Why is my water heater's pressure relief valve leaking?

The T&P valve is the single most important safety part on your water heater. A sealed tank full of water heated past boiling can fail with the force of a small bomb if pressure has nowhere to go. The valve prevents that. When internal pressure hits 150 psi or the water hits 210 degrees Fahrenheit, the valve lifts off its seat and lets water and steam escape, dropping the pressure back to a safe level.

Manufacturer Watts describes T&P valves as critical safety devices that protect equipment from excess pressure and catastrophic failure. That is why discharge deserves attention. It may reflect excessive pressure or temperature, or a valve that no longer seals correctly. A drip alone does not tell you which condition applies.

One rule comes before everything else. Never plug, cap, or screw a fitting onto a T&P valve to stop the drip. Doing so disables the one part standing between your tank and a rupture. If the valve is leaking, the answer is always to fix the cause or replace the valve, never to block it.

Thermal expansion is one possible cause. Water expands when heated, and the amount depends on the tank volume and temperature change. That added volume has to go somewhere. If a gauge on a hose bibb also shows pressure climbing after the heater runs, why water pressure suddenly gets too high walks through telling expansion apart from a failed pressure reducing valve.

In an open system the extra volume can move back into the supply. A check valve, backflow preventer or pressure-reducing valve (PRV) can restrict that path. In a closed system without effective expansion control, heating can raise pressure enough to trigger the relief valve. It does not necessarily reach 150 psi on every cycle, and a worn or dirty seat can leak below the valve's rated opening pressure. Commercial buildings with an RPZ at the meter are a common example, and our page on commercial backflow and expansion tanks explains where the tank goes.

A properly sized and charged thermal expansion tank is a common solution when expansion is the cause. It connects to the cold-water supply and uses an air cushion to accept expanded water and limit pressure rise. IPC Section 607.3 requires expansion control for a storage water heater supplied through a check valve, PRV or backflow preventer. Check any existing expansion device before adding or replacing equipment. A tank does not repair an overheating heater or a defective relief valve; our page about whether you need an expansion tank covers that distinction.

Sometimes excessive incoming pressure contributes to the problem. A high static reading leaves less room for pressure to rise during heating, but it does not prove the tank crosses the 150 psi relief point on every cycle. Measure the pressure and check expansion control as separate parts of the diagnosis.

Plumbing code (IPC Section 604.8) requires a pressure-reducing valve wherever incoming static pressure exceeds 80 psi. The PRV steps high street pressure down to a safe range and protects the whole house, not only the water heater. If you do not have one and your pressure runs high, the heater is one of the first places you notice it. A pressure reducing valve with a built-in bypass can ease this, but it doesn't replace an expansion tank; see whether a PRV needs a thermal expansion bypass.

You can check this yourself. Screw an inexpensive pressure gauge onto a hose bib or the heater's drain valve with all fixtures off, and read the dial. A reading well above 80 psi points to a PRV problem or a missing PRV. For the full picture, see our page on what home water pressure should be. The fix here is installing or repairing a PRV, which often resolves the T&P drip and protects your faucets, fill valves, and pipes at the same time.

The T&P valve also responds to heat. If the tank water reaches 210 degrees Fahrenheit, the valve opens whether or not pressure is high. A thermostat set too high, or a thermostat that has failed and is letting the burner run long, can push the water that hot.

The U.S. Department of Energy's water heating fact sheet says a 120 degrees Fahrenheit setting generally meets most household needs, and advises setting the thermostat low enough to minimize scalding concerns. That does not make hot water incapable of causing burns, and a thermostat setting alone does not confirm the actual tank temperature. If the valve discharges steam or unusually hot water, have the heater checked rather than treating it as an ordinary thermostat adjustment.

A valve discharging steam or unusually hot water can signal a thermostat that is not shutting off heat. Shut off the heater's power or fuel supply if it is safe to do so and call for service. Stay clear of the discharge, and never block it.

If a pressure reducing valve sits upstream of the heater, a dripping relief valve can point to thermal expansion. Our page on why pressure rises after a Watts pressure regulator shows how Watts describes the problem and the cold-tap test.

If your pressure is normal, your temperature is set right, and you already have an expansion tank, the valve itself may simply be worn out. T&P valves contain a spring and a rubber seat that age over time. Mineral scale from Phoenix's hard water builds up on the seat, and once it stops sealing cleanly the valve weeps even at safe pressure. A valve that has opened many times over the years eventually fails to reseal.

Here is how to work through the diagnosis and fix:

• Test the pressure. Record the static reading and whether it rises during heating. Above 80 psi needs correction, but a pressure problem and a defective relief valve can coexist.
• Check the temperature. Compare the actual water temperature with the heater's setting and manufacturer's guidance. Steam or unusually hot discharge calls for shutting down and having the heater checked.
• Check expansion control. Identify any expansion tank or other approved device and check its condition, sizing and setup. Absence of a tank alone does not establish the cause of the drip.
• Have the valve checked. If pressure, temperature and expansion control are appropriate, a worn or dirty seat may be preventing the valve from sealing. Replacement requires shutting down and depressurizing the heater and following its manufacturer's instructions. If the valve is releasing a lot of water, see how to shut off a leaking water heater.

Pay attention to the discharge tube as well. Code (IRC Section P2804) requires a rigid pipe routed from the valve down to within a few inches of the floor or to an approved drain, so escaping hot water flows safely downward and never sprays a person. The tube must run downhill, stay the same size as the valve outlet, and have no threads on the end and no valve in its path. If your heater is missing this tube or it points the wrong way, fix that no matter what else is going on.

If your home is in unincorporated Maricopa County, the county says a replacement done to manufacturer specifications needs no permit, and we cover that in our page on water heater permits in the county.

A dripping T&P valve needs attention, and steam or unusually hot discharge can indicate overheating that calls for prompt shutdown and service. If the drip is persistent or you cannot find the cause, our licensed Phoenix team (Arizona ROC #355170) can test the pressure, check expansion control and the relief valve, and recommend a repair. If you also see water pooling under the heater, read our page on a water heater leaking from the bottom to check for tank failure.

The relief valve is one of several safety and shutoff valves in a home. For how they all fit together, see the different types of water valves. The code expects a few other safety devices on every home too, listed in the plumbing safety devices a home needs.

If you hear hissing rather than see a drip, see why a water heater is hissing.

Water that appears on top of the tank but not at the valve usually comes from the inlet or outlet joints; leaks from the top of a water heater walks through each part.

Where the relief valve discharge pipe goes and when a pan is required are covered in commercial water heater relief valve discharge and pan rules.