Water Heater T&P Relief Valve: Testing and Replacement
The relief valve on a water heater, the T&P valve (temperature and pressure relief valve), opens to dump water if the tank gets too hot or the pressure climbs too high. Test it once a year by lifting the lever, and replace it if it won't reseat, keeps dripping, or hasn't been looked at in years. Watts, a major valve maker, says to operate the lever at least once a year and have the valve inspected at least once every three years.
What the T&P valve does
The valve threads into the top or upper side of the tank, with a sensing probe that sits in the hottest water. It has two triggers. A typical residential valve like Watts' LF100XL opens at 210°F, and at a set pressure, with standard settings of 75, 100, 125 or 150 psi. Watts says the temperature probe must sit in the top 6 inches of the tank.
A.O. Smith says the valve's relief pressure must not exceed the working pressure on the water heater's rating plate. That's why you replace it with the same pressure and temperature rating, not whatever's on the shelf.
A pressure relief valve is not the same thing as a pressure reducing valve. The relief valve on the heater is an emergency dump. A pressure reducing valve (PRV) on the main line lowers the house water pressure all the time. If your water pressure runs high, the PRV fixes the cause; the relief valve only reacts to it.
Where it is and what it looks like
Look on the top or the upper side of the tank for a brass valve with a small flip-up lever. A pipe runs from it down the side of the heater, ending a few inches off the floor or running outside. That discharge pipe is part of the safety system too.
How to test a T&P relief valve
Put a bucket under the end of the discharge pipe. Check that a discharge pipe is actually connected before you touch the lever; Watts says to confirm this first.
Stand to the side and lift the lever. Hot water should rush out of the pipe.
Let it run for a few seconds, then snap the lever back down.
Watch the pipe for a few minutes. The flow should stop completely.
A.O. Smith says operating the valve by hand at least once a year keeps the waterways clear. If the valve won't seat after a couple of flips, or it doesn't open at all, replace it.
Why a relief valve drips or keeps opening
A.O. Smith says a dripping T&P valve is usually caused by house water pressure that's too high. The same manual says most codes allow a maximum incoming pressure of 80 psi, and recommends no more than 50 to 60 psi. Watts says a pressure regulator is needed when pressure is typically more than 75 psi.
The other big cause is thermal expansion. Heated water expands, and in a closed system (one with a check valve or a pressure reducing valve on the supply) it has nowhere to go. Watts' guide gives the example of a 40-gallon tank making about a half gallon of extra water as it heats, and says thermal expansion relief typically releases about one cup of water per 10 gallons of tank size. The fix is a properly charged expansion tank. See water heater expansion tank.
A big gush of very hot water is different. A.O. Smith says that can mean the tank is overheating, and to shut off the gas supply and call a qualified person. More on drips on our relief valve leaking page.
How to replace a water heater relief valve
Parts and tools
- A new T&P valve with the same pressure and temperature rating and the same probe length as the old one. Read the tag on the old valve and the heater's rating plate. Many residential valves have a 3/4 inch inlet; the Watts LF100XL is a 3/4 inch valve with a male inlet and female outlet.
- A pipe wrench.
- Thread sealant tape or pipe dope rated for potable water.
- A garden hose and a bucket.
Steps
Turn off the heat source
Gas control to OFF or vacation, or the breaker off on an electric heater.
Close the cold water supply
valve to the heater.
Relieve the pressure
Open a hot tap somewhere in the house, then lift the relief valve lever briefly.
Drain some water
Hook a hose to the drain valve at the bottom and drain until the water level is below the relief valve opening. On a side-mounted valve that's a few gallons; on a top-mounted one, less.
Disconnect the discharge pipe
from the valve outlet.
Unscrew the old valve
counterclockwise with the pipe wrench. Old valves can be stubborn; brace the tank so you don't twist it.
Install the new valve
Wrap the threads with sealant, thread it in by hand, then snug it with the wrench so the outlet points down toward the discharge pipe route.
Reconnect the discharge pipe
Refill
Close the drain valve, open the cold supply and keep the hot tap open until water runs without spitting air.
Restore heat
and check the valve and its threads for leaks.
Discharge pipe rules
Most leaks and code problems are in the pipe, not the valve. Per Watts and the International Plumbing Code:
- The pipe must be the same size as the valve outlet for its full length.
- It must drain by gravity and can't end in a threaded fitting.
- It has to end within 6 inches of the floor or the flood rim of a receptor, or outside.
- No shutoff valve between the relief valve and the tank, or in the discharge line.
- Watts flags discharge lines over 30 feet long or with more than four elbows as excessive.
Local rules vary. Boise, for example, requires copper, CPVC, galvanized or listed pipe, a drain indicator hole near the valve, and forbids running the T&P line into the drain pan. Check with your local building department before you change the discharge piping.
When it's a pro job
Call a plumber if the valve keeps dripping after a new valve and an expansion tank, if it ever discharges a lot of hot water, or if the old valve is seized into a rusty tank. HomeGuide puts a pressure relief valve replacement at $50 to $300. If the tank threads are corroded or the tank is old, ask about the cost of a new heater too.
FAQ
How much does it cost to replace the T&P valve on a water heater?
HomeGuide's range is $50 to $300 with a plumber. The valve itself is a modest part, and most of the cost is labor and the service call.
When should I open the pressure relief valve on a water heater?
Once a year as a test, per both Watts and A.O. Smith. Also open it briefly to let pressure off before you remove or replace the valve. Don't use it as a way to bleed pressure day to day; fix the pressure instead.
What size fitting is a water heater relief valve?
Most residential tank valves have a 3/4 inch threaded inlet, like the Watts LF100XL. Match the old valve and the heater's manual to be sure.
Can you adjust a pressure relief valve?
No. The T&P valve on a water heater has a fixed factory setting. If it's opening too often, lower the house pressure or add an expansion tank.
Sources
- WattsSeries LF100XL Temperature and Pressure Relief Valves Specification Sheet (opens in a new tab)
- A. O. SmithWater Heater Not Working? Troubleshooting Guide (opens in a new tab)
- A. O. SmithProMax Atmospheric Vent Gas Water Heater Instruction Manual (opens in a new tab)
- WattsTemperature and Pressure Relief Valves for Water Heaters (Watts guide, hosted by City of Tavares) (opens in a new tab)
- International Code Council2018 International Plumbing Code Essentials, pp. 40-41 (opens in a new tab)
- City of Boise Planning and Development ServicesWater Heater Installation Guide (opens in a new tab)
- HomeGuideWater Heater Repair Cost (opens in a new tab)