Guide to Hot Water Heater Temperature Pressure Relief Valves

Water Heater Safety Inspections - check the TP Relief Valve
- BLEVEs: Boiling Liquid Expanding Vapor Explosions at Water Heaters (or heating boilers)
- How to use, test, inspect water heater temperature and pressure safety relief valves
- How to fix or replace leaky water heater relief valves
- ANSI and water heater standards and advice: water heater relief valve requirements
- Water heaters: how to inspect, test, adjust, repair, and choose among types of hot water heaters
WATER HEATER SAFETY - separate article
Questions & Answers about water heater pressure/temperature relief valves or TP valves on water heating appliances
References

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InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

Water heater safety valves: this article explains 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.

Water Heater Temperature & Pressure Relief Valves

Here at RELIEF VALVES - Water Heaters we discuss temperature and pressure relief valves used on residential water heaters. 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. Water tank pressure relief valves are discussed at WATER PUMP SAFETY. At HOT WATER PRESSURE EXPANSION RATE we discuss how we measure water pressure and how temperature changes affect water pressure in a closed water heater tank or heating boiler. See WATER HEATER SAFETY for our complete list of water heater safety devices and water heater safety inspection advice.

Suggestions for Testing the Water Heater Temperature Pressure Relief Valve (TP Valve or "Safety Valve") on a Water Heater

Requirement for Installation of Water Heater T&P Valves

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.

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 Co'.s installation guide. [1]

Testing the T&P Valve on a Water Heater

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 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.

Other Versions of Pressure or Temperature & Pressure Relief Valves on Water Heaters

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.

T&P Valve Installation Location, Position, & Drain Discharge Line Routing Rules

Proper location for water heater TP relief valve

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 [10]

Do not depend on a pressure-relief only valve to protect a hot water heating system. Pressure relief alone can't prevent a water heater tank explosion. See BLEVE EXPLOSIONS for details.

Pressure relief valves are only intended for protection of tanks that are not heated - such as a well water pressure tank or a cold water storage tank. Where a pressure-only relief valve is to be used it should be installed as close as possible to the equipment it's protecting.
Shutoff valves should never be installed between a relief valve and the tank it is designed to protect

Proper Temperature & Pressure Relief Valve Size for Water Heaters

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. [10]

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.

Proper position for water heater TP relief valve

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, left, 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.

Missing Water Heater Relief Valve Discharge Tube

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

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.

Leaky or Dripping Water Heater Pressure & Temperature Relief Valves

Evidence of dangerous leaks and corrosion at water heater temperature/pressure safety valves

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 some common water heater relief valve leak causes:

The water heater temperatures are excessive
The building water supply pressure is too high or periodically water pressure fluctuates and is too high at times
Periodic discharge of the temperature and pressure relief valve may be due to thermal expansion in a closed water supply system. Details are at Closed Hot Water Systems A closed water system with thermal expansion and no means of relief can cause leaks at the pressure/temperature relief valve, such as
- on a hot water heater with some building piping arrangements
- on a hot water heating boiler at which its expansion tank has become waterlogged - see EXPANSION TANKS for diagnosis & repair procedures
Dirt or debris are preventing proper closure of the relief valve valve
A deteriorated gasket inside the relief valve or corrosion on the valve seat can cause leaking at the valve; we find this mess occurring when someone lifts the "test lever" on a older P/T valve that has not been tested or operated for some time. A brittle piece of gasket can be spit out of the valve and it will then keep leaking.
Water expansion pressure (thermal expansion) increases in a closed system with a check valve installed close to the water heater. The increase in plumbing system pressure to a level that opens the TP valve is called "thermal expansion pressure". Watts suggests installing a bypass model water pressure regulator that lets the excessive pressure head back to the street main or building water supply system - a solution that only works if the supply pressure is lower than the T&P relief valve spill pressure - which it usually is. (See THERMAL EXPANSION of HOT WATER for further explanation in horrible detail. )
The T&P valve is installed on a sidearm gas heater that is causing an overheat condition at the sensor point of the valve (This thermal expansion is discussed at T&P Valves Installed on Gas Sidearm Heaters)
Watch out: a dripping or frequenly 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.

