Guide to Heating System Boiler Pressure & Temperature Relief Valves
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Heating Boiler Pressure & Temperature Relief Safety Valves
Attic boiler pressure control tanks
Guide to relief valve inspection, testing, installation, repair
Photo examples of unsafe, dangerous pressure/temperature relief valve installations
Troubleshooting heating system boiler controls and switches

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This website 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. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.

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

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. 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.

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 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 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 is a Safety Hazard

The 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.

Leaking or Previously Leaking Pressure/Temperature Relief Valves are Dangerous

If 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.

Relief Valves Connected to Shutoff Valves or Piped to Hidden Locations are Unsafe

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

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.

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.

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. Here is a list of common defects where this equipment is installed:

Pressure relief valve outlet has been blocked off - such as by addition of the drain valve (closed of course) in our photo at left. This is just asking for a catastrophic, dangerous, boiler explosion.
Temperature/Pressure relief valve missing or installed too remote from the boiler
Temperature/Pressure relief valve missing its discharge tube
Temperature/Pressure relief valve leaking, corroded, needs test or probably replacement
Temperature/Pressure relief valve piped to a hidden location (making it impossible to notice that the valve is leaking and thus may be unsafe)
Temperature/Pressure relief valve has been modified - such as cutting off of the temperature sensing tip in order to install the valve at an improper location

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:

See that the boiler pressure is at or below the rated valve-opening pressure or temperature by checking the (imprecise) boiler gauge readings against the valve tag data when the boiler has heated itself up to its "shut off" point.
Inspect the Temperature/Pressure relief valve for evidence of tampering
Inspect the Temperature/Pressure relief valve for evidence of a history of leaks by observing the following
- any leak or corrosion stains around the mouth of the valve
- any drip stains on the floor below the valve discharge tube (photo at left)
- Using your finger, feel the inside of the tip of the discharge tube and check for water - it should be dry
- Check for leaks around the valve where it is mounted on the boiler or boiler piping
Check that the Temperature/Pressure relief valve has a discharge tube properly installed and that the discharge is not blocked by anything whatsoever
Check that the Temperature/Pressure relief valve data tag is in place

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BOILERS, HEATING
  BACKDRAFTING HEATING EQUIPMENT
  BACKFLOW PREVENTER VALVE, HEATING SYS
  BANGING HEATING PIPES RADIATORS
  BOILER COMPONENTS & PARTS
  BOILER CONTROLS & SWITCHES
  Air Bleeder Valves
  Air-bound boilers & radiators
  Air Scoops Purgers Separators
  AQUASTAT CONTROL Functions
  Boiler Pressure Controls & Settings
  CAD CELL RELAY SWITCH
  CHECK VALVES, HEATING SYSTEM
  Chemical treatments, Boiler
  Circulator Pumps & Relays
  Draft Regulators, Dampers
  ELECTRIC MOTOR OVERLOAD RESET SWITCH
  ELECTRICAL POWER SWITCH FOR HEAT
  Expansion Tanks
  FIRE SAFETY CONTROLS
  Gauges on Heating Equipment
  Limit Switches, Boilers
  Low Water Cutoff Valves, Boilers
  MIXING / ANTI-SCALD VALVES
  OIL LINE QUICK STOP VALVES
  OIL LINE SAFETY VALVES
  Pressure Gauges, Boilers
  Pressure & Temperature Settings, Controls
  RELIEF VALVES - TP VALVES
  Spill Switches
  Stack Relay Switch
  Zone Valves
  BOILER LEAKS CORROSION STAINS
  BOILER LEAKS, HOW TO LOCATE
  BOILER NOISE SMOKE ODORS
  BOILER OPERATING PROBLEMS
  BOILER OPERATION DETAILS

Thanks to Henry Torres (Boiler Inspector), for calling to our attention an error in our BTU rating statement about relief valves.
Thanks to Ron Wells, Wells Inspection Services, Inc., Chapel Hill, NC 27515, Tel: 919-968-4981, Cell: 919-302-2652 License Number: 75 - Mr. Wells is a professional home inspector and a member of ASHI, the American Society of Home Inspectors. He can be reached at Ron@weelsinspections.com

For details about the setting, re-setting, or function of the controls and switches commonly found on hot air heating systems see these articles:

Hot Air Heating Furnace Basic Operating Steps
Guide to Heating System Components:
Air Bleeder Valves for Hot Water Heating Systems: Radiators, Baseboards, Convectors
Aquastats Heating Boiler Primary Controls: A guide to Aquastats for control of hot water heating boilers and tankless coils
BLUE vs YELLOW COMBUSTION FLAMES - the Blueray history and blue flame vs. yellow flame combustion, flame color & combustion efficiency, & how oil competes with gas as a heating fuel.
CAD CELL RELAY SWITCH on oil fired furnaces or boilers as flame sensors & safety devices
Check Valves Guide to check valves and backflow preventers on hydronic (hot water) heating systems (gas or oil fired)
Draft Hoods on gas fired heating equipment, function and safety
Draft Regulators & barometric dampers on oil fired heating equipment
Expansion Tanks Guide to pressure control and expansion tanks on hydronic (hot water) heating systems (gas or oil fired)
Fan Limit Switch on hot air furnace heating systems (gas or oil fired)
Flue Gas Spill Switch on gas fired heating equipment sense combustion and protect from blocked flues
Gas Piping, Valves, Controls including gas regulator assembly and other controls
Heat Won't Turn Off - Stop Unwanted Heat explanation of why heat may continue to come out of radiators or baseboards even though you have turned down the thermostat
MIXING / ANTI-SCALD VALVES: Guide to mixing valves on hot water heating systems and radiant heat systems
Power Switches to turn on or off heaters for furnaces & boilers, oil & gas fired, heat pumps or electric furnaces or boilers
RELIEF VALVES - TP VALVES: A guide to temperature and pressure relief valves & safety controls on hot water and steam heating systems
Stack Relay Switches Explained on oil fired furnaces or boilers as flame sensors & safety devices
Thermostats & Heat Controls for furnaces & boilers, oil & gas fired, heat pumps or electric furnaces or boilers
Water Feed Valves: A guide to water-feeding/pressure-reducing valves on hot water and steam heating systems
Zone Valves: A guide to zone valves for heating zone control on hot water heating systems

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair

Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.

