Water heater safety valves:
Here at we discuss temperature and pressure relief valves used on residential water heaters. We explain the function, inspection, and maintenance of temperature & pressure relief valves on water heaters used to produce hot water for washing and bathing.
We describe how to diagnose and cure just about every problem with T&P valves on water heaters and we describe how to inspect the hot water supply system for unsafe or improper Temperature & Pressure Relief Valve installation.
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To reduce the risk of abnormally high and dangerous pressures in the hot water system and to reduce chances of a dangerous explosion, water heater manufacturers, building codes, and independent standards require that a temperature and pressure relief valve be installed on the water heater - a T&P Relief Valve.
[Click to enlarge any image]
The tag providing information on any T&P relief valve describes its operating pressure and temperature rating.
The T&P Valve installed on your water heater must be marked with a "set pressure" (the pressure at which the valve will open to relieve pressure) must be equal to or less than the maximum working pressure of the water heater.
The Temperature/Pressure Relief valve installation sketch shown at left is from American Water Heater Company's installation guide. 
The funny little device on the elbow seen at the left hot water riser on this water tank is in common use in Mexico.
Covered-Over relief valves:
The relief valve on this water heater has been covered by owner-installed water heater insulation.
This is an unsafe condition as the operation of the temperature or pressure relief valve may be interfered with by the insulation and also because the valve cannot be inspected for evidence of leaks or failure.
The water heater temperature/pressure relief valve is installed into an opening directly on the water heater at a location marked for that purpose, usually at or near the top of the heater (where water temperatures will be greatest).
Watch out: for these unsafe water heater relief valve or pressure-only relief valve installation mistakes 
(Mar 7, 2013) Christian said:
Question: I went and looked at a house today. On the water heater, there was a tag, it was red and white. It stated "notice of unsafe condition". However on the check boxes there was nothing checked IE stuff like CO or ignition risk. However, on the bottom it said "no water, copper through floor". What does this mean? the house is vacant and has been for some time. Thanks
Christian, Without some photos or other details, I don't know, but I speculate that if the discharge tube on the relief valve is piped down through the floor to a non-visible location, that's improper & unsafe and may be what the tagged meant.
All pressure/temperature relief valves include a tag or label that indicates the valve discharge capacity in BTUs - heat energy discharge rate. The installation or technical data or data tags for all modern water heaters include the water heater's BTU input rate capacity (and some indicate the specifications for the TP valve too.)
The Temperature/Pressure relief valve for a water heater must have a BTUh discharge rate (BTUs per unit time, such as BTUs per hour) that is equal to or greater than the BTU input rate of the heating appliance the valve is supposed to be protecting. 
As we discuss at BLEVE EXPLOSIONS, it is the release of heat (or "temperature" or "energy") that prevents a water tank from exploding when it is being overheated, not the release of pressure.
Details abour the required relief valve size are at WATER HEATER TPR VALVE SIZE.
The Temperature/Pressure relief valve mouth must point down, and a 3/4" I.D. (or greater) discharge tube must be attached to the T&P Valve's opening and routed down close to but not touching the floor level, typically near a floor drain and stopping 6" above the floor. This permits emergency hot water to be discharged without risk of burning the face and body of a bystander.
Our photo above illustrates a T&P valve installed on the water heater top and connected to a discharge tube that passes outside the building through the wall. This Tucson Arizona home installation disposes of any water heater T&P discharge outside to about 6" from the ground surface - not to a hidden location. And because there is no freeze risk in this climate the local installation was approved.
T&P valve discharge tubes are permitted to discharge below the floor of a structure but should not be piped to a hidden location where you won't notice that the valve is dripping or leaking. Never close off or block the discharge opening of the discharge tube, just as you wouldn't plug the discharge opening of the T&P valve itself.
Reader Question: (Aug 21, 2014) Julie said:
I have a hot water heater with the temperature relief valve on the top. It is a short 30-gal water heater that is under a counter top. What is the minimum clearance distance that is required from the bottom of the countertop to allow the temperature relief valve adequate clearance? Is there a code spec for this?