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.

Accidental or deliberately plugged Temperature & Pressure Relief Valves

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. The discharge opening must not be blocked or reduced in size under any circumstance. [1] [2]

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.

Other safety valves and devices found on some water heaters include the following:

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.

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:

WATER PRESSURE REDUCER / REGULATOR
  What's a Water Pressure Regulator?
  WATER PRESSURE VARIATION CAUSES
  Diagnose Poor Municipal Water Pressure
  WATER PRESSURE REGULATOR ADJUSTMENT

Expansion tanks to relieve high water pressure:

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.

What is the Proper Size & Rating for Temperature/Pressure Relief Valves on Water Heaters?

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. Othewise 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.[10]

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 accumlation 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.

How Mineral Deposits and Heating May Weaken a Water Heater Tank Bottom, Contributing to BLEVE Explosion:
What is a BLEVE or Boiling Liquid Expanding Vapor 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 VALVES - TP Valves on Boilers and at RELIEF VALVES - Water Heaters.

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.

Using the Temperature & Pressure Valve to Aid Draining a Hot Water Storage Tank

Water heater pressure temperature safety valve (C) Daniel Friedman

How to Let Air Into the Hot Water Tank to Speed the Tank Draining Process

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.

At HOT WATER PRESSURE EXPANSION RATE we discuss how we measure water pressure and how temperature changes affect water pressure in a closed water heater tank or heating boiler.

See WATER HEATER SAFETY for our complete list of water heater safety devices and water heater safety inspection advice.

Frequently Asked Questions and Comments about water heater pressure/temperature relief valves or TP valves on water heating appliances.

Some of the FAQs discussed below are adapted from information provided by the Watts Regulator Company in a 1973 publication.[10]

Question: Leaky pressure relief valve blamed on bad expansion tank bladder ?

I have a weil McLain hot water boiler, which is about 20 years old. It was leaking out the pressure relief valve, so we changed out the expansion tank which had a bad bladder in it, as well as the pressure relief valve, but now I noticed it has continued to leak out of the new pressure relief valve when the boiler is running. Any ideas? - Vincent Nizzardi

Reply:

A bad expansion tank bladder would certainly be a cause of relief valve leakage. As you replaced both the tank and the TP valve, and now see a leak at the new valve, there are other possible explanations such as:

dirt on the valve seat due to crud that flowed into the valve during testing after installation. A technician might try flushing the valve and gently tapping the lift rod to be sure the valve is seated properly.
high water pressure in the boiler - check the water pressure with a reliable gauge
high boiler temperature
a defective automatic water feeder that is putting too much water or water at too high a pressure into the system
a leaky tankless coil that leaks into the boiler
you may need an expansion tank on your water heating system. See the explanation and cure described in the article above at Closed Hot Water System & Thermal Expansion Problems.

Keep us posted - what you learn may help other readers. - Editor

Question: Do I also need a TP valve on the cold water line? What about a check valve on the cold water supply main line?

Do I need a pressure-relief valve on the cold waer pipe that feeds my hot water heater tank? - Watts

Do I need to install a check valve on the cold water supply main line? Is it safe to do so? - Watts

On a municipal water supply system, do I need a check valve on the cold water line if a TP valve is installed on the water heater? - Watts

Reply: no

No, there is no need for a separate pressure relief valve on the cold water line feeding the water heater. That's because water pressure within the system is common throughout all of the plumbing system: cold water line into the tank, hot water in the water heater, and hot water in the hot water lines leaving the water heater tank all see the same pressure. [10]

... the original purpose of a check valve was to protect the [water] meter by preventing superheated water from backing up into the cold supply main from range boilers and heaters. However, the very condition that can cause this, can also cause explosions and the temperature and pressure relief valve principle protects against both excessive temperature and [excessive] pressure, thereby eliminating the need for a check valve. A check valve closes the [plumbing] system, thereby allowing pressure to build up from thermal expansion higher than the city main pressure, and therefore should not be used except where required by local codes. [10]

