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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.
Green links show where you are. © Copyright 2014 InspectApedia.com, All Rights Reserved.
A Guide to Heating Boiler Temperature & Pressure Relief Valves, Inspection, Defects, Testing, Repair
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. Readers should also see BOILER PRESSURE & TEMPERATURE SETTINGS and BOILER OPERATING PROBLEMS.
At RELIEF VALVES - Water Heaters we discuss temperature and pressure relief valves used on residential water heaters. Pressure relief valves (that sense pressure only, not temperature) are also required on pressurized tanks such as water tanks in buildings.
Building 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 tank or heating boiler.
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.
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.
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.
Missing Relief Valve Extension Discharge Tube is a Safety Hazard
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.
Leaking or Previously Leaking Pressure/Temperature Relief Valves are Dangerous
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.
Unsafe TP Relief Valves: Connected to Shutoff Valves or Piped to Hidden Locations
Only a complete fool would do what we found on this boiler. To "stop" an annoying boiler drip at the pressure temperature relief valve, the mechanic installed a short length of pipe capped by a drain valve which he could simply shut.
This might have been installed on a system for other reasons, such as connecting a hose to permit easy draining of pressure off of the boiler through the TP valve.
But it is in all events dangerous, illegal, and plain stupid to ever install a shutoff valve or any other sort of "cap" on a pressure/temperature relief valve.
Old Heating Boilers (steam or hot water) may have No Relief Valve at All - Check the Attic
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.
List of Common Boiler Temperature/Pressure Relief Valve Defects
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.
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.
Temperature/Pressure Relief Valve Testing Advice
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:
Continue reading at BLEVE EXPLOSIONS
or at BOILER CONTROLS & SWITCHES or select a topic from the More Reading links shown below.
Green link shows where you are in this article series.
Frequently Asked Questions (FAQs) About Heating Boiler Temperature & Pressure Relief Valves: installation, inspection, maintenance, repair or replacement
Question: does the relief valve discharge tube have to be of copper? Is PVC ok?
Thanks for the information. Is there any requirement that the fitting and pipe extension is required to be made from copper? I came across a boiler where the fitting off the T&P and the extension to the floor is made of PVC. - Kevin 7/26/2011
Kevin the relief valve would itself tap into a metal fitting on the boiler or on older installations on metal heating piping. But the discharge tube on many new installations is plastic; it's a pipe that rarely sees service and whose job is to divert hot water to the floor rather than onto a bystander. In that application most code officials accept PVC.
Question water is shooting out of a valve at our well head.
Water is shooting out a valve on the well head in the front yard. What might be the problem - Sue 8/21/11
Sue: regarding "Water is shooting out a valve on the well head in the front yard. What might be the problem"
Question: Pressure keeps going up in my hot water boiler - how do I diagnose this?
Hot water furnace. My gauge goes up to 32 pounds then trips. Shut of furnace and still happens. My hot water tank is heated by furnace. I shut off valves on lines going to my hot water tank and problem stops. Should I suspect internal leak in hot water heating core allowing internal pressure in tank to leak back to furnace causing pressure to rise. - Will 8/22/11
Question: water heater pressure goes up too high and the TP valve opens
I have a water heating system with radiators and no tank. My pressure goes to over 40 psi and the safety valve opens. I touched the pipe going to the expansion tank and it's cold so it looks like no water goes in there when it should. If I shut off the water supply line and drain some water the pressure reduces, however after draining it seems as the amount of water is not enough to go all the way up to the 2nd floor radiator. I checked it by opening the bleed valve and nothing comes out since there is not enough water to fill it all the way up. It heats great when the supply line is open but the pressure goes way over normal. Based on the scenario below I am suspecting a bad feeder/pressure reducer valve. Do you think this might be causing the problem. Any info would be greatly appreciated. Thank you. - May0 12/4/11
I ran a test for the feeder/pressure reducer valve and it's fine. It shuts off the water exactly at 12 psi. Once the systems turns on and starts heating the water, the pressure keeps rising way over 30 psi. I watched it and it went over 40 psi when the water temperature was 150 F. I really don't know what might be causing it. I looked at the boiler unit and I can't see any signs of leaks. Shouldn't the expansion tank take care of this? Thinking logically the water should escape there and therefore reduce the pressure in the system. I don't think any water goes in that tank. I shut off the valve on the pipe going to the expansion tank. I drained it - there was hardly any water in there. I opened the valve and I was touching the pipe to see if it will get hot once the water goes in there. All this was done while the pressure was over 40 psi. The pipe didn't get hot making me think no water went in there. After that I opened the drain valve on the tank and the water started coming out reducing the pressure immediately. What could be causing the pressure to rise so high? Is the expansion tank's job to at least partially help regulate the pressure in the system? Thanks for your advise.
Question: the pressure valve leaks when my boiler runs for a long time
I have a 20 year old SlantFin cast iron oil boiler. I have had 3 different technicians clean and set it up and I still have the same problem.
