The water heater T&P valve deals with two things at once: temperature and pressure. It sits on the top or side of the tank, and its job is to prevent the vessel from rupturing. It is a component that spends almost its entire service life doing nothing at all, and that is precisely what makes it easy to neglect.
This guide covers what the valve does, how to test it, what the failure signs mean, and the discharge pipe requirements that are as important as the valve itself.
Why the Valve Exists
Water expands as it heats. In a sealed vessel that expansion has nowhere to go, so pressure rises. If the thermostat fails and the heater continues warming water past its normal limit, both temperature and pressure climb together.
At that point the water inside is above its atmospheric boiling point but held liquid by pressure. If the vessel then fails, that superheated water flashes to steam instantly and expands enormously. The result is not a leak but an explosion, and the consequences are severe.
The T&P valve prevents this by opening before the tank reaches that condition, releasing water and pressure so the vessel never approaches failure. It responds to whichever limit is reached first, temperature or pressure, which is why it carries both in its name.
How the T&P Valve Works
Two independent mechanisms operate inside a single body.
The pressure element is a spring holding the valve closed. When system pressure exceeds the spring’s rating, it lifts the valve off its seat and water discharges until pressure falls back and the spring reseats it.
The temperature element is a wax-filled probe extending into the tank. As water temperature rises, the wax expands and drives a piston that opens the valve mechanically, independently of what the pressure is doing. When the water cools, the wax contracts and the valve closes again.
Neither mechanism requires electricity, a sensor circuit, or a controller. The valve senses conditions directly and acts on them, which means it continues protecting the tank during a power failure, and there is nothing to be accidentally switched off or misconfigured.
Typical operating parameters
| Parameter | Example, Legom 930004NT |
|---|---|
| Opening temperature | 93°C to 99°C |
| Resetting temperature | ≥70°C |
| Pressure ratings | 500 to 1400 kPa across seven options |
| Resetting pressure | ≥90% of opening pressure |
| Water-touching material | DZR brass |
| Sizes | DN15 (1/2″), DN20 (3/4″) |
| Standard | AS1357.1 |
The pressure rating must be matched to the vessel it protects. A valve rated above the tank’s design pressure may never open before the tank reaches a dangerous condition, which defeats the entire purpose of fitting it. This is the specification most worth confirming when replacing one.
Testing the T&P Valve
Because the valve spends years dormant, the only way to know it still works is to test it. A valve that has never been tested and never operated is indistinguishable from one that has quietly seized.
The procedure
Place a container beneath the discharge pipe outlet, or confirm the pipe discharges somewhere safe. Lift the test lever on the valve briefly and release it.
What should happen: water flows out while the lever is lifted, then stops completely when released. The valve should reseat cleanly with no continuing drip.
No flow at all means the valve has seized closed. It is providing no protection whatsoever and needs immediate replacement.
Continued dripping after release means the valve failed to reseat, usually because scale or debris around the seat prevents it closing fully. It also needs replacement.
A caution about testing
Take care during the test, since the discharged water is at tank temperature and will scald. Keep hands and body clear of the discharge outlet, and be aware that the water may emerge with force.
The objection people raise, and why it is backwards. An old valve sometimes leaks after being tested, because scale around the seat prevents it closing again. This leads some owners to avoid testing altogether, reasoning that testing caused the problem. It did not. A valve that fails during a controlled test was already unreliable and would have failed when it was genuinely needed. Discovering that with a container in your hand is far preferable to discovering it during an actual overpressure event.
How often
Annually is the general recommendation, and in commercial and institutional settings the check is usually part of a documented maintenance schedule with recorded results. Test more frequently in hard water areas, since scale accumulation around the seat is the main cause of a valve failing to operate or failing to reseat.
The Discharge Pipe: As Important as the Valve
The valve is useless without a correctly arranged discharge pipe, and this is where installations most often fall short.
It must run downward. An upward or horizontal run traps water in the pipe, which prevents full drainage and can allow scale to accumulate or the pipe to freeze in cold conditions.
It must not be reduced in diameter. The pipe should be the same size as the valve outlet or larger. Narrowing it restricts the flow the valve was designed to release, which means pressure may not fall fast enough during an actual event.
It must terminate safely. The outlet should discharge where escaping water causes no harm and where discharge is visible, since discharge that nobody notices is a warning nobody acts on. It should also terminate near floor level rather than at head height, given the water is scalding.
It must have no valve, cap, or restriction of any kind.