Julie a look at installation details for TP valves on water heaters didn't come up with an over-head clearance, though some common sense would indicate you'd need enough space for the test lever to be operated AND enough space to remove and replace the valve when needed. Since a top-mounted TP valve has a temperature sensing stem that protrudes downwards into the water heater the removal space will be more than you think
On a small water heater (cylinder) the extension of the sensor is about 3" while on larger water heater cylinders that added length could be as much as 9" below the inlet. Adding that 9" of sensor to the TP valve body and lever height (anywhere from 5 5/8" to 9 1/4") means the minimum (for the smallest valve size) valve total length is about 9" and the larger valve could be much larger.
Now we can slightly tip a valve to get it out of the heater, buy - I'd like to see 12" or more to give both working space and space to remove the valve.
Relief valve missing its discharge tube or drip line, also called an extension tube: this is an unsafe condition - someone may be scalded should the relief valve open.
Relief valve installed through an elbow, coupling, or pipe nipple: do not install a temperature & pressure relief valve through any intermediary plumbing fittings. Doing so can cause the T&P valve to fail to operate safely when it should. For example, an air pocked in a coupling can prevent proper sensing of water temperature.
We inspected a hot water tank installation at which the installer actually cut off the temperature sensing probe from the T&P valve so that he could install it through an elbow. This was a stupid and dangerous modification. - Ed.
Constricted, Under-sized, or Mis-Routed Relief Valve Discharge Tubes are also dangerous. The discharge tube attached to the water heater pressure/temperature relief valve should be the same diameter as the opening of the valve itself. In our photo someone has fastened a 1/2" pipe discharge tube onto a 3/4" diameter relief valve.
See a complete catalog of water heater TPR valve drain piping specifications & defects
at RELIEF VALVE DISCHARGE TUBE
Watch out: As our photo shows (above left), mineral salts left behind as hot water evaporates from the mouth of a pressure or temperature relief valve can completely clog the spring that is intended to allow the relief valve to open under excess (unsafe) pressure. This relief valve needs to be replaced immediately and the cause for the valve's leakage also needs to be determined.
Temperature & Pressure Relief valves may open, leak, or drip for a variety of reasons that we list and explain in detail
at RELIEF VALVE LEAKS. You should review that article. But here we list some common water heater relief valve leak causes:
Watch out: a dripping or frequently spilling T&P valve is dangerous because those very leaks can eventually cause the valve to clog and then to fail to open when it should. The result cause dangerous, even fatal
The discharge opening of a TPR Valve must not be blocked or reduced in size under any circumstance.  
Technical note: why must the TP Valve point "down"? Take a look at the photo above. If a relief valve is dripping the deposit of minerals inside the valve will accumulate still more rapidly if the valve points to the side or upwards. The result is a clogged valve as we explain above - a dangerous situation that risks an explosion.
Watch out: Never remove nor plug a pressure/temperature relief valve. This [water heater pressure/temperature relief] valve must be marked with the maximum set pressure not to exceed the marked maximum working pressure of the water heater. Install the valve into an opening provided and marked for this purpose in the water heater, and orient it or provide tubing so that any discharge from the valve exits only within 6 inches above, or at any distance below, the structural floor, and does not contact any live electrical part.
At RELIEF VALVES - TP VALVES we explain that in the photograph above there are two blockages of the test leve on the TPR valve that can prevent it from opening in response to pressure or temperature: the plaster wall and the foam insulation sprayed around the lever.
A relief valve on a water heater should have a BTU or heat spillage release capacity greater than the BTU input rating of the water heater. Otherwise even if the valve opens it can't release heat fast enough and there is a real risk that the water heater could explode.
The pressure-relief component of a TP valve or of a separate additional pressure-relief valve should be at least 20 to 30 psi above the maximum working pressure in the system. Otherwise you will find occasional "nuisance dripping" at the relief valve due to normal water pressure variations or due to water hammer.
The standard opening pressure for TP valves on residential water heaters is usually 150 psi, and most water heater tanks also have a standard operating pressure of 150 psi. If the building water supply pressure is above 125 psi, Watts recommends that a pressure-reducing valve be installed to reduce pressure to 40-45 psi so that a standard TP valve set can be used.
Note: we recommend that for most conditions residential water pressure inside the building should not exceed 70 psi. We observe an increase of leaks and drips at faucets and toilets at higher building pressures as the pressure may exceed the design pressure of some plumbing fixtures.
See WATER PRESSURE REDUCER / REGULATOR.
