Awaiting expert review. This page covers safety or regulatory rules that change over time. It is kept out of search results until a qualified reviewer has checked it; the sources below are dated so you can verify each point.
Quick answer
A temperature and pressure relief valve (T&P valve) is a spring-loaded safety valve on a water heater that opens automatically and discharges water if the tank gets too hot or the pressure inside climbs too high.
A typical residential valve opens at 210°F or at a set pressure such as 150 psi. It is an emergency device, not a control, and it must never be capped, plugged or removed.
Quick facts
The key facts about Temperature and Pressure Relief Valve, with sources:
- Job
- Emergency relief of excess temperature and pressure; not an operating control 1
- Temperature setting (one common model)
- 210°F (99°C) 1
- Pressure settings offered
- 75, 100, 125 or 150 psi on one ¾-inch model 1
- Sensor location
- Within the top 6 inches of the tank 1
- Lever test
- Operate at least once a year (Watts) 1
Key takeaways
- The T&P valve is the part that stops a water heater from becoming a pressure vessel with no way out.
- It opens on high temperature (210°F on a common model) or on its set pressure, often 150 psi.
- A drip after each heating cycle usually means the system lacks expansion control or pressure is too high, not that the valve is bad.
- Its discharge pipe must run full size, downward, with no valve or cap, and end where a discharge can be seen.
- Never cap, plug or remove it; A.O. Smith warns an explosion could occur if the valve or its pipe is blocked.
Safety first: never cap or plug a T&P valve
Never cap, plug or remove a T&P valve
A.O. Smith warns that an explosion could occur if the T&P relief valve or its discharge pipe is blocked, and says not to cap or plug either 2.
If your valve is dripping, leave it in place, put a bucket under the discharge pipe and call a licensed plumber.
If the valve is discharging hot water hard and steadily, turn the heater off (gas knob to off, or the breaker for electric) and call for service.
If you smell gas, leave the house and call the gas utility or 911 from outside.
What a T&P relief valve is: two safety valves in one body
A temperature and pressure (T&P) relief valve is the safety valve near the top of a tank water heater that opens if the water gets too hot or the pressure gets too high.
A water heater is a closed steel tank of water heated by a burner or element: if the thermostat sticks on, the water keeps getting hotter, and if pressure has nowhere to go, it keeps climbing. The T&P valve answers both problems with one fitting.
Watts, a major valve maker, describes its LF100XL as a combined “2 in 1” valve that provides “fully automatic temperature and pressure relief protection” for hot water storage tanks 1. A brass body holds a spring-loaded disc against a seat.
A temperature probe reaches down into the hottest water at the top of the tank. Either too much heat or too much pressure lifts the disc and lets water escape through the discharge pipe.
Watts is blunt about its role: the device “is designed for emergency safety relief and shall not be used as an operating control” 1. In plain terms, if it opens, something else is wrong.
How a T&P valve opens and reseats
- Normal running: A spring holds the disc shut. Watts says maximum system operating pressure must stay under 75% of the valve’s set pressure 1.
- Pressure trip: If tank pressure reaches the set point, such as 150 psi, the force on the disc beats the spring and the valve opens 1.
- Temperature trip: If water at the top of the tank reaches 210°F, the thermostat element expands and opens the valve 1.
- Discharge: Hot water flows down the discharge pipe. Releasing water drops both pressure and temperature, because cold water refills the tank.
- Reset: The valve is self-closing once conditions fall back 1. Repeated opening is a symptom to fix, not normal operation.
