LARGER VIEW of a heating boiler cad cell relay switchTemperature & Pressure Relief Valves
TPR Valves or T&P Valves used on boilers, calorifiers, geysers, water heaters & all water heating equipment

  • RELIEF VALVES - TP VALVES - CONTENTS: TPR valves or Pressure & Temperature Relief Safety Valves: what are they, where are they installed, how do they work. Missing Relief Valve Extension is a Safety Hazard.
  • POST a QUESTION or READ FAQs about pressure or pressure & temperature safety relief valves: inspection, diagnosis, installation requirements, relief valve testing, and relief valve repair procedures.

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Guide to TP valves:

How to install, inspect, troubleshoot, repair pressure / temperature relief valves or straight pressure relief valves used on heating boilers, steam boilers, water heaters, and even on water pressure tanks. This article explains what TP or pressure relief safety valves are, why they are needed, how they work, and what goes wrong.

We describe how to test (or when not to test) relief valves, how to know that this important safety device is in trouble, including by simple visual inspection, and we answer just about any question about these important safety devices.

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A Guide to Temperature & Pressure Relief Valves, Inspection, Defects, Testing, Repair

Relief valve missing extensionThis guide to relief valve inspection, testing, installation, repair, answers most questions about the use of these safety devices on heating equipment such as central heating system boilers, water heaters, steam generators and even pressurised water tank. This information aids in troubleshooting, inspection, diagnosis, and repairs.

We include a discussion of temperature and pressure relief valves used as safety devices on heating boilers and other pressurized vessels.

We also discuss:

Leaking or Previously Leaking Pressure/Temperature Relief Valves are Dangerous. Attic boiler pressure control tanks - Old Heating Boilers (steam or hot water) may have No Relief Valve at All - Check the Attic.

List of Common Boiler Temperature/Pressure Relief Valve Defects. Temperature/Pressure Relief Valve Testing Advice. Photo examples of unsafe, dangerous pressure/temperature relief valve installations.

Troubleshooting heating system boiler controls and switches.

Article Series Contents

Lots of controls are installed on modern water heaters, hot water cylinders, hot water and steam heating boilers. Many of these devices are principally concerned with safety. The combination of these devices provide a tremendous margin of safety on home and commercial heating boilers, as evidenced by the rarity with which we read in modern times of boiler explosions.

Before these devices were in common use, and even today if the devices are improperly installed, poorly maintained, or damaged, the heating systems they are supposed to protect are in fact un-protected.

Watch out: A defective relief valve is a latent safety hazard in that the valve does not by itself cause a water heater or heating boiler to explode, but it may fail to protect against that event should other dangerous conditions causing over temperature or over pressure arise in a heating boiler or water heater.

Examples of TPR Valve Safety Hazards

Obsolete boiler pressure relief valveCheck the article series links given above to find details for each type & application of temperature & pressure safety valve used on plumbing & heating systems found in or around buildings. Below are a few examples to whet your interest.

Pressure and Temperature Relief Valve on heating boilers: a TP valve is installed on all modern heating boilers to release hot water and pressure should the boiler's internal pressure or temperature rise to an unsafe level.

Our photo at page top shows what the typical boiler relief valve looks like. You may find this valve connected at the top of a heating boiler, at its side, or (less desirable) very nearby on boiler hot water piping.

Our photo at left shows an older (obsolete) type of pressure relief safety device that may be mounted close to the boiler but not right on it. This type of pressure relief device may not sense boiler temperature, just boiler pressure.

You can see that this pressure relief valve (shown above) has been leaking - it may be unsafe, as we discuss further below. Both the page top relief valve and the one in this photo are missing their discharge tubes.

Watch out: shown above is a single example of an unsafe condition at an antiquated pressure safety valve. A more complete explanation and illustration of safety hazards around TPR valves are provided in detail in each of the TPR valve categories listed above in this article.

Temperature Pressure Relief valve opening level blocked by foam insulation (C) Daniel Friedman

Above is another unsafe temperature/pressure relief valve installation: the test lever that would be used to manually open, test or flush the TP valve is blocked first by having been placed up against a plaster wall and second by having had foam insulation sprayed into an opening that some smarter plumber cut into the wall to permit the lever to move.

Watch out: any obstruction that prevents the test lever on a TPR valve from being lifted or opened can also prevent the valve from opening in response to excessive pressure or temperature in the heating equipment it is supposed to protect. The result risks an exploding water heater or boiler.