Watch out: as we discuss above, dripping TP relief valves are unsafe. The hazard is that minerals in the water supply accumulate inside the valve during the passage of hot water through the valve assembly. That accumulation of scale will eventually block operation of the relief valve, causing the relief valve to fail to operate properly if unsafe pressure or temperature occurs in the water heater in the future. The risk, ultimately, is a water heater tank or heating boiler explosion.
BLEVE EXPLOSIONS or boiling liquid vapor explosions can occur at both domestic water heaters (calorifiers or geysers) and at hot water heating boilers (hydronic heating systems). In a BLEVE explosion, the state change of superheated water from a hot liquid to a vapor (steam) form releases an enormous amount of energy.
We can understand the huge energy release involved in a BLEVE explosion by a quick review of the extra energy required to change the state of water from a liquid to a gas.
At SEER RATINGS & OTHER DEFINITIONS in our discussion of BTUs (British Thermal Units, a measure of energy), we point out that while only 180 BTUs of energy are needed to raise one pound of water at 32 degF to 1 pound of water at 212 degF, a much larger amount of energy, 970 BTUs, are needed to raise 1 pound of water at 212 degF to 1 pound of steam vapor at 212 degF.
Notice that the temperature has stayed the same! What has changed is the state of our pound of water: from a liquid form to a vapor form. This figure is the latent heat of vaporization, the number of BTUs of energy used to raise one pound of water at 212 degF to one pound of steam vapor at the same temperature; in other words, the temperature is unchanged but the state of matter is changed from liquid to vapor. State changes involve large amounts of energy.
We discuss the role of pressure/temperature relief valves in protecting against these hazards
at RELIEF VALVE, TP VALVE, BOILER and
at RELIEF VALVE, WATER HEATER - this article.
Water heater tank explosions are rare thanks to the widespread requirement for and use of pressure and temperature relief safety valves.
But if the safety valve has been damaged, modified, or even omitted (as we saw on our neighbor's water heater), that condition, combined with overheating can cause a water tank to explode, creating a BLEVE - Boiling Liquid Expanding Vapor Explosion that releases tremendous force and causing extreme damage to a building.
Because repeated heating of the water tank bottom may combine with other conditions (such as corrosion or excessive heating due to mineral deposits on the tank bottom) to produce a weak water heater tank bottom, that is the part more likely to fail in an overheat and overpressure condition.
A failure at the water tank bottom may explain why a BLEVE can produce a water tank explosion that behaves like a rocket, sending the water tank skyrocketing up through a building.
It is possible to speed the heater draining and also to ease the flushing procedure if you open a hot water tap nearby so that you can let air into the heater as water leaves it
Some plumbers open the water heater pressure/temperature relief valve for this air-in purpose - but there is risk that you'll later be unable to get the valve to shut fully again - debris can clog the valve seat - sometimes we can stop a relief valve from dripping by tapping on the valve lift rod that protrudes through the valve lift lever
Other plumbers simply remove the relief valve entirely - this is the process recommended by some water heater manufactures such as A.O. Smith. Removing the relief valve makes it easy to inspect this critical safety component itself, and it's easy to clean or replace the safety valve at that time.
When replacing the relief valve use an approved teflon tape or pipe sealant and work neatly and with care so that there will be no leaks at this location.
Safety Warning- a damaged, improperly-selected model, or modified pressure/temperature relief valve is dangerous and could lead to a serious BLEVE explosion should the water heater later overheat.
For a general discussion of temperature and pressure relief valves used as safety devices on heating boilers and other pressurized please
see RELIEF VALVES - TP VALVES.
Pressure relief valves (that sense pressure only, not temperature) are also required on pressurized tanks such as water tanks in buildings.
Some manufacturers (Rheem) recommend that the temperature and pressure safety relief valve be tested once every six months. To test the valve the lever on the valve is lifted, which should result in (hot) water flushing through the valve and its discharge tube into a bucket placed below the discharge tube for that purpose or into a floor drain.
In Australia and New Zealand the test lever on a temperature/pressure relief valve is called an easing lever but performs the same purpose.
After lifting the easing lever or relief valve opening lever, lower it gently back to its closed position. Rheem points out that rough handling of the relief valve test lever, such as allowing it to "snap" closed, can damage the temperature and pressure relief valve - a dangerous condition that could lead to a water heater explosion
or BLEVE EXPLOSION.
If water does not flow freely when the temperature/pressure relief valve is opened, the valve should be replaced by a licensed plumber, making sure that an approved, properly-rated TP valve is installed.