- FaultStuck thermostat or rising pressure
- Set point reached210°F or set pressure, e.g. 150 psi 1
- Disc liftsWater discharges down the pipe
- Tank cools and depressurizesCold water refills
- Valve self-closesThe fault still needs a fix 1
Source: [1]
Types of relief valves on water heaters
| Valve | Responds to | Where it is used |
|---|---|---|
| Combination T&P valve | Temperature and pressure 1 | Standard on residential tank water heaters |
| Pressure-only relief valve | Pressure only | Some tankless heaters and boilers; follow the maker’s manual |
| Extension-thermostat T&P | Both, with a longer probe | Watts uses extra-length probes for side tappings on external-flue heaters 1 |
| Hot-outlet-mounted T&P | Both | Watts allows the hot outlet line only when no tank tapping is provided 1 |
Example: the 75% rule for system pressure vs. valve setting
Watts says maximum system operating pressure must not exceed 75% of the valve’s set pressure 1. Applying that rule to its four standard settings:
| Valve set pressure | 75% ceiling for system pressure | Example: house at 85 psi static |
|---|---|---|
| 75 psi | 56.25 psi | Too high; the valve will weep |
| 100 psi | 75 psi | Too high |
| 125 psi | 93.75 psi | Within the ceiling, but above the 80 psi PRV trigger in most codes 6 |
| 150 psi | 112.5 psi | Within the ceiling; still above the 80 psi code trigger 6 |
The 85 psi house is an example. The point: a valve can sit inside its own margin while the house still needs a pressure-reducing valve. Heating then adds thermal expansion on top of static pressure in a closed system, which is when drips start.
Where the T&P valve and discharge pipe are on a water heater
Look at the top or the upper side of the tank for a brass valve with a small lever and a pipe running down the side of the heater.
A.O. Smith’s parts diagram labels it the temperature-pressure relief valve, next to the rating plate 2. The tag wired to the valve lists its pressure and temperature settings and its BTU rating.
Two numbers on your equipment should line up. A.O. Smith says the valve’s relief pressure must not exceed the working pressure on the heater’s rating plate 2.
Watts adds that the valve’s BTU discharge capacity must exceed the heater’s input rating; its LF100XL is rated for heaters up to 105,000 BTU per hour 1.
If a plumber’s quote says “replace T&P,” ask for those two checks.
What a T&P valve protects, and what it cannot do
What it does
Why a T&P valve can fail without warning
The danger is not a dripping valve. It is a valve that cannot open. Watts warns that naturally occurring mineral deposits may adhere to the valve and render it inoperative 1. Hard water and long gaps between checks raise that risk.
That is why both makers call for a physical inspection, not just a glance. Watts says relief valves should be removed and inspected at least once every three years by a licensed plumbing contractor or qualified technician 1.
A.O. Smith says at least every five years, and that damage from corrosive water or mineral deposits can only be found when a qualified person removes the valve 2. The two intervals differ; follow the tag on your valve.
What drives the cost of replacing a T&P valve
We do not quote prices, because fair numbers need dated local quotes.
A T&P replacement is a small part with real labor: the tank must be cooled and partly drained, old threads may be corroded, and the discharge pipe may need to be rebuilt to current rules. The bigger cost driver is the cause.
If the valve dripped because pressure is high or expansion control is missing, the real fix may be a pressure-reducing valve or an expansion tank as well.
Lifespan depends on water quality and is not rated in years; the makers set inspection intervals instead 12. Our plumbing cost guide explains how plumbers price small parts and service calls, and the repair vs. replace tool helps if the heater itself is near the end of its life.
How a licensed plumber replaces a T&P valve
This is plumber work: it involves draining a hot, pressurized tank. In outline:
- Shut off the heater’s fuel or power and the cold water supply, and let the water cool.
- Relieve pressure and drain enough water to drop the level below the valve opening.
- Remove the old valve and inspect the tank opening for corrosion.
- Fit a valve rated for the heater: pressure not above the rating plate’s working pressure, BTU capacity above the heater’s input 21.
- Check the probe sits within the top 6 inches of water 1.
- Run or rebuild the discharge pipe to the local code and the maker’s instructions 13.
- Refill the tank completely, check for leaks, then restore fuel or power.
- Check static pressure and expansion control so the new valve does not start dripping too.
The yearly T&P lever test
Watts says the lever must be operated at least once a year after installation to keep the waterways clear 1. A.O. Smith also says to operate it annually 2. If you choose to do it yourself:
- First confirm a discharge pipe is connected and runs to a safe place; Watts warns of injury otherwise 1.
- Stand clear of the pipe outlet; the water is scalding hot.
- Lift the lever briefly and let it snap back. Water should flow out of the pipe.
- If no water flows, Watts says the valve is inoperative: turn off the heater and call a plumber immediately 1.
- If the valve keeps dripping after the test, call a plumber; debris may be holding it open.
T&P valve warning signs, and when to call
- Drips after the heater runs: often thermal expansion in a closed system with no working expansion tank 5.