Never plug the discharge pipe to stop a drip. This is the most dangerous thing an owner can do to a water heater, and it happens because the reasoning seems sound: the valve is dripping, so stop the drip. What actually happens is that the tank loses its only protection against overpressure while the condition causing the drip remains. The drip was a symptom and a warning. Capping the pipe removes the warning and the protection together.
Installation
Being clear about the boundary here matters, because the two jobs are frequently confused.
Fitting the discharge pipe
Attaching a discharge pipe to an already-installed valve is straightforward. Thread it onto the valve outlet, tighten by hand, then finish with a wrench. Control the tightness, since overtightening damages the brass threads and a cracked fitting on a safety device defeats its purpose. Confirm the pipe runs downward and terminates safely.
Replacing the valve itself
This is a different job, and the article you may have read suggesting it can be done without shutting the system down is mistaken. Replacing a T&P valve requires the tank to be depressurised and partially drained first, because the valve threads into a pressurised vessel containing water at scalding temperature.
The sequence is: isolate power or gas to the heater, close the cold water inlet, open a hot tap to relieve pressure, drain the tank below the level of the valve, then remove and replace the valve, and finally refill and check for leaks before restoring power.
In many jurisdictions this work is regulated, and on gas-fired units it may legally require a certified person regardless of competence. Where you are uncertain, this is a job worth passing to a plumber, since the consequences of getting it wrong involve a pressurised vessel of very hot water.
Failure Signs and What They Mean
The valve gives several signals when something is wrong, and interpreting them correctly determines whether you address the valve or the condition behind it.
| Sign | Likely meaning | Action |
|---|---|---|
| Occasional discharge during heating | Thermal expansion in a closed system | Check whether an expansion vessel is needed |
| Continuous dripping | Failed to reseat, or persistent excess pressure | Investigate pressure, replace if the valve is at fault |
| Heavy discharge or spraying | Genuine overpressure or overheating event | Shut off water and power, call an engineer |
| Whistling or high-pitched noise | Water forcing past a partially open seat | Test the valve, replace if it will not reseat |
| No flow when tested | Valve seized closed | Replace immediately, there is no protection |
| Corrosion or scale around the body | Long-term seepage or water quality issues | Inspect and consider replacement |
Distinguishing normal discharge from a fault
Some discharge can be normal. As water heats it expands, and in a closed system, meaning one with a non-return valve or pressure limiting valve on the incoming supply, that expansion has nowhere to go. Pressure rises during each heating cycle until the T&P valve relieves it.
This is the valve working correctly, but it indicates the system needs an expansion vessel to accommodate the expansion instead. Without one, the valve discharges on every heating cycle, which wears it out prematurely and wastes hot water.
Continuous dripping, by contrast, is not normal. It points to system pressure that is too high, water temperature set too high, or a valve that has failed to reseat. All three need investigating rather than tolerating.
Common Causes of Failure
Scale accumulation. Mineral deposits around the seat prevent the valve closing fully after it operates, and in hard water areas this is by far the most common cause.
Excessive system pressure. A valve that operates frequently because incoming mains pressure is too high wears out far faster than one that rarely acts. Fitting a pressure limiting valve addresses the cause rather than the symptom.
Age. The valve has a finite service life, and manufacturers typically recommend replacement at intervals regardless of apparent condition, because a seized valve gives no indication that it has stopped protecting anything.
Never being tested. The failure mode that matters most, because a valve that has silently seized looks exactly like one that works.
Replacement Considerations
When replacing, match the specification rather than choosing the nearest equivalent.
The pressure rating must correspond to the tank’s design pressure. The temperature rating should match the original. The connection size and thread must match the tank fitting, and the probe length should suit the tank so the temperature element sits properly in the water rather than in a pocket.
Body material matters for longevity, with DZR brass resisting dezincification in aggressive water. And certification should cover the standards applicable in your market, since this is a safety component subject to regulation in most jurisdictions.
Our guide to TP valve replacement cost covers what affects the price, and our article on TP valves on water heaters covers the component in wider context.
“The thing about a T&P valve is that it fails silently. A pump seizes and you know within a day because nothing works. A relief valve seizes and absolutely nothing changes, right up until the day it was needed. That is why the annual test matters more than any inspection: you cannot look at one and tell whether it works. The other thing I would say to anyone whose valve is dripping is not to cap the pipe. It seems like the obvious fix and it is the single most dangerous thing you can do to a water heater, because you have removed the protection and left the condition that caused the drip in place.”