If your temperature/pressure relief valve has not been tested or replaced sooner than five years, Rheem recommends that the TP valve should simply be replaced. More frequent replacement of the safety valve may be needed in areas where hard water is found.
One has to consider that the manufacturer would not be likely to include the test-lever feature on relief valves if it had no intended uses whatsoever.
Water heater manufacturers' installation instructions for at least some water heaters advise building owners or maintenance personnel to manually operate the TP valve at least once a year to make sure it is working properly.
Recommendations for testing versus inspecting temperature and pressure relief valves vary.
Nevertheless, Ken, I agree with you that while the "easing lever" can be used to open or "operate" a TP valve, that does not promise that the valve will necessarily open at the rated pressure or temperature.
But by making this test you might find by that the valve is "stuck" or clogged and will not operate. Not finding that the valve is stuck is not a 100% promise of proper function but the risk is reduced. Here are detailed TP valve testing instructions from American Water Heater:
Standing clear of the outlet (discharged water may be hot), slowly lift and release the lever handle on the temperature and pressure relief valve to allow the valve to operate freely and return to its closed position.
If the [relief] valve fails to completely reset and continues to release water, immediately shut-off the electrical power and the cold water inlet valve and call a qualified service technician. 
Watch out: Opening or operating the pressure relief valve using the lever is not a complete test and it should not be used by home inspectors nor by anyone who is not prepared to shut down the water heater and replace the TP valve immediately should the valve fail to operate or should it fail to close and stop leaking or dripping after it has been tested using the lever.
And in sum, I agree completely that a questionable or used or leaky TP valve should be replaced immediately.
More water heater temperature & pressure relief valve (TPR) test recommendations are in the Residential Gas and Electric Water Heater Handbook [PDF] provided by A.O. Smith and linked-to at REFERENCES
A Temperature Limiting Valve that limits the outgoing water temperature to 50 degC in order to prevent scalding. This valve should be checked annually by measuring water temperature at a nearby water taps, making sure that the water delivered closest to the water heater is not hotter than 50 degC.
See ANTI SCALD VALVES.
An Expansion Control Valve is used on water heaters in south and Western Australia where hard water is found, and in some other countries or other jurisdictions. The Expansion Control Valve discharges water into a drain to relieve excess pressure in the hot water tank. The purpose of this valve is to release pressure through a separate control so that the safety provided by the Temperature and Pressure Relief valve is not compromised by clogging from minerals should that valve frequently open.
The expansion control valve should be tested every six months, following the same procedure as for temperature and pressure relief valves as we described above.
Readers of this article should
see RELIEF VALVES - TP VALVES where we include additional relief valve information including for hydronic heating and steam heating boilers used for central heating.
And see WATER HEATER SAFETY for our complete list of water heater safety devices and water heater safety inspection advice.
At TUNDISH used in PLUMBING we describe special devices designed to provide an air gap in the relief valve discharge tube and piping: a feature that can permit visual detection of a spilling TP valve if its discharge has been piped to an otherwise not visible location, and a feature that can protect the potable building water supply system against contamination from cross connections.
If the building water pressure gauge reading is ever found at 80 psi or higher, you will want to install a water pressure regulator at the point where water supply enters the building.
If your building already has a water pressure gauge installed, it may be defective or it may be set too high. The articles listed just below discuss how to adjust a water pressure regulator:
An alternative to installing or changing a water pressure regulator when building water pressure is occasionally 80 psi or higher is the installation of an expansion tank to temporarily absorb that pressure increase. Proper use of an expansion tank can help avoid unnecessary opening of the pressure/temperature relief valve on a hot water heating tank or a hot water heating boiler.
Continue reading at RELIEF VALVE, WATER HEATER DIAGNOSTIC FAQs or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.
Or see HOT WATER EXPANSION TANKS - how to cure the problem of leaks in heated hot water systems, calorifiers, water cylinders
Or see HOT WATER PRESSURE EXPANSION RATE - how much does water expand when you heat it?
Or see RELIEF VALVE DISCHARGE TUBE - important safety feature avoids getting scalded if the TPR valve opens
Or see RELIEF VALVE LEAKS - all of the casues and cures
Or see RELIEF VALVES - TP VALVES - home
Or see WATER HEATER SAFETY
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