 Guide to Heating System Boiler Pressure & Temperature Relief Valves
HeatAPedia ©
- Heating Boiler Pressure & Temperature Relief Safety Valves
- Attic boiler pressure control tanks
- Guide to relief valve inspection, testing, installation, repair
- Troubleshooting heating system boiler controls and switches
Our site offers impartial, unbiased advice without conflicts of interest.
We will block advertisements which we discover or readers inform us are associated with bad business practices,
false-advertising, or junk science. Our contact info is at
inspect-ny.com/appointment.htm.
This website answers most questions about Heating System Boiler Controls on central heating systems to aid in troubleshooting, inspection, diagnosis, and repairs.
Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.
© Copyright 2008 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.
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.
 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 5000 BTUs but the heating boiler was rated for 4,000 BTUH! The system was unsafe - it was a simple repair to install the proper valve.
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.
Common Boiler Temperature/Pressure Relief Valve Defects
- 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
 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
Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.
BOILERS, HEATING
BOILER CONTROLS & SWITCHES
Air Bleeder Valves
Aquastat Functions
BOILER LEAKS CORROSION STAINS
BOILER OPERATING PROBLEMS
BOILER PARTS LIST
Cad Cell Relay Switch Flame Sensors
Circulator Pumps & Relays
Expansion Tanks
Gauges on Heating Equipment
Limit Switches, Boilers
Low Water Cutoff Valves, Boilers
Mixing Valves
Pressure Gauges, Boilers
Relief Valves - TP Valves
Spill Switches
Stack Relay Switch
Zone Valves
BOILER LEAKS CORROSION STAINS
BOILER NOISE SMOKE ODORS
BOILER OPERATING PROBLEMS
BOILER OPERATION DETAILS
BOILER PARTS LIST
For details about the setting, re-setting, or function of the controls and switches commonly found on hot air heating systems
see these articles:
Technical Reviewers
Particular thanks are due to experts and also consumers who read these articles and suggest corrections, changes, and additions to
the material. Content suggestions, technical corrections and content critique are invited for any of the content at our website.
More expert information on this topic |
Building & Environmental Inspection, Testing, Diagnosis, Repair, & Problem Prevention Advice 
Directory of Professionals to Inspect or Test a Building 
Mobile View
InspectAPedia TM Home & Site Map
The Mold Information Center:
Home Inspection Construction Consulting Services
Contact Daniel Friedman