LARGER VIEW of a heating boiler cad cell relay switchGuide to Heating System Boiler Pressure & Temperature Relief Valves

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Guide to TPR valves on hot water heating boiler:

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.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

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

Relief valve missing extensionArticle Contents

This article series answers most questions about Heating System Boiler Controls on central heating systems to aid in troubleshooting, inspection, diagnosis, and repairs. Here we include a discussion of temperature and pressure relief valves used as safety devices on heating boilers and other pressurized vessels.

Lots of controls are installed on modern hot water and steam heating boilers and many of them 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.

A defective relief valve is a latent safety hazard in that the valve does not by itself cause a 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.

Obsolete boiler pressure relief valvePressure 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.

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.

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 system was unsafe - it was a simple repair to install the proper valve.

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.



List of Common Boiler Temperature/Pressure Relief Valve Defects & Hazards

Our photographs below show an older type of pressure relief valve used on hot water heating systems. Our photo at below left shows a bronze-colored pressure reducer, followed by a red pressure relief valve. That valve is unsafe because of the drain valve installed at its outlet pipe.

Our second relief valve photograph (below right, contributed by home inspector Ron Wells) shows the same combination of equipment with the pressure reducing automatic water feeder valve located to the right of the pressure relief valve.

Installed in the proper order, the pressure reducer/water feeder is installed closest to the water supply source and the pressure relief valve follows the pressure reducer/water feeder and so is installed between that device and the heating boiler.

Unsafe pressure relief valve (C) Daniel Friedman Unsafe pressure relief valve (C) Daniel Friedman

This type of relief valve responds only to water pressure and may be located at some distance from the heating boiler itself.

Watch out: this older equipment does not provide the same protection as a pressure/temperature relief valve.

Checklist of Temperature/Pressure Relief Valve Problems & Safety Hazards

Here is a list of common defects where boiler or water heater temperature/pressure relief valves are installed. All of these are unsafe conditions.

Boiler TP valve rating data (C) Daniel Friedman 2009 Relief valve discharge tube reduced or constricted (C) Daniel Friedman 2005

LARGER VIEW of a heating boiler cad cell relay switch

TP valve piped into an ell (C) Daniel Friedman 2006


Leaking or Previously Leaking Pressure/Temperature Relief Valves are Dangerous

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.

See RELIEF VALVE LEAKS for an explanation of all of the possible causes of a leaking TPR valve and thus of the suggested remedies.

Missing Relief Valve Extension Discharge Tube is a Safety Hazard

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.

Old Heating Boilers (steam or hot water) may have No Relief Valve at All - Check the Attic

Attic pressure tank for heating systems

Some very old heating boilers may not have a relief valve installed.

These systems used a pressure relieving overflow tank located high in the building, above any upper floor radiators or baseboards, often in the building attic.

We illustrate an attic heating system overflow tank in our photo at left.

Hot water inside the hydronic heating system is piped from a heating boiler located in the building below, upwards through radiators or baseboards, and connected upwards still to this attic water tank.

The attic pressure tank was open to the atmosphere and often itself included an overflow pipe which would permit any excess water (or pressure) to flow out of the tank and out of the building, perhaps through a building wall to the outdoors.

While these attic systems for boiler pressure relief safety worked well for decades, placing a temperature relief valve right on or very close to the heating boiler is a safer installation.

Heating Boiler 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. Just confine your check to the following steps:

Watch out: use the Correct TP Valve Type & Capacity

Reader Question: can I use a hot water heater relief valve on an oil fired home heating boiler?

Boiler TP valve rating data (C) Daniel Friedman 2009Can I use a hot water heater pressure relief valve on a oil fired boiler? - Aaron 9/13/12

Reply: heck no, what you propose is unsafe


NO. The operating pressure & temperature relief range on a water heater is too high to use on a domestic or residential heating boiler.

In our photo at left [click to enlarge] you can see that this heating boiler TP relief valve is rated at 30 psi (it opens if boiler pressure reaches or exceeds 30 psi) and at 535,000 BTUH (it is capable of releasing 535,000 BTUs of energy per hour) - these are matched to protect a heating boiler that normally operates at pressures below 30 psi and whose BTU input rate is below 535,000 BTUH.

A typical domestic water heater TP valve like the Watts M6 is set to open at 150psi of water pressure (five times higher than on a heating boiler) and at 210 degrees F of temperature (higher than the normal operating temperature of a heating boiler).

Watch out: Installing a water heater TP valve on a home heating boiler would be UNSAFE and risks a very catastrophic explosion should an overpressure condition occur in the boiler.

Watch out: also if you see leak stains on the pressure relief valve such as those shown in our photo. This valve is leaking around the stem that operates the TP valve test lever. This valve is unsafe and needs to be replaced.


Continue reading at RELIEF VALVE LEAKS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.





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Publisher - Daniel Friedman