The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 2010, $69.00 U.S., is available from Carson Dunlop. The Home Reference Book is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. InspectAPedia.com ^® author/editor Daniel Friedman is a contributing author. Field inspection worksheets are included at the back of the volume.

Carbon Monoxide Gas Toxicity, exposure limits, poisoning symptoms, and inspecting buildings for CO hazards
DUST CONTAMINATION FROM HVAC? An Investigation of Indoor Dust Debris Blamed on a Heating/Cooling System Reveals Carpet Dust
Fuel Oil & Oil Heating Magazine, 3621 Hill Rd., Parsippany, NJ 07054, 973-331-9545
Goodman Furnace High Temperature Plastic Vent HTPV safety recall US CPSC notice
Home Heating System Should Be Checked [for proper venting and for CO Carbon Monoxide Hazards - DJF]
Inspection Procedures for Oil-Fired Heating Systems Detailed step by step approaches for inspecting complex systems]
Lennox Pulse Furnace Safety Inspection/Warranty Program: Carbon Monoxide Warning
Oil Tanks - The Oil Storage Tank Information Website: Buried or Above Ground Oil Tank Inspection, Testing, Cleanup, Abandonment of Oil Tanks
Oil Tanks Above Ground, UL Standards, guidance for home owners, buyers, and inspectors
Plastic Heating Vent Pipe & Other Heating Safety Recall Notices
Weil McLain Model GV Gas Boiler/gas valve CPSC recall/repair
Domestic and Commercial Oil Burners, Charles H. Burkhardt, McGraw Hill Book Company, New York 3rd Ed 1969.
National Fuel Gas Code (Z223.1) $16.00 and National Fuel Gas Code Handbook (Z223.2) $47.00 American Gas Association (A.G.A.), 1515 Wilson Boulevard, Arlington, VA 22209 also available from National Fire Protection Association, Batterymarch Park, Quincy, MA 02269. Fundamentals of Gas Appliance Venting and Ventilation, 1985, American Gas Association Laboratories, Engineering Services Department. American Gas Association, 1515 Wilson Boulevard, Arlington, VA 22209. Catalog #XHO585. Reprinted 1989.
The Steam Book, 1984, Training and Education Department, Fluid Handling Division, ITT [probably out of print, possibly available from several home inspection supply companies] Fuel Oil and Oil Heat Magazine, October 1990, offers an update,
Principles of Steam Heating, $13.25 includes postage. Fuel oil & Oil Heat Magazine, 389 Passaic Ave., Fairfield, NJ 07004.
The Lost Art of Steam Heating, Dan Holohan, 516-579-3046 FAX
Principles of Steam Heating, Dan Holohan, technical editor of Fuel Oil and Oil Heat magazine, 389 Passaic Ave., Fairfield, NJ 07004 ($12.+1.25 postage/handling).
"Residential Steam Heating Systems", Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
"Residential Hydronic (circulating hot water) Heating Systems", Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
"Warm Air Heating Systems". Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
Heating, Ventilating, and Air Conditioning Volume I, Heating Fundamentals,
Boilers, Boiler Conversions, James E. Brumbaugh, ISBN 0-672-23389-4 (v. 1) Volume II, Oil, Gas, and Coal Burners, Controls, Ducts, Piping, Valves, James E. Brumbaugh, ISBN 0-672-23390-7 (v. 2) Volume III, Radiant Heating, Water Heaters, Ventilation, Air Conditioning, Heat Pumps, Air Cleaners, James E. Brumbaugh, ISBN 0-672-23383-5 (v. 3) or ISBN 0-672-23380-0 (set) Special Sales Director, Macmillan Publishing Co., 866 Third Ave., New York, NY 10022. Macmillan Publishing Co., NY
Installation Guide for Residential Hydronic Heating Systems
Installation Guide #200, The Hydronics Institute, 35 Russo Place, Berkeley Heights, NJ 07922
The ABC's of Retention Head Oil Burners, National Association of Oil Heat Service Managers, TM 115, National Old Timers' Association of the Energy Industry, PO Box 168, Mineola, NY 11501. (Excellent tips on spotting problems on oil-fired heating equipment. Booklet.)
Links to our list of additional information on heating system inspection, repair, maintenance
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Guide to Heating System Boiler Pressure & Temperature Relief ValvesInspectAPedia® -

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

Missing Relief Valve Extension is a Safety Hazard

Leaking or Previously Leaking Pressure/Temperature Relief Valves are Dangerous

Relief Valves Connected to Shutoff Valves or Piped to Hidden Locations are Unsafe

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

Questions & Answers regarding this article

Ask a Question or Search InspectAPedia

Technical Reviewers & References

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair

Guide to Heating System Boiler Pressure & Temperature Relief Valves
InspectAPedia^® -