Question? Just ask us!
Free Encyclopedia of Building & Environmental Inspection, Testing, Diagnosis, Repair
InspectAPedia ® Home
PLUMBING SYSTEM INSPECT DIAGNOSE REPAIR
AGE of PLUMBING MATERIALS & FIXTURES
AIR DISCHARGE at FAUCETS, FIXTURES
ANTI SCALD VALVES
ANODES & DIP TUBES on WATER HEATERS
BACKUP PREVENTION, SEPTIC
BACKUP PREVENTION, SEWER LINE
BACKWATER VALVES, SEWER LINE
BATH & KITCHEN DESIGN GUIDE
CHEMICAL CONTAMINANTS in WATER
CHEMICAL ODOR SOURCES
CHLORINE IN DRINKING WATER
CLOGGED DRAIN DIAGNOSIS & REPAIR
DEBRIS in WATER SUPPLY, Water Heater
DEPTH of SEPTIC TANK
DRAIN & SEWER PIPING
FAUCETS & CONTROLS, KITCHEN & BATH
FAUCETS, OUTDOOR HOSE BIBBS
FLOOD DAMAGE ASSESSMENT, SAFETY & CLEANUP
FLOOR DRAIN / TRAP ODORS
FLUSHOMETER VALVES for TOILETS URINALS
GAS PIPING, VALVES, CONTROLS
GALVANIC SCALE & METAL CORROSION
HARD WATER - SOFTENERS
HEAT TAPES, Heat, Insulation prevent Freeze-Up
LEAD POISONING HAZARDS GUIDE
LEAD IN DRINKING WATER, HOW to REDUCE
METHANE GAS SOURCES
MIXING / ANTI-SCALD VALVES
MUNICIPAL WATER PRESSURE IMPROVEMENTS
NOISE / SOUND DIAGNOSIS & CURE
ODORS GASES SMELLS, DIAGNOSIS & CURE
ODORS IN WATER
ODORS, SEPTIC or SEWER
ODORS SEWER GAS in COLD WEATHER
ODORS, SULPHUR SMELL SOURCES
ANIMAL or URINE ODOR SOURCE DETECTION
PIPING IN BUILDINGS, Clogs Leaks Types
PLUMBING FIXTURES, KITCHEN, BATH
PLUMBING NOISE TRANSMISSION CONTROL
PLUMBING VENT DEFINITIONS & CODES
PLUMBING VENT DEFECTS & NOISES
PUMPS USED in BUILDINGS
PUMPS, WATER REPAIR
RELIEF VALVE LEAKS
RELIEF VALVES - TP Valves on Boilers
RELIEF VALVES - STEAM TP VALVES
RELIEF VALVES - Water Heaters
RELIEF VALVES - Water Tanks
REPAIR BURST LEAKY PIPES
METHANE GAS HAZARDS
SEPTIC SYSTEM INSPECT DIAGNOSE REPAIR
SHUTOFF VALVE LOCATION, USE
SULPHUR & SEWER GAS SMELL SOURCES
SWEATING (CONDENSATION) on PIPES, TANKS
TOILETS, INSPECT, INSTALL, REPAIR
WATER, WELLS, WATER TANKS: TESTING GUIDE
WATER PRESSURE LOSS DIAGNOSIS & REPAIR
WATER PUMPS & TANKS
WATER SOFTENERS & CONDITIONERS
WATER SOURCE ALTERNATIVES
WATER SUPPLY & DRAIN PIPING
WATER SHUTOFF VALVE LOCATION, USE
WATER SHUTOFF VALVE, WELL PUMP
WATER TESTS, CONTAMINANTS, TREATMENT
WELLS CISTERNS & SPRINGS
WINTERIZE A BUILDING
This article describes the causes of leaks, drips, or discharges from pressure relief valves, temperature/pressure relief valves, or TP valves found on heating boilers, water heaters, or the simpler pressure relief valves found on water pressure tanks. We list the wide variety of possible TP Valve leaks and how to find and fix each of those problems.
