A dripping temperature and pressure (T&P) relief valve almost always means real pressure inside the tank reached its limit. The usual cause is thermal expansion on a closed system, fixed with an expansion tank. High incoming pressure, too-high temperature, or a worn valve can also cause it.
What the T&P valve does and why a drip matters
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 puts the rule plainly: a relief valve "is the only mechanism on a water heater to relieve excess pressure and temperature." That is why a dripping valve is worth your attention. A valve that opens now and then is doing its job, but a valve that drips repeatedly is reacting to a condition that keeps coming back. Find that condition and you stop the drip at its source.
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.
Cause one: thermal expansion on a closed system
This is the most common reason a T&P valve drips, and it surprises a lot of homeowners. Water expands when you heat it. A 40-gallon tank heating cold water to your setpoint produces roughly half a gallon of extra volume. That water has to go somewhere.
In an open system the extra volume pushes back into the city main. But most Phoenix homes are closed systems. A check valve, a backflow preventer, or a pressure-reducing valve (PRV) on the incoming line stops water from flowing backward. With the expanded water trapped, pressure spikes every time the burner or element runs. When the spike tops 150 psi, the T&P valve opens, drips, and reseals. You see a little water near the discharge tube, often after the heater has been idle and then fires up.
The fix is a thermal expansion tank. This small tank installs on the cold-water supply line near the heater. It holds a cushion of air behind a rubber bladder that absorbs the extra half gallon, so pressure never reaches the relief point. Plumbing code requires expansion control on closed systems for exactly this reason. If your drip happens during heating cycles and your home has a PRV or backflow device, an expansion tank is almost certainly your answer. We cover this in more depth on our page about whether you need an expansion tank.
Cause two: high incoming water pressure
Sometimes the problem is not inside the tank at all. It is the pressure coming in from the street. If your home's static water pressure is already high, every heating cycle pushes the tank past the 150 psi relief point with little room to spare.
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.
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.
Cause three: temperature set too high
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 recommends a setpoint of 120 degrees Fahrenheit, which is well below the relief temperature, safe from scalding, and easier on your energy bill. Many tanks ship from the factory set near 140 degrees, and the DOE notes that the higher setting wastes energy on standby losses. If your relief valve discharges hot or near-boiling water, suspect temperature. Turn the thermostat down to 120 and watch whether the drip stops.
A valve discharging steam or very hot water is the more concerning case, because it can signal a runaway thermostat that no longer shuts the burner off. If lowering the setpoint does not stop hot discharge, shut the heater down and have it checked. A stuck thermostat is a genuine hazard, not a slow leak you can sit on.
Cause four: a worn or failed valve, and how to fix the drip
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. Put a gauge on a hose bib with everything off. Above 80 psi points to a PRV or expansion issue, not a bad valve.
- Check the temperature. Lower the thermostat to 120 degrees and see whether the drip stops over the next day.
- Look for an expansion tank. No expansion tank on a closed system is the leading cause. Adding one is the usual fix.
- Replace the valve last. If pressure and temperature are fine and expansion is handled, the valve is the culprit. Replacing it is a low-cost job, but the tank must be shut off, depressurized, and partly drained first.
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.
A dripping T&P valve is rarely an emergency by itself, but it points to one of these four conditions, and one of them, a stuck thermostat venting hot water or steam, can be dangerous. If the drip is hot, persistent, or you cannot find the cause, our licensed Phoenix team (Arizona ROC #355170) can test the pressure, add an expansion tank, or replace the valve and confirm your tank is safe. If you also see water pooling under the heater, read our page on a water heater leaking from the bottom to rule out tank failure.