- Steady trickle all the time: high static pressure, a worn seat or debris.
- Hot water gushing or steam from the pipe: overheating; turn the heater off and call now.
- Crusty mineral buildup or corrosion on the valve body.
- A discharge pipe that has been capped, plugged, threaded at the end or reduced in size.
For water on the floor, see our plumbing emergencies guide; for service, see water heater repair and replacement.
ANSI Z21.22, IPC 504 and state discharge-pipe rules
Product standard: ANSI Z21.22, Relief Valves for Hot Water Supply Systems. A.O. Smith says a nationally recognized testing laboratory certifies its valve to it, and Watts’ specification requires it; the Watts valve is also ASME rated and CSA listed 21.
Model code: The International Plumbing Code governs relief valves and their discharge piping in Chapter 5 (Section 504 in the editions we checked through state adoptions). Ohio adopts Chapter 5 of the 2021 IPC with amendments 4. Local adoption and amendments vary.
State amendment example (Michigan): Mich. Admin. Code R 408.30785 amends Section 504.6.
It requires a separate discharge pipe for each valve that ends open to the air not more than 4 inches above the floor with an unthreaded end; no interconnection with other discharge pipes; no valves in the line.
Rigid pipe rated for at least 210°F; full valve-outlet size, draining by gravity; discharge to the floor, the heater pan or a waste receptor; and no direct connection to the drainage system 3.
Manufacturer rule: Watts says the discharge line must be the valve outlet size its full length, pitch downward, and end at least 6 inches above a drain where any discharge is visible 1.
Where the code and the maker differ, your inspector decides; the model-code text itself was not readable at source and is not quoted.
T&P valve vs. pressure relief valve, expansion tank and PRV
| Device | What it does | Main difference |
|---|---|---|
| T&P relief valve | Dumps water at a set temperature or pressure 1 | Emergency release; not a control |
| Expansion tank | Absorbs the extra volume from heating 5 | Prevents the pressure rise that makes a T&P drip |
| Pressure-reducing valve | Lowers incoming street pressure | Controls static pressure; required above 80 psi in most codes 6 |
| Thermostatic mixing valve | Blends hot and cold at a set outlet temperature | Scald control at the tap, not tank safety |
| Water heater thermostat | Turns heating on and off | The operating control the T&P backs up |
T&P valve myths
- Myth A dripping valve should be capped.
- Reality Capping it removes the heater’s last protection; A.O. Smith warns of explosion 2.
- Myth A drip always means the valve is bad.
- Reality Often the system lacks expansion control or pressure is too high 56.
- Myth If it has never opened, it works.
- Reality Mineral deposits can stick it shut; Watts calls for yearly lever tests 1.
- Myth Any pipe will do for the discharge.
- Reality It must be full size, unvalved and rated for heat; Michigan requires 210°F-rated rigid pipe 3.
- Myth A higher-pressure valve fixes the drip.
- Reality The valve must not exceed the heater’s rated working pressure 2. Fix the pressure instead.
How state codes change the discharge pipe
The valve is national; the discharge pipe is local. Michigan amends the model code’s discharge rule to a 4-inch floor clearance with an unthreaded end 3, Ohio adopts the 2021 IPC Chapter 5 with its own amendments 4, and states on the Uniform Plumbing Code number the rule differently.
Where a heater sits in an attic or a closet with no floor drain, the allowed endpoints (floor, pan, waste receptor) decide whether a pan and drain line are needed. Ask your building department before moving or rebuilding a discharge pipe.
What to do when a T&P valve drips or discharges
A dripping T&P valve is usually reporting a system problem, not a bad valve. The guide below reads the most common patterns. The order that works: read static pressure, check whether the supply has a check valve, PRV or backflow device, check the expansion tank, then look at the valve itself.
Next step: put a bucket under the pipe, note when it drips (after heating, or all the time), and call a licensed plumber with that detail. If hot water is pouring out and you cannot reach a regular plumber, an emergency plumber can make the heater safe.