Green links show where you are. © Copyright 2014 InspectApedia.com, All Rights Reserved.
Causes of leaky dripping Pressure/Temperature Relief Valves
Temperature & Pressure Relief valves may open, leak, or drip for a variety of reasons including:
Safety Hazard Warnings About Dripping or Leaking Pressure Relief Valves
As our photo shows (above left), mineral salts left behind as hot water evaporates from the mouth of a pressure or temperature relief valve can completely clog the spring that is intended to allow the relief valve to open under excess (unsafe) pressure.
The impaction of the relief valve spring with copper and calcium salts in this photo means that the valve is almost certainly not going to open should the heater's internal pressure become unsafe.
The drip shown at the mouth of this relief valve demonstrates that a relief valve can drip and leak for a long time without anyone observing this dangerous condition.
This relief valve needs to be replaced immediately and the cause for the valve's leakage also needs to be determined.
Accidental or deliberately plugged Temperature & Pressure Relief Valves
Watch out: Never remove nor plug a pressure/temperature relief valve. This [water heater pressure/temperature relief] valve must be marked with the maximum set pressure not to exceed the marked maximum working pressure of the water heater. Install the valve into an opening provided and marked for this purpose in the water heater, and orient it or provide tubing so that any discharge from the valve exits only within 6 inches above, or at any distance below, the structural floor, and does not contact any live electrical part. The discharge opening must not be blocked or reduced in size under any circumstance.  
In explaining why the relief valve on a water heater may be dripping, American Water Heater Co.  explains:
Periodic discharge of the temperature and pressure relief valve may be due to thermal expansion in a closed water supply system. The water utility supply meter may contain a check valve, backflow preventer or water pressure reducing valve. This will create a closed water system.
During the heating cycle of the water heater, the water expands causing pressure inside the water heater to increase. This may cause the temperature and pressure relief valve to discharge small quantities of hot water. To prevent this from happening, there are two recommendations:
Watch out: get advice from a licensed plumber and your local water supplier. Never remove or plug a leaky pressure/temperature relief valve on a water heater - doing so risks dangerous or even fatal BLEVE EXPLOSIONS.
Also see the FAQs below where we discuss closed plumbing systems caused by check valves on the cold water line.
Readers of this article should see RELIEF VALVES - TP VALVES where we include additional relief valve information including for hydronic heating and steam heating boilers used for central heating.
And see WATER HEATER SAFETY for our complete list of water heater safety devices and water heater safety inspection advice.
If the building water pressure gauge reading is ever found at 80 psi or higher, you will want to install a water pressure regulator at the point where water supply enters the building.
If your building already has a water pressure gauge installed, it may be defective or it may be set too high. The articles listed just below discuss how to adjust a water pressure regulator:
Expansion tanks to relieve high water pressure
An alternative to installing or changing a water pressure regulator when building water pressure is occasionally 80 psi or higher is the installation of an expansion tank to temporarily absorb that pressure increase. Proper use of an expansion tank can help avoid unnecessary opening of the pressure/temperature relief valve on a hot water heating tank or a hot water heating boiler.
Watts Regulator Co. offers this explanation of T&P leak problems that may be encountered on gas fired sidearm heaters. 
Question: What would cause an automatic T&P valve to open and close repeatedly when there is very little hot water in the storage tank?
Reply: troubles at the gas sidearm heater
Continue reading at THERMAL EXPANSION of HOT WATER or select a topic from the More Reading links shown below.
Suggested citation for this web page
Green link shows where you are in this article series.
Frequently Asked Questions (FAQs)
Question: no dischartge tube on TP valve, steam coming out; Wrong TP valve rating?
(Oct 8, 2012) Filipe Bernardino said:
I have a relief valve right on top of my steam boiler, it dosen't have a discharge outlet but it has openings right on top of it where i see some, if not too much steam coming out, it should be replaced or it is normal?
(Oct 8, 2012) raul said:
if the installation for my boiler said use 50psi and the tech put in 30psi relief valve. do you think thats the reason it keeps leaking.
