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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.
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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. 
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
T&P Valve Installation Location, Position, & Drain Discharge Line Routing Rules
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 
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.)
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
Constricted, Under-sized, or Mis-Routed Relief Valve Discharge Tubes
Evidence of dangerous leaks and corrosion at water heater temperature/pressure safety valves
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.  
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:
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.
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.
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:
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.
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.
Some of the FAQs discussed below are adapted from information provided by the Watts Regulator Company in a 1973 publication.
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
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:
Keep us posted - what you learn may help other readers. - Editor
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
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. 
... 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. 
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.
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
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.
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.
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
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.
Will a "straight" pressure-only relief valve prevent overheating and thus keep my hot water system safe if a check valve is also used?
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.
Does TP valve on residential water heater use a p-trap - Xavier
No Xavier, the discharge from a TP valve should not be trapped it's discharge end should be visible
Can the relief valve affect the pressure of water coming out of faucets? - Pam Gregg
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.
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
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. 
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.
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?
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.
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.
I am ... in the process of selling a condo I own. I got this request for repairs for the hot water heater with a picture of the heater. On the picture it shows the that the discharge line is above the TPR valve, and that this is a problem (see description on attachtment). This doesn't make any sense to me. Can you help me decide what the best action would be? thanks. - R.N. 7/11/2013
Quoting from the inspection report:
The overflow line is higher than the Temperature and Pressure Relief (T & P) valve. The over flow line should always allow to water to drain to a point lower than the valve without backing up to the valve. Water trapped in the discharge line may cause the T & P valve to corrode and malfunction. (The concern is if the water overheats and turns to steam, and the temperature control valve fails to function, the over pressure may cause the heater to burst or explode.) Lowering of the discharge line to a point below the valve and replacement of the valve should solve the problem.
The photo is a bit difficult to read but if you look closely where the two flexible copper lines enter the wall behind the water heater, you'll see that the smaller leftmost flexible tube, connected back to the water heater TP valve, enters the wall at a height above the valve outlet opening. What the home inspector said was perfectly correct and represents a safety hazard.
Perhaps if I explain the concern in my own words it will be more clear:
The temperature/pressure relief valve on a water heater is connected to a drain line so that if the valve opens someone nearby is not shot in the face with hot water. The discharge drain extension is typically taken to just a few inches above the floor or in some jurisdictions it may be directed outdoors - a solution that I think is risky because IF the valve should be leaking, dripping, etc., one wants to notice that and fix it to keep the system safe.
The inspector's report makes a valid point: we should never route the discharge tube "up" from the actual outlet opening of the TP valve. That's because if the valve should develop a small leak or be discharged on occasion, the up-routed discharge tube will keep water and debris remaining in the tube at the valve outlet where debris or mineral accumulation clog the valve or interfere with its operating spring. The result over time could be that the valve becomes clogged and would then fail to open in a true emergency - risking, ultimately a dangerous BLEVE or water heater explosion.
Watch out: ALSO, I suspect from the photo that your water heater has a discharge tube that directs the valve outlet into a wall and going to who knows where. If the other end of that line is not already readily visible and in a location where it would be noticed, that too would be unsafe and improper.
The FIX for this unsafe condition is usually trivial: the discharge tube must be routed only "downwards" from the TP valve outlet opening, and the end of the discharge tube must be in a readily accessible, visible, and safe location. You'd probably find these same instructions in the installation manual for the water heater.
The COST for this repair should be no more than a simple plumbing service call and perhaps a few piping connections. What would make sense to me and what would be most economical would be to combine this repair with any other plumbing repairs that are needed at the home.
The TIME to fix this problem is as soon as possible, without handling it as an extra-cost "immediate-emergency" plumbing repair. That means, call a plumber and schedule the work for as soon as possible and practical, but not sounding so scared that the plumber gouges the customer with extra fees.
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