— Maggie Shen, Director of Legom
Sourcing T&P Valves
Legom manufactures temperature and pressure relief valves as part of a 68-model HVAC valve range at its facility in Jiaxing, Zhejiang Province, supplied to partners in more than 90 countries.
Because the wax thermostatic elements inside these valves are produced in the same facility rather than sourced externally, switching characteristics are specified at the source. Every element is individually tested and rated for 100,000 operating cycles.
When evaluating any supplier for a safety component, ask for the full datasheet rather than a summary, confirm the certification covers the specific model and your market, and ask what accuracy the valve retains after extended thermal cycling rather than what it achieves when new. OEM and ODM services cover pressure rating, temperature threshold, connection size, and body material.
Frequently Asked Questions
What does a water heater T&P valve do?
It prevents the tank from rupturing by releasing water when either temperature or pressure inside reaches a dangerous level. Two independent mechanisms operate in one body: a spring that lifts when pressure exceeds its rating, and a wax-filled probe extending into the tank that expands when water temperature rises and opens the valve mechanically. Neither requires electricity, so the valve continues protecting the tank during a power failure.
How do I test a T&P valve?
Place a container beneath the discharge outlet or confirm the pipe discharges somewhere safe, lift the test lever briefly, then release it. Water should flow while the lever is lifted and stop completely when released, with no continuing drip. No flow at all means the valve has seized closed and provides no protection. Continued dripping means it failed to reseat. Take care, since discharged water is at tank temperature and will scald.
Should I test it if testing might make it leak?
Yes. It is true that an old valve sometimes leaks afterwards, because scale around the seat prevents it closing again, and this leads some owners to avoid testing. That reasoning is backwards. A valve that fails during a controlled test was already unreliable and would have failed when genuinely needed. Finding out with a container in your hand is far better than finding out during an actual overpressure event. If it leaks after testing, replace it.
Is it normal for a T&P valve to drip?
Occasional discharge during a heating cycle can be normal thermal expansion, particularly in a closed system with a non-return or pressure limiting valve on the supply. Water expands as it heats, and with nowhere to go, pressure rises until the valve relieves it. This indicates the system needs an expansion vessel rather than that the valve is faulty. Continuous dripping is not normal and points to excessive pressure, temperature set too high, or a valve that has failed to reseat.
Can I cap the discharge pipe to stop the dripping?
No, and this is the most dangerous thing you can do to a water heater. Capping, plugging, or fitting a valve to the discharge pipe removes the tank’s only protection against overpressure while leaving the condition causing the drip in place. The drip is a symptom and a warning. Removing the warning does not remove the danger, and the consequence of a tank rupturing under pressure is severe. Investigate the cause instead.
Can I replace a T&P valve myself?
Fitting a discharge pipe to an existing valve is straightforward. Replacing the valve itself is not, because it threads into a pressurised vessel containing scalding water. The tank must first be isolated from power or gas, the cold inlet closed, pressure relieved through a hot tap, and the tank drained below the level of the valve. In many jurisdictions this work is regulated, and on gas-fired units it may legally require a certified person. If uncertain, pass it to a plumber.
How often should a T&P valve be replaced?
Manufacturers typically recommend replacement at intervals regardless of apparent condition, because a seized valve gives no indication that it has stopped protecting anything. Beyond that schedule, replace immediately if it produces no flow when tested, fails to reseat after testing, drips continuously, or shows corrosion around the body. In hard water areas expect a shorter service life, since scale accumulation around the seat is the main cause of failure.
What size and rating do I need?
The pressure rating must correspond to the tank’s design pressure, since a valve rated above it may never open before the tank reaches a dangerous condition. Match the temperature rating to the original, confirm the connection size and thread suit the tank fitting, and check the probe length so the temperature element sits in the water rather than in a pocket. Confirm certification covers the standards applicable in your market, as this is a regulated safety component.
Why does my valve keep discharging?
Three common causes. Excessive incoming mains pressure makes the valve operate frequently, which also wears it out prematurely, and fitting a pressure limiting valve addresses that at source. Water temperature set too high brings the tank closer to the valve’s temperature threshold. And a closed system without an expansion vessel forces the valve to relieve thermal expansion on every heating cycle. Establish which applies rather than replacing the valve repeatedly.
Reviewed by Maggie Shen, Director at Legom, on August 4, 2026. This guide to the water heater T&P valve was reviewed for technical and safety accuracy, including the testing procedure, discharge pipe requirements, and the depressurisation required before valve replacement.