- If Drips right after the heater runs
- Then Check expansion control and closed-system devices 5
- If Drips all the time
- Then Check static pressure; above 80 psi needs a PRV in most codes 6
- If No water during a lever test
- Then Valve is inoperative: turn off the heater, call a plumber 1
- If Hot water or steam pouring out
- Then Turn the heater off now and call for service
Questions about Temperature and Pressure Relief Valve
What does a temperature and pressure relief valve do?
It protects the water heater from too much heat or pressure. If tank water reaches about 210°F, or pressure reaches the valve’s set point such as 150 psi, the valve opens and discharges water through its pipe. That drops temperature and pressure before the tank can fail. It then closes on its own.
Why is my water heater relief valve dripping?
Most often because pressure rises when the heater runs and has nowhere to go. A check valve, pressure-reducing valve or backflow preventer closes the system, and without a working expansion tank the T&P valve weeps. High street pressure or debris on the valve seat are other causes. Never cap it; call a plumber.
Is a leaking T&P valve dangerous?
The drip itself is a warning, not the danger. The danger is a valve that is blocked, capped or stuck shut, because then nothing protects the tank. A.O. Smith warns that an explosion could occur if the valve or discharge pipe is blocked. Have a plumber find out why it is dripping.
How often should a T&P valve be tested?
Watts says the lever must be operated at least once a year to keep the waterways clear, and A.O. Smith also calls for yearly operation.
For a deeper check, Watts says a licensed contractor should remove and inspect the valve at least every three years; A.O. Smith says every five. Follow the tag on your valve.
What should I do if no water comes out when I lift the lever?
Watts says that means the valve is inoperative. Turn off the water heater, gas knob to off or breaker off for electric, and call a plumber immediately. Do not keep running a heater whose relief valve does not open.
Where should the T&P discharge pipe go?
To a safe, visible spot set by your local code. Michigan’s amended rule allows the floor, the water heater pan or a waste receptor, ending not more than 4 inches above the floor with an unthreaded end. Watts says the pipe stays full size, pitches down and has no valve or cap.
Can I replace a T&P valve myself?
It is a job for a licensed plumber in most cases. The tank must be cooled and drained below the valve, the new valve must match the heater’s working pressure and BTU input, and the discharge pipe must meet local code. Many jurisdictions require a licensed installer for water heater safety work.
What pressure is a water heater relief valve set to?
Residential valves commonly come set at 150 psi, and Watts also offers 75, 100 and 125 psi on one model. The setting must not exceed the working pressure on the heater’s rating plate. Watts says system pressure should stay under 75% of the set pressure, or about 112 psi for a 150 psi valve.
Do tankless water heaters have a relief valve?
Many do require a pressure relief valve, but the type and placement come from the maker’s manual and your local code, not the tank-heater rules. Check the installation manual for your model and ask the installer to show you where it is and where it discharges.
Sources
- Watts, Series LF100XL Temperature and Pressure Relief Valves, specification sheet ES-LF100XL, retrieved .
- A.O. Smith, Residential Standard Gas Ultra Low NOx Water Heater Installation Instructions and Use & Care Guide (100283020), retrieved .
- Michigan Admin. Code R 408.30785, amending plumbing code Section 504.6, Requirements for discharge piping (Cornell LII), retrieved .
- Ohio Admin. Code 4101:3-5-01, adopting Chapter 5 of the 2021 International Plumbing Code (Cornell LII), retrieved .
- North Carolina Office of State Fire Marshal, Interpretation: Thermal Expansion Control, 2018 NC Plumbing Code §607.3 (January 3, 2019), retrieved .
- Pacific Northwest National Laboratory, Building America Solution Center: Service Water Pressure, retrieved .
Expert review
Written by the HyrePlumber Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: HELD under G-REV until a licensed plumber signs.
Valve settings, the 75% rule, probe position, lever test, inspection interval and discharge-line guidance are from the Watts LF100XL specification sheet; the ANSI Z21.22 listing, working-pressure match, five-year inspection and the do-not-cap warning are from an A.O. Smith use and care guide; both labeled as manufacturer guidance.
Discharge-pipe code text is Michigan’s amended Section 504.6; Ohio’s rule confirms adoption of 2021 IPC Chapter 5. ICC model text was not readable and is not quoted. Thermal-expansion cause from the NC fire marshal; 80 psi trigger from PNNL. The 85 psi house is an example. No prices.
Suggest a correction. We fix errors and say what changed.