Note: We add that there are other reasons for a check valve or backflow preventer valve on water systems: to prevent water from the building from flowing backwards into the city supply mains during an interval of loss of pressure in the mains. Should such a pressure loss occur the risk is that unsanitary water from buildings connected to the mains could flow backwards into and thus contaminate the water supply mains in a community. Most community water supply systems include a pressure regulator combined with a backflow preventer or check valve at the water meter. Watts' note above was penned in 1973. The company may have more to say on this topic today.

When a hot water system includes a temperature & pressure relief valve installed on the water tank or heater there is no advantage to installing a check valve to protect the water meter - as Watts explained in the quotation above. Watts points out that if a check valve is installed on the cold water line to the water heater, a result is a pressure rise in the system each time water is heated in the hot water tank.

Question: What is an "open main" water system and does it have safety implications for water heaters?

Does the water pressure in my house plumbing system ever increase under any circumstance (hot water heater or something else) on an open main water supply system? - Watts

Reply:

In an "open main" system nothing is closed between the house [plumbing] system and the street main. Therefore, water pressure [in the house] cannot increase above the street main pressure from the reservoir regardless of any expansion [in the hot water system] from temperature. Consequently, a straight pressure relief valve cannot operate unless a system is closed.[10]

Note: By "straight pressure relief valve" we think Watts means a valve that responds only to water pressure and that does not include a response to hot water temperature. Such a system could be unsafe.

Question: Why is hot water pressure going up to 150 psi in my water heater tank?

I had to install a new temp controller in my hot water tank. It is a bourdon tube type and I installed exactly like the original one. The water in the tank is heated by a coil as a separate zone from the furnace. The house water pressure is about 50 psi. I have the tank temp set at 125 degrees.

I installed a pressure gauge between the pressure relief valve and the top of the tank. When the hot water from the furnace enters the tank the water pressure goes from 50 psi to 150 psi and the relief valve discharges a little and then stops. The water temp is 101 degrees at this time. Why is the hot water tank pressure increasing to 150 psi? - Alan

Reply:

Alan you may need an expansion tank on your water heating system. Heating water in a closed container increases the pressure. See our discussion above about Closed Hot Water System & Thermal Expansion Problems.

Question: can I use a combination of check valve and pressure-only relief valve to protect against overheating my hot water tank?

Will a "straight" pressure-only relief valve prevent overheating and thus keep my hot water system safe if a check valve is also used?

Reply: No. Such a system is unsafe

No. Overheating in the hot water system depends entirely on the BTU input rate to the water heater. A relief valve that operates on pressure only (ignoring temperature), regardless of its size or rated discharge capacity, can't prevent overheating nor reduce temperature. Such a sysetm is unsafe.

Question: Should TP Valve Discharge Lines be Trapped?

Does TP valve on residential water heater use a p-trap - Xavier

Reply:

No Xavier, the discharge from a TP valve should not be trapped it's discharge end should be visible

Question: will the relief valve affect building water pressure delivered to faucets?

Can the relief valve affect the pressure of water coming out of faucets? - Pam Gregg

Reply:

No, Pam, not under normal circumstances. That's because hot water leaving the water heater may pass BY a relief valve en route to your faucets (depending on where it is installed) but the water is not passing THROUGH it.

But if a water heater pressure-relief valve were stuck wide open flushing hot water continuously down a drain, building water pressure would probably fall noticeably - that would be an unusual circumstance and surely you would know it, from the flooding water out of the valve and quickly by the loss of hot water.

Question: using the "easing lever" on TP Valves? When should a TP valve be replaced?

I don't understand how simply lifting the "easing lever" verifies that the temp./psi. relief valve is intact and will function properly at factory set limits. Most water supplies have trace elements , including some non- toxic metals.

