Water Heater Pressure Relief Valve Leaking

What the T&P valve does

The T&P valve (temperature and pressure relief valve) sits on the top or upper side of the tank, with a pipe running down toward the floor. It opens on either of two triggers. A common Watts residential valve relieves temperature at 210°F and comes in pressure settings of 75, 100, 125 and 150 psi. Watts also says system operating pressure should stay under 75% of the valve's set pressure.

That last point matters. If your house runs at high water pressure, the valve can weep long before the tank is in real danger.

Why the relief valve leaks

1. 1

   Cause 1: Thermal expansion in a closed system

   Water expands when it heats. In older houses that expansion pushed back into the city main. Many homes now have a check valve or pressure-reducing valve at the meter, which closes the system off. With nowhere to go, pressure climbs every time the burner or elements run, and the T&P valve lets a little out. A.O. Smith names thermal expansion as one cause of T&P leaks and recommends a thermal expansion tank where needed.

   The giveaway: the valve drips during or right after a heating cycle, then stops. See water heater expansion tank for how they work.

2. 2

   Cause 2: House water pressure is too high

   A.O. Smith says household water pressure should be 50 to 60 psi and no higher than 80 psi. A hose-bib pressure gauge from a hardware store screws onto an outdoor faucet and tells you in a minute. Leave it on overnight if you can, since city pressure often rises when demand drops. If you read high, A.O. Smith says a pressure-reducing valve or expansion tank may be needed. Installing a pressure-reducing valve is a plumber job.

3. 3

   Cause 3: The valve is worn or has debris in the seat

   Old valves corrode, and mineral scale can hold the seat open a crack. Watts calls for an inspection at least once every three years by a licensed plumber or qualified technician, who removes the valve and checks it for mineral deposits and corrosion. If yours has never been touched and the drip is steady regardless of heating, the valve itself is the likely problem.

4. 4

   Cause 4: The water is too hot

   A thermostat stuck on, a failed gas control or a high setting can drive the tank toward the temperature trigger. This is the dangerous version. If the discharge is very hot or steamy, shut the heater off and call a plumber. A.O. Smith calls 120°F the right setting for most homes, so check where yours is set once things cool down.

Checks you can do

1. Note when it drips. Only while heating points to expansion or high pressure. All the time points to a worn valve.
2. Test house pressure with a gauge on an outdoor faucet. Compare it to the A.O. Smith range above.
3. Look for an expansion tank on the cold line near the heater. If there is one, tap it. A tank that sounds full of water all over has likely failed.
4. Exercise the valve. A.O. Smith says to lift the test lever and let it snap back, and to replace the valve if it keeps leaking afterward. Watts says to operate the lever at least once a year, and only after confirming a discharge line is attached. If no water comes out when you lift it, the valve is stuck. Turn the heater off and call a plumber.

Lifting the lever on a valve that hasn't moved in years can unseat debris and leave it dripping. Have a bucket ready and know where the cold shutoff is.

The discharge pipe: what code expects

Where the drip ends up matters too. The model codes (IRC Section P2804.6.1 and IPC Section 504.6) set the rules for the discharge pipe:

• It must be at least the size of the valve outlet, with no valves or tees in it.
• It must drain by gravity with no traps or sags.
• It must end in the same room, in view, through an air gap, and no more than 6 inches above the floor unless it discharges to a drain or receptor.
• The end must not be threaded, so nobody can cap it.

Check with your local building department for the edition your area uses.

Fixes and what they cost

• Replace the T&P valve. HomeGuide lists the job at $50 to $300, with the part itself around $10 to $25. The new valve must match the heater's pressure and BTU rating on the label, so take the old one or the label info to the store.
• Add an expansion tank. HomeGuide puts expansion tank replacement at $300 to $800 for materials and labor.
• Install or replace a pressure-reducing valve if house pressure is high. Get a plumber's quote, since the price depends on access at the main.

Replacing the valve means shutting off the water and the heater, relieving pressure and draining some water, then threading in the new valve with sealant. It's doable for a confident DIYer, but if the old valve is seized into a steel tank fitting, a plumber has the tools and the replacement parts on the truck.

RV and boiler relief valves

An RV water heater's relief valve drips for the same reason: pressure and heat. Many RV manufacturers count on an air pocket at the top of the tank to absorb expansion, and that pocket fills in over time. Check your RV heater manual for how to restore it.

A boiler relief valve is a different device on a heating system, with a lower setting. A leak there usually means boiler pressure or the boiler's expansion tank, and a heating technician should look at it.