as adopted locally, also says the pressure setting is not less than 25 psi above the system pressure and not greater than 150 psi.

The sensing element has to be in the top of the tank. The outlet is a pipe, full size, with no valve and no threads on the end for someone to cap, pitched to drain, ending close to the floor or an appropriate receptor where you can see it.

International Code Council notes explain why: you should notice the discharge, the pipe should not trap scalding water, and nobody should be hit in the face by a discharge at chest height.

If your pipe does not look like that, fix the pipe while you fix the drip. A pipe that runs uphill can hold water, corrode, freeze, and hide a valve that has been opening for months. A pipe that dumps into a pan which has no drain of its own only relocates the puddle.

Thermal expansion on a closed system

Water expands as it is heated. Heat a typical residential tank through a normal rise and the volume increase is on the order of half a gallon to a gallon, depending on gallons and degrees.

In an open plumbing system that extra volume sneaks back toward the city main. In a closed system it cannot. A check valve, a pressure-reducing valve, or a backflow preventer at the meter makes the house a closed bottle. Every heating cycle raises the pressure.

The relief valve opens a spoonful, the pressure drops, the valve tries to close, and you find a drip or a nightly puddle. The heater is not “overpressurizing” in the sense of a failed control. The plumbing has no elasticity.

The usual cure is a potable-water expansion tank on the cold side, sized from the heater volume and the static pressure, precharged to match the static pressure when the tank is installed. Manufacturers publish a chart.

A tank that is the wrong size, or that was precharged to 40 psi in a house that sits at 80 psi, goes waterlogged and the drip returns. Checking the precharge means isolating the tank and using a tire gauge on the Schrader valve, which is a plumber’s ten-minute test and a homeowner’s test only if you already understand which valve you are isolating.

Do not add a second relief valve and consider the expansion solved. Do not set the water heater’s valve to a higher rating than the tank allows so it stops dripping. You would be defeating the tank’s protection.

High street pressure makes all of this worse. If static pressure is over about 80 psi, a pressure-reducing valve is often required and, once installed, it is exactly the device that closes the system and creates the need for the expansion tank.

The two pieces belong together. If you have banging pipes as well as a drip, mention both. Water hammer and high pressure share a plumber and not a heater warranty.

A valve that drips because it is tired

Relief valves sit for years, then are asked to seal perfectly after a speck of scale lands on the seat. The act of testing can cause the drip.

If the drip started the day someone lifted the lever, the seat is fouled or nicked.

The repair is a new valve, not another test and not a cap.

Some manufacturers say so directly and prefer scheduled replacement over annual exercising. The test page holds that nuance. Here the action is the same: a valve that does not shut off completely is replaced.

Replacement means draining the tank below the valve, breaking the connection, installing the correct pressure and temperature rating with the probe reaching into the tank, and rebuilding the discharge pipe to the same rules.

The tank is refilled and bled before any electric power returns. It is a routine service call and a miserable flood if the old valve’s threads take the tank port with them. On an old tank, price a new heater against a new valve.

A valve on a tank that is about to leak at the bottom is a short comfort. Repair or replace is the comparison. Do not reuse the old discharge pipe if it is corroded, reduced in size, or fitted with a valve.

Temperature, not just pressure

A thermostat failed closed can drive the tank toward the valve’s temperature rating. The drip is then a high-limit event, and on electric heaters the red button may have tripped or may be about to.

You may see very hot water at the tap, well above what the dial claims.

Measure it. If the tap is far above 120°F and the valve is opening, shut the heater down and treat it as a control failure. Do not just add an expansion tank and walk away.

The expansion tank does not stop a runaway thermostat. Electric high-limit behavior is on the electric no-hot-water page. Gas controls fail too. A burner that does not shut off is a service emergency, not a drip.

A set point that is merely high, without a failed control, can still nudge a marginal valve if the pressure is already near the edge from expansion. Lowering the set point to the Consumer Product Safety Commission’s 120°F neighborhood is worth doing for scalding reasons anyway.

It is not a substitute for an expansion tank on a closed system. Confirm the cause. Otherwise you turn the dial down, the drip pauses for a week, and it returns on the next long recovery.

Checklist: a dripping relief valve

• Steam or a heavy discharge: heater off, then a plumber. Do not cap.
• Look at the pipe. No valve, no cap, no uphill loop, visible end near the floor. Fix the pipe if someone “improved” it.
• Did the drip start right after a lever test? Replace the valve.
• Is there a pressure-reducing valve, a check valve, or a backflow preventer at the meter? Assume a closed system until proved otherwise. Size an expansion tank.
• Measure static pressure and the tap temperature. High temperature points at the control. High pressure with normal temperature points at expansion.
• Do not raise the valve’s rating to hide the symptom.
• On an old tank, compare the valve job with a replacement heater before you invest in a stack of parts.

After the repair

The new valve should be dry during a normal recovery. A spoonful on the first heat as it seats is a reason to watch, not a reason to declare victory, and a continued drip means the expansion tank’s precharge or size is still wrong, or the new valve was damaged by debris still in the port.

Have it corrected. A relief valve is not a decorative fitting you get used to. The International Code Council’s notes say a discharge means something is wrong with the system, the heater, or the valve.

That sentence is the whole diagnosis, in order: system, heater, valve. Work it in that order when the pipe is already correct, and work the pipe first when the pipe is illegal. Then take a shower without wondering what the puddle is for.

Sources

International Code Council, “CodeNotes: Water Heater Safety in the I-Codes” (iccsafe.org), explaining IRC P2804 and IPC relief-valve discharge rules. IRC P2804.1 through P2804.6.1 as adopted locally: valve requirements, the 210°F temperature setting limit, the pressure setting window, and the discharge-pipe list.

U.S. Consumer Product Safety Commission publication 5098, for the tap-temperature context. Expansion-tank sizing charts are published by the tank manufacturers and should be followed for precharge and heater volume. ASME and the valve maker’s tag set the rating that may be installed on a given heater.