These elements usually collect and build up on the sensing stem of a water heater relief valve. If a relief valves "easing lever " is tripped, valves with lime and trace metal accumulation often will not re-seat, resulting loss of heated water,shutting off the water supply to the unit, shutting down the power to the unit, ordering a new relief valve and installing the replacement. It's seems to thoroughly test a water heater relief valve, Psi. and temp. limits would have to applied that would require the relief valve to engage.

Once a relief valve has released at its set point it should never be put back into service. - Ken Hansen

Reply: when should the temperature/pressure relief valve on a water heater be tested? And what is a "test"?

One has to consider that the manufacturer would not be likely to include this 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.

Neverthess, 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 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. [1]

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.

Question: since a TP valve releases excessive pressure in a water heater or boiler, isn't pressure-release enough protection against a water heater tank explosion?

If the condition of too much hot water pressure is corrected by the automatic opening of the pressure/temperature relief safety valve or "blowoff valve", why was I told that a water tank can still blow up?

Reply: relieving pressure alone won't prevent a hot water tank from exploding - that's why a TP valve also responds to temperature and the TP valve has to release heat at a rate equal to or greater than the BTU input rate for the water heater

One would think that the TP valve alone would always be enough protectin on a water heater tank. But as Watts Regulator Company explained back in the 1970's and as was demonstrated in the film "Explosion Danger Lurks", even a wide-open pressure relief valve can not prevent water in a water heater tank from becoming overheated.

At WATER TANK PRESSURE CALCULATIONS we discuss the calculations behind the increase in temperature and pressure in an enclosed water tank or water heater tank. In the case of a hot water storage tank, Watts points out that

The reason that the volume of pressure discharge [through the relief valve] is not enough to overcome the BTU heat input [into the hot water heater tank] is because thermal expansion pressure is approximately 2 1/2% of volume for every 100 degree rise. [The] fluid heat discharge ... necessary to relieve the extra BTU heat input for every 1000 heat units is about 20 times greater in volume.

In a [hot water heating] system with a check valve and a pressure relief valve, the relief valve opening by pressure increase due to expanding water will discharge approximiately 1/2 lbs. of water for each 1,000 heat units (BTUs) put into the water - because that is the rate at which the water expands when heated.

To release 1,000 heat units when the temperature is at 210-212 degF, there must be approximately 6 2/3 lbs. of water released from the system. In other words, to prevent overheating [water in the hot water storage tank] a means must be provided to relase about 20 times as much water from the system as a pressure relief valve can discharge from thermal expansion [alone], when there is a check valve in the supply line or [when there is] an accidental stoppage [of the water piping] to make the system a closed one.[10]

In other words, in a closed hot water piping system (closed by the presence of a check valve or a blockage in the water piping to prevent expanding hot water from "pushing" water volume back out of the watertank, to prevent a hot water tank from overheating, the TP valve has to discharge heat [not pressure] at or greater than the rate that the heater's burner or electrodes are putting heat into the water in the tank.

But on a typical water heater, the heat input rate in BTUs is about 20 times more than the heat output rate through a TP valve if it operated on pressure alone.

Watch out: the conclusion of this technical discussion is that it is absolutely essential that the BTU (heat or energy) dumping rate for a TP valve must be properly matched to the BTU input rate of the heater. And a pressure-only relief valve on a water heater, that is excluding a valve that also responds to temperature, would be an unsafe installation.