Possibly. But it's easy to check. Compare the actual boiler pressure when leaks are seen with the valve rating. Normally a hydronic heating boiler doesn't run over 30 psi
Fillip, the valve should not be leaking, but before replacing it you need to know why it's leaky, as the problem may not be the valve. Call your service company as thisisva safety concern
Watch out: a discharge tube is required on TP valves to avoid scalding burn hazards.
Question: replaced defective pressure relief valve - not tight enough?
(Apr 21, 2014) Jeff said:
I put in a new water heater and when it came the pressure relief valve was bent and defective,the place I bought it gave new pressure relief valve I put it in but did not tighten it as far as it could go in because than I would have a problem of putting the line into pressure relief valve.I can turn it by hand but with line in you can not turn it.It does not leak and valve works.will this be a problem down the road?
Jeff, I am nervous about promising the safety of such a critical device in a DIY installation that I cannot see.
If the valve is easily rotated by someone bumping the TP valve extension tube it's too loose. Or, as up surmise, it's too loose if it leaks.
But I understand very well the problem of where you end up when rotating the TP valve into place on a heating appliance, as we for sure want the valve body facing down.
You have two options:
1. check the valve several times over the next week with the heating system at full operating temperature and pressure - looking for leaks and check again at periodic service or if you see mineral salts build-up around the valve or water on the floor or boiler side.
2. remove the valve and us additional turns of teflon tape so that the valve is adequately snug while pointing down properly.
Question: how to diagnose dripping from the pressure relief valve
(June 1, 2014) Frank 06/01/2014 said:
I have a 31-yeay oil boiler (tankless water heater), which had small water dripping from the pressure relief. The pressure and temperature was 20 psi and 180 degree. The boiler does not ignite frequently now because of the beginning of summer season. Water dripping from the pressure relief significantly increased when the boiler was tested to heat the house, and the pressure reading went up quickly to 30 psi. The water released from the pressure relief contained a lot of yellow residue.
Greatly appreciate if you have any thoughts.
If the TP valve is leaking and the boiler temperature is BELOW the 200degF limit I suspect either overpressure in the boiler or a defective valve.
The fact that leaks increase when the boiler is heated up is consistent with either of those conditions.
Pressures close to or over 30 PSI will spill the valve.
If your boiler has a tankless coil the coil could be leaking water into the boiler raising its pressure.
With the boiler OFF and cooled down to room temperature, remove water to drop pressure to 12 psi (or slightly higher if your home is more than 3 floors tall).
Leave the boiler off and watch for a pressure incrase - that'd indicate a leak into the boiler or a faulty water feed valve.
8/17/14 Donald Fletcher said:
The high low limit device on my Weil Mclain natural gas boiler failed. Temp went well over 200F. This set the boiler relief valve off. I replaced the high/low device and the well the copper sensor end fits in. There was no scale on the old well or the sensor end of the high/low that was replaced. There was just a black film coating, which was easily wiped off. This new high/low device seems to work but it is not accurate. In order to get the boiler to shut off at 180F the setting on the high/low must be set to 160F.
In Aug 2013 TP valve activation was a real problem which led to new TP, Expan tank, auto water feeder valve, back flow valve and as mentioned above the last item the high/low limit replaced Oct 2013.
All was from OCT 2013 till Aug 13 2014 and then the TP was activated for reasons unknown other than the pressure gauge read 30lbs which is the trigger point for the TP. Expan tank checked out at 12 PSI. I do have a street fill valve and this valve was closed since OCT 2013 and boiler pressure was checked every month or so and was a constant 14/17 PSI. There is a Bock indirect water heater that I thought could have a coil pin hole that opens when it has a mind to, allowing potable street water into the boiler through the coil. System is up a running now for the last several days and the pressure is at a steady 17psi. Hot or cold the pressure is always the same 17psi. I have verified the pressure with a separate 0-30psi gauge.
