If your water heater’s pressure relief valve is dripping, don’t tape over it or ignore it. The T&P (temperature and pressure) valve is a safety device. When it leaks, something in the system is pushing it open, and that something needs a licensed plumber to diagnose and fix.
What the T&P Valve Actually Does
The temperature and pressure relief valve sits on the side of your water heater tank, usually with a small pipe running down toward the floor or a drain. Its job is simple: if the pressure or temperature inside the tank climbs too high, the valve opens to release some water and prevent a catastrophic failure. Most residential units are rated to open at 150 psi or 210°F, whichever comes first.
Occasional brief dripping during a heating cycle can be normal in certain system configurations. A valve dripping constantly, or releasing a steady stream, means the system is regularly hitting those limits, or the valve itself has worn out.
The Most Common Causes, in Order of Likelihood
Thermal expansion. This is the most frequent culprit, especially in homes with a closed plumbing system. When cold water heats up, it expands. In an open system, that expansion can push back toward the street main. But if you have a check valve, pressure-reducing valve, or backflow preventer on your supply line, that expanded water has nowhere to go. Pressure builds, the T&P valve opens, water drips out. You’ll often see this happen in the morning after the heater runs overnight.
The valve itself is failing. T&P valves are mechanical and they wear out. After years of mineral buildup and thermal cycling, the seat inside can corrode or degrade. A worn valve may weep at pressures well below the setpoint. General guidance is to replace these valves every three to five years; if yours is older than that and dripping, the valve is often the problem itself.
Thermostat set too high. If your water heater thermostat is set well above 120°F, the heater runs hotter and approaches the temperature limit more often. Some units ship with the thermostat set higher than 120°F. Worth checking, but understand the tradeoff: lower temperatures reduce scalding risk, while higher temperatures (around 140°F) help control Legionella bacteria growth. That’s a conversation worth having with a plumber if you’re unsure what setting is right for your household.
Actual overpressure from the supply. Street water pressure varies by neighborhood and time of day. In some areas, supply pressure can spike above 80 psi during low-demand hours. If your pressure-reducing valve is set too high or has failed, incoming pressure alone can push your T&P valve to open. Plumbing codes generally require supply pressure to be kept at or below 80 psi.
How a Plumber Diagnoses It
A licensed plumber will check a few things in a specific order. First, they’ll measure supply pressure with a gauge at a hose bib, looking for anything above 80 psi. Then they’ll check whether your system is open or closed and look for a thermal expansion tank. No expansion tank on a closed system is almost always the root cause.
They’ll also test the T&P valve itself by briefly lifting the test lever (with the discharge pipe pointed safely away) to see if it reseats properly. A valve that doesn’t reseat cleanly after a test needs replacement.
If pressure is normal and the valve reseats fine, they’ll look at the thermostat setting and check whether the heater is actually reaching high temperatures. A temperature and pressure gauge can be installed temporarily to log what the system does over a heating cycle.
What You Can Safely Do Yourself
You can check the thermostat setting on your water heater without special tools. On an electric unit, the thermostat is usually behind an access panel. On a gas unit, the dial is on the gas valve. If it’s set noticeably above 120°F, lowering it is a reasonable first step, though you should understand the Legionella tradeoff mentioned above before changing it.
You can also check your water pressure using a simple gauge that screws onto an outdoor hose bib. These run under $15 at any hardware store. If the gauge reads above 80 psi consistently, you have a pressure problem that a plumber needs to address.
That’s really where the DIY stops. Don’t replace the T&P valve yourself. It requires draining part of the tank, working around scalding water, and installing the discharge pipe to code. Getting it wrong creates a real safety hazard. And don’t cap or plug a leaking valve. Ever. That valve exists to prevent the tank from rupturing.
When You Need to Call a Licensed Plumber
If the valve is dripping steadily or running water more than briefly, call a plumber. If you check the thermostat and pressure and nothing obvious stands out, call a plumber. If you need an expansion tank installed, that’s a licensed job in most jurisdictions. If the valve needs replacement, a plumber will ensure the new one is properly rated for your heater and that the discharge pipe is installed correctly per code.
Because this site does not hold a plumbing license, we don’t do this work. Our purpose here is to help you understand what’s happening so you can have an informed conversation with someone who does.
When you’re ready to hire, verify the contractor’s license at cslb.ca.gov before anyone starts work. A licensed plumber will pull the right permits where required and carry liability insurance. That matters more than a low bid.
A T&P valve repair is generally not a major expense, but the cost of ignoring one can be. Get a quote from a licensed plumber in your area, and don’t let a slow drip become a bigger problem.