Temperature and pressure relief valve on a water heater (C) Daniel Friedman

Guide to Hot Water Heater Temperature Pressure Relief Valves
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Water Heater Safety Inspections
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
Questions & answers about water heater pressure/temperature relief valves or TP valves on water heating appliances

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.

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

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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.

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

The 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).

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&) 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.

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.

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.

Repairing Leaky Water Heater Pressure Relief Valves: Evidence of dangerous leaks and corrosion at water heater temperature/pressure safety valves

As our photo shows (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.

The impaction of the relief valve spring with copper and calcium salts in this photo means that the valve is almost certainly not going to open should the heater's internal pressure become unsafe.

The drip shown at the mouth of this relief valve demonstrates that a relief valve can drip and leak for a long time without anyone observing this dangerous condition.

This relief valve needs to be replaced immediately and the cause for the valve's leakage also needs to be determined.

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.

Closed Hot Water System & Thermal Expansion Problems

In explaining why the relief valve on a water heater may be dripping, American Water Heater Co. [1] explains:

Periodic discharge of the temperature and pressure relief valve may be due to thermal expansion in a closed water supply system. The water utility supply meter may contain a check valve, backflow preventer or water pressure reducing valve. This will create a closed water system.

During the heating cycle of the water heater, the water expands causing pressure inside the water heater to increase. This may cause the temperature and pressure relief valve to discharge small quantities of hot water. To prevent this from happening, there are two recommendations:

Install a diaphragm-type expansion tank that is suitable for potable water on the cold water supply line. The expansion tank must have a minimum capacity of 1.5 U.S. gallons for every 50 gallons of stored water.
Install a 125 PSI pressure relief valve in the cold water supply line. Make sure the discharge of this valve is directed to an open drain and protected from freezing. [1]
Note: While a 125 psi pressure relief valve on cold water supply piping is fine to protect piping from rupturing, if building incoming water pressures are too high, rather than relying just on a relief valve (wasting water) we prefer to install a pressure reducing/control valve at the building entering water supply piping. Limit incoming water pressure to 60 psi at a typical one or two story residential home.

Watch out: get advice from a licensed plumber and your local water supplier. Never remove or plug a leaky pressure/temperature relief valve on a water heater - doing so risks dangerous or even fatal BLEVE EXPLOSIONS.

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.

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.

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.

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: 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: 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.

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RELIEF VALVES - TP Valves on Boilers
RELIEF VALVES - Water Heaters
RELIEF VALVES - Water Tanks

Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. (416) 964-9415 1-800-268-7070 info@carsondunlop.com. Thanks to Alan Carson and Bob Dunlop, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education including the ASHI-adopted Home Inspection Training Program (home study course), publications such as the Home Reference Book, report writing materials including the Horizon report writer, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home 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
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.
"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.
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.
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
"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.
Rheem Electric Water Heater "Owners Guide and Installation Instructions", (Australian models) rheem.com.au/images/pdf/owners_dom-elec_121996C_0707.pdf
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

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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

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

Repairing Leaky Water Heater Pressure Relief Valves: Evidence of dangerous leaks and corrosion at water heater temperature/pressure safety valves

Closed Hot Water System & Thermal Expansion Problems

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

Expansion tanks to relieve high water pressure:

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?

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

Reply:

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"?

Questions & Answers regarding this article

Ask a Question or Search InspectAPedia

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?