So do you have any thoughts on the matter that can I apply. Thanks
Boch indirect and boiler 19 yrs old
For temperature sensing errors on typical modern aquastats and limit switches:
Do you think the problem could relate to poor thermal contact between the Aquastat's sensor probe and the surface of the well into which it is inserted?
Did you use the manufacturer's recommended thermal conductive grease? (Some heating techs don't)
Thermal grease is recommended by limit switch manufactures for units whose sensor probe fits into an immersion well in a heating boiler or water heater. Absence of good contact between the sensor tip and the sensor well sides can explain off-spec temperature sensing as well as (I think) odd burner on-off cycling.
Dan, I was not aware of the thermal grease. Did not see it mentioned in the directions. It would seem the grease could help get the high limit set at 180F and a actual boiler flame off at 180F. However it is worth noting that the boiler does flame back on at 140F. I would gladly pull the probe and use thermal grease in an attempt set things to right. Well worth trying. :) - Don Fletcher
For a limit control sensor to work properly when inserted into the immersion well on a boiler, it must be in contact with the inner surface of the immersion well; the grease assures good thermal contact. The limit controls I've bought and installed generally came with a little tube of the special grease. Some time ago some heating techs avoided using the grease claiming it solidified and made later service difficult. That problem was corrected. In sum , the control should be installed according to the manufacturer's specs. Check with the company who made your unit. Let me know if youre' told something different. The thermal grease was commonly provided with older Honeywell aquastats and is also sold at plumbing and heating suppliers - IF your unit's manufacturer specifies its use (not all do) you'll want to add it.
Well, I it's a Honeywell L4080D1226 and I called them today and was told heat conductive compound is not supplied with the limit assembly. There is no mention of the grease in the Weil McLain parts list. So I took the sensor body out of the well and tilted the sensor in an effort to get good metal to metal contact. But, the results changed nothing. I did not call Weil Mclain yet but I will. I will also get some compound to put in there as well. The well is not a snug fit to the sensor, that is to say there is air space. So there is room for thermal compound.
This morning with everyone taking showers I took pressure readings while the boiler was fired up. What was 17psi cold or hot has become 17psi cold & 21psi hot. Higher than needed but OK I think if it stays that way. Pressure release valve is not dripping.
Well the boiler is at 100F and I just checked the pressure. It is 21psi. That makes what I said earlier (17psi cold 21psi hot) very wrong. The street fill valve is closed and as far as I know the indirect water heater is the only other place the extra pressure can be coming from. Although it would seem it is doing so in short intervals.
Reply: other sources of abnormal boiler or water heater tank (cylinder) pressrure
If your boiler or your hot water system includes a tankless coil or any equivalent (a coil containing house water (to be heated) immersed in the vessel whose TP valve is leaking, shut OFF water entering that coil - to see if house pressure in the coil is sending water through a coil leak into the boiler.
Coil contains circulated boiler water. Hose water at street pressure may be leaking into the coil by way of a pin hole in the coil. So we are on the same page so to speak.
I have shut of boiler supply to the coil numerous times. But it is not easy to prove out the problem because it happen very intermittently. I will monitor it overnight and see the PSI in the early morning. If it's like past history it will still be 21 lbs come morning. If I wait it out it could weeks or months before the hole will open up and cause it to happen again. There are no leaks from any pressure relief valves at this time. The Bock indirect water heater TP has never leaked. Thanks for helping me with Dan, I really appreciate it.
Usually house water pressure is higher than boiler water pressure - but not always.
If the boiler is COLD its internal pressure may be just 12 psi. In that case house water pressure inside the indirect water heater tank, sitting at say 30 psi to 60 psi, might indeed leak INTO the coil through which boiler water is circulating.
You should see that as a pressure increase on the boiler pressure gauge as long as you've left the system off long enough to have a boiler-cold-before and boiler-cold-after pressure reading.
The case you describe, house water pressure leaking INTO the heating coil inside of an indirect-fired water heater is similar to the tankless coil leak case I describe in the article linked below.
Keep us posted.
Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.
Search the InspectApedia website
HTML Comment Box is loading comments...
Technical Reviewers & References