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- The Illustrated Home illustrates construction details and building components, a reference for owners & inspectors
[1] "Residential Electric Water Heater Installation Instructions and Use & Care Guide", American Water Heater Co., October 2001, American Water Heater Co., Johnson City, TN, [manufacturer of residential & commercial water heaters, also manufacturer of Polaris/Commercial water heaters], Tel: 800-999-9515, web search 1/12/2012, original source: americanwaterheater.com/support/manuals/res-elect.pdf [copy on file] [Technical review requested 1/25/2011]
[2] ANSI Z21.22 - 1986, provides requirements for pressure/temperature relief valves and automatic shutoff devices for hot water supply systems
[3] Pounds of lime deposited vs. temperature and hot water usage: see Purdue Bulletin #74 (also provided by A.O. Smith in the article below). Purdue's chart shows the number of pounds of lime deposited per year as a function of the water usage in gallons per day, with an assumed 10 grains of water hardness.
[4] "When, Why, and How to Remove Water Scale from Tank Type Glass-Lined Water Heaters (for non glass-lined tanks, consult water heater manufacturer)" PDF provided by A.O. Smith Water Products Company - hotwater.com/lit/training/4800r9.pdf 800-433-2545 - 01/07/2009.
[5] A.O. Smith's Form No. 4800 Rev. 8 Why? When? & How? /UN•LIME Specific De liming Instructions for use with Up-N-Down Transfer Kit for Tank Type Heaters. (Normally supplied in UN-LIME shipping cartons), Supersedes Form Nos. 4800 Rev. 7 and 4813-100.
[6] A.O. Smith's Form No. 4778* All about Deliming Coil-Type/Tube-Type Commercial Water Heaters and Hydronic Boilers *Normally supplied when ordering Part No. 4930 Motorized Deliming Pump Kit
[7] "Building Owner Water Heater Safety Notice", Building Department, City of Colleyville TX, web search 09/24/2010, original source: http://www.colleyville.com/dmdocuments/Building%20
Owner%20Water%20Heater%20Safety%20Notice%203-6-102.pdf
Quoting:
Water heater safety is imperative to the occupants of a building or structure. If improperly installed, water heaters can be detrimental to the structure, as well as being potentially fatal to its occupants. The proper installation of a water heater is so important that according to Texas State Law all water heater installations must be inspected by a state licensed plumbing inspector.
[8] Rheem Electric Water Heater "Owners Guide and Installation Instructions", (Australian models) rheem.com.au/images/pdf/owners_dom-elec_121996C_0707.pdf
[9] Watts, 815 Chestnut Street, North Andover, MA, USA 01845-6098, web search 09/18/2010 original source: http://www.watts.com/pages/learnAbout/reducingValves.asp?catId=64
[10] 52 Questions and thier Answers [about] Hot Water [Heaters, Explosions & Water Heater Safety], Watts Regulator Company (1973)
[11] "Four hurt as water heater explodes", Elaine Porterfield, Paul Shukovsky, Lewis Kamb, Seattle Post Intelligencer, 28 July 2001, web search 25 Sept 2010, original source: http://www.seattlepi.com/local/33094_boom28.shtml
Quoting from the original news article:
The powerful blast occurred at 10:23 a.m. at Plaza Zacatecas -- a small strip mall that includes four Mexican American-run businesses: a video store, a butcher shop, a Mexican restaurant and a shop that sold clothing and other items. ... The steam-powered tank hurtled across the busy intersection at First Avenue South and South 152nd Street -- over at least six lanes of traffic -- before landing more than 439 feet away in the parking lot of a Pizza Hut. ... The tank then tumbled another 25 feet before coming to rest. Four people were injured in the blast. Three were treated and released from a local hospital, but one woman was admitted to Harborview Medical Center with second-degree burns to her arm.
[12] Water heater rocket myth: Mythbusters water heater explosion video produced ihn 2007, web search Sept 25 2010, original source: http://www.youtube.com/watch?v=JmJoyuUJj2Q
Website: http://dsc.discovery.com/tv/mythbusters/
Quoting from Mythbusters: (Discovery Communications,
In small scale testing, the Mythbusters started with a small six gallon water heater and disabled all of its safety features under the theory of poor installation or neglect. While the water heater eventually ruptured, it did not explode like a rocket. The Mythbusters then upgraded to larger thirty gallon water heater which exploded with significantly greater force, sending the water heater several hundred feet into the air. In order to confirm the stated myth, the Mythbusters obtained a full size fifty two gallon water heater and built a shack around it with a roof that followed standard California building codes. The water heater eventually exploded, shooting through the roof five hundred feet into the air and disintegrating the shack. In light of these results, and the fact that there is documented evidence corroborating the myth, the Mythbusters deemed it confirmed.
[13] Water heater video #2, Mythbusters exploding water heater demonstration, a 50 gallon water tank explosion
http://www.youtube.com/watch?v=0rXwcDkobUY&feature=related