The pressure valve leaks pressure/water when the boiler runs for a long time. When just using hot water or if only one zone calls for heat, it is fine. However, if multiple zones call for heat such as first thing in the morning when the programmable thermostats turn up, the pressure rises to about 30 PSI after about 10-15 minutes which is enough to make the valve open and trickle water. The colder it is in the house the bigger the puddle. Any help or suggestions would be appreciated. - Tom Dionne 3/28/12
Dionne, check the temperature on your boiler when the TP valve is leaking - if it is too high that could be the problem - fixed by adjusting or repairing the aquastat control. The diagnostic procedure you need is at RELIEF VALVE LEAKS.Also see WATER PRESSURE TOO HIGH: DANGERS for other causes and effects of of abnormal water pressure.
Question: loud whooshing noise after we take a shower
After showers we hear a loud "ccssshhh" noise which sounds like it may be from the hot water tank. What could this be. It happens a couple of times after shower - Julia 8/21/12
Julia, have someone standing at the water heater and other water handling equipment in the building so that first we know where the noise is coming from. That's the first step in diagnosing this problem. Otherwise an answer here would be just too-wild speculation.
Question: can I use a hot water heater relief valve on an oil fired home heating boiler?
Reply: hell no, what you propose is unsafe
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.
Question: I keep having to change my pressure relief valve due to leakage
I regularly have to have my pressure relief valve changed after a year or so because it starts to leak. My oil company says that happens with anti-freeze. Is there a special one to use with anti-freeze? FYI: I am on city water but do not have a pressure relief valve. - Mike 10/22/12
Balderdash. Either you're getting a series of defective TP valves installed (unlikely) or there is a cause of abnormal temperature or pressure at your heating boiler that needs to be properly diagnosed and fixed. The diagnostic procedure you need is at RELIEF VALVE LEAKS.
Question: my heating boiler shuts off at a temperature below the setting
My boiler is set at 190 degrees, but always shuts off at 170. Any ideas? - Mark 10/26/12
Mark, your aquastat may be improperly installed (lacks temperature sensing grease or has bad contact between the aquastat temperature sensor and the sensor well sides in the boiler) or the device may be defective, or your temperature gauge may be off. See AQUASTAT CONTROL for details.
Question: what kind of pressure relief valve do I need on my water heater?
hi i have water heater 40000 btu natural gas what kind of pressure relief valve i need ? thank you - Paul 12/11/12
Paul, the installation instructions for your water heater will tell you exactly what temperature/pressure valve is required for your unit. The specifications will include:
Watch out: installing a TP valve that does not meet the water heater's specifications is dangerous and risks an explosion
Question: shouldn't I follow the manufacturer's instructions and test my boiler's TP valve every year?
I like this set of sensible and logical inspection steps. However, the Watts 374A 30PSI pressure relief valve clearly states on its paper owners/users tag atop the metal specifications tag that the homeowner should activate it yearly to ensure proper function. I'd rather do what you are suggesting because it avoids the possibility of trapping any boiler water sediment/foreign material in the valve and preventing it from seating fully. But, wouldn't further 'flushing' of valve with more water usually rectify this? Like with Schrader valve with air in inner tube of bicycle tire or for auto tubeless tire valves? - Christopher 1/9/2013
Reply: better to ask your HVAC service tech to test the TP valve
Thanks for the good question Christopher.
Normally we always want to follow the product manufacturer's directions - they have a great interest in safe proper use of their equipment. And I agree that sometimes you can stop a leaky TP valve by repeated flushing of the valve to get debris off the valve seat. But I also have opened a TP valve to flush it and had the valve spit out part of the valve seat washer - leaving me with gushing hot boiler water at scalding temperatures.
The advice to "test" the TP valve by opening its test lever once a year is "safe" advice for the manufacturer to give. But it's not so safe for the homeowner because in my OPINION:
Therefore a safer suggestion for all homeowners is to ask their heating service tech to check the TP valve at the time of annual service. The service tech has a replacement TP valve right with him or her, in the service truck, and s/he knows how to replace the valve it if keeps on leaking.
Question - Comment: relief valve drain froze leading to an explosion at the heating boiler
One danger (I may have missed) happened to me. The T & P valve had a slow leak which drained outside building. At a persistent 4 degrees above 0 F, the water drip outside eventually froze and sealed the drain pipe at the exit point. Once sealed the pressure built until the T & P valve exploded and the final six week repair cost came in at just over $36,000. T & P valves are a real hazard to home owners! - B.T. 1/27/2013
Excellent point; We've talked about the issues of draining to a remote or unobserved location, but not about the freeze risk. Thank goodness no one was killed.
Actually you are describing a triple fault that led to an explosion:
Had your boiler had a different weakest-link than the TP valve itself, the whole boiler could have exploded, causing still more damage and risking injury or even death to building occupants. For example, had your device been draining a water heater the unit would become a rocket, shooting up through the building - details are at BLEVE EXPLOSIONS.
Questions & answers or comments about pressure or pressure & temperature safety relief valves: inspection, diagnosis, installation requirements, relief valve testing, and relief valve repair procedures. .
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