Be sure that the Relief Capacity of the TPR Valve Is Properly Matched to the Heating Appliance

Boiler relief valve data tag

The data tag that should be found on a boiler relief valve (photo at left) gives key information and lets the owner or inspector know if the proper type of safety device has been installed.

The maximum pressure and/or temperature that the relief valve will allow is marked on the valve's metal tag.

Compare this data with the boiler capacity which will be given on the heating boiler or water heater's data tag. Use the Input BTUh value of the heating appliance when comparing to the venting capacity of the T&) valve.

At an inspection of Vassar Temple in Poughkeepsie, NY we observed that a pressure relief valve with capacity to handle 40,000 BTUs but the heating boiler was rated for 4,000,000 BTUh!

The heating boiler was unsafe - it would normally be a simple repair to install the proper valve. The service tech had simply installed a residential-range TPR valve on a huge commercial hydronic boiler.

We told Rabbi Steve Arnold that we were worried that if the boiler exploded it would kill all of the Reform Jewish worshippers in Poughkeepsie.

He replied, no, only during the high holy days.

See RELIEF VALVE LEAKS for details.

Check for Leaking Pressure/Temperature Relief Valves

Leaky relief valve showing corrosion and debrisIf a relief valve has been leaking it is unsafe. Above on this page we showed an obsolete relief valve with leak stains down its front.

We don't know if the valve has stopped leaking because a problem has been fixed (such as something else causing boiler overpressure) or if the valve has stopped leaking simply because its internals have become clogged with mineral debris which has been left behind as hot water evaporated.

The pressure temperature relief valve shown at left was dripping, but visual inspection showed that it was clogged with mineral debris left behind as boiler water leaked out and evaporated.

The mineral debris can, as you see in this photo, obstruct movement of the spring and valve internal parts, preventing it from opening when it's needed.

At RELIEF VALVE DISCHARGE TUBE we include a catalog of methods to detect past or ongoing or current leakage at the TPR valve.

Check for Defects in the TPR Valve Drain Piping

Relief valve missing extensionThe TP valve shown in our photo at left displays the most common safety defect found with this equipment - failure to pipe the valve's potential discharge of hot water to a safe location.

The relief valve should be piped to a few inches from the floor with the end of the discharge tube always in a visible location so that if it is leaking or open the building owner or manager can observe that (unsafe) condition.

A client described finding her son and his friends in the basement playing "steam boat". They had tied a string through the little hole in the relief valve discharge lever, running the string up over a boiler pipe near the ceiling.

By pulling on the string the boys created an exciting blast of hot steamy water coming out of the boiler. Luckily none of them was scalded by this game.

But when the TP relief valve discharge extension is missing from a heating device, someone can be badly scalded.

At a home inspection in New York a real estate agent burst into tears while telling us how her son had lost an eye when he and friends played with a boiler relief valve and he was shot in the face with scalding water. A proper discharge tube could have prevented this tragedy.

See RELIEF VALVE DISCHARGE TUBE for complete details about the installation specifications & inspection defects of TPR valve drain piping.

Temperature/Pressure Relief Valve Testing Advice

Evidence of relief valve leaking

Watch out: While it is possible to "open" a boiler TP relief valve by lifting its "test" lever, unless you are a trained heating service technician or plumber, and unless you have a spare TP valve of the proper size in your hand, we advise against "testing" a TP relief valve by opening this lever during a building inspection.

And in our OPINION, even though T&P valve instructions provided by the manufacturer recommend annually testing the valve using its try-lever, that's better done by your trained heating service technician or plumber during annual service. Besides a leak problem that could require you to shut down the equipment and make you sorry you ever touched the valve, there are scalding-burn hazards.

Either ask your service technician to test the TPR valve or just confine your own TPR valve check to the following steps:

Below: a temperature and pressure relief valve that opens at 15 psig, used on a Mr. Steam MS90E series steambath generator. That installation is described at STEAM BATH / SHOWER GENERATOR INSTALLATION.

Temperature Pressure relief valve on the Mr. Steam MS90E series steam generator (C) Daniel Friedman

Automatic Gas Shutoff Valves

Watts S210-5 automatic thermostatic gas water heater shutoff valve - at

Some equipment such as some gas fired water heaters may use an automatic gas shutoff valve instead of a conventional water-dumping pressure/temperature relief valve.

An automatic gas shutoff valve will close to stop the fuel supply to a gas fired water heater if the temperature is found to be too high and thus unsafe.

WATTS Water Heater Controls - automatic gas shutoff - provides details about that automatic gas shutoff control that uses a manual reset

Details about the installation, testing, and functions of this device are at AUTOMATIC GAS WATER HEATER SHUTOFF


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