http://www.waterheaterblast.com/
Because of built in safety devices most water heaters safely operate day in, day out without any major problems. But don't let the excellent safety record of water heaters lull you into forgetting about the explosive potential of these marvels of convenience. When a water heater explodes, it releases a tremendous blast force which can easily demolish a building.
Randall Hilton and crew, with help from the Service Roundtable has prepared this video of a water heater explosion as a demonstration of the explosive power of a simple water heater. The hot water tank explodes using the steam pressure that any water heater can generate when the thermostat and temperature pressure relief valve (T&P valve or PT valve) malfunction. We were impressed by how far the tank flew after the water heater exploded. Click on the links below to view the video. Then, visit the Q&A page for warning signs as well as simple steps which can help you prevent your own water heater from exploding.

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Water Heater Temperature & Pressure Relief Valves

Suggestions for Testing the Water Heater Temperature Pressure Relief Valve (TP Valve or "Safety Valve") on a Water Heater

Requirement for Installation of Water Heater T&P Valves

Testing the T&P Valve on a Water Heater

Other Versions of Pressure or Temperature & Pressure Relief Valves on Water Heaters

T&P Valve Installation Location, Position, & Drain Discharge Line Routing Rules

Proper location for water heater TP relief valve

Proper Temperature & Pressure Relief Valve Size for Water Heaters

Proper position for water heater TP relief valve

Missing Water Heater Relief Valve Discharge Tube

Constricted, Under-sized, or Mis-Routed Relief Valve Discharge Tubes

Leaky or Dripping Water Heater Pressure & Temperature Relief Valves

Accidental or deliberately plugged Temperature & Pressure Relief Valves

Other safety valves and devices found on some water heaters include the following:

Expansion tanks to relieve high water pressure:

What is the Proper Size & Rating for Temperature/Pressure Relief Valves on Water Heaters?

How Mineral Deposits and Heating May Weaken a Water Heater Tank Bottom, Contributing to BLEVE Explosion: What is a BLEVE or Boiling Liquid Expanding Vapor Explosion?

Using the Temperature & Pressure Valve to Aid Draining a Hot Water Storage Tank

How to Let Air Into the Hot Water Tank to Speed the Tank Draining Process

Frequently Asked Questions and Comments about water heater pressure/temperature relief valves or TP valves on water heating appliances.

Question: Leaky pressure relief valve blamed on bad expansion tank bladder ?

Reply:

Question: Do I also need a TP valve on the cold water line? What about a check valve on the cold water supply main line?

Reply: no

Question: What is an "open main" water system and does it have safety implications for water heaters?

Reply:

Question: Why is hot water pressure going up to 150 psi in my water heater tank?

Reply:

Question: can I use a combination of check valve and pressure-only relief valve to protect against overheating my hot water tank?

Reply: No. Such a system is unsafe

Question: Should TP Valve Discharge Lines be Trapped?

Reply:

Question: will the relief valve affect building water pressure delivered to faucets?

Reply:

Question: using the "easing lever" on TP Valves? When should a TP valve be replaced?

Reply: when should the temperature/pressure relief valve on a water heater be tested? And what is a "test"?

Question: since a TP valve releases excessive pressure in a water heater or boiler, isn't pressure-release enough protection against a water heater tank explosion?

Reply: relieving pressure alone won't prevent a hot water tank from exploding - that's why a TP valve also responds to temperature and the TP valve has to release heat at a rate equal to or greater than the BTU input rate for the water heater

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Technical Reviewers & References

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair

How Mineral Deposits and Heating May Weaken a Water Heater Tank Bottom, Contributing to BLEVE Explosion:
What is a BLEVE or Boiling Liquid Expanding Vapor Explosion?