Reuben's Home Inspection Blog


Why the relief valve at the water heater is leaking, and what to do about it

Leaking T&P relief valveWhen a temperature and pressure  (T&P) relief valve at a water heater leaks, it's usually a simple fix; just replace the valve.   These valves cost less than $15, and replacing the valve is a very basic job - just drain some water out of the water heater, remove the discharge tube, and replace the valve.  No big deal.

Unless it starts leaking again.

If a recently replaced T&P relief valve starts leaking again, it probably means that the relief valve is only doing it's job; it relieving excess pressure in the water heater.  When this happens, the fix gets a little bit more involved.  I'll explain.

When a water heater heats up the water in the tank, the water expands.  When this happens, the water typically ends up expanding back out the cold water inlet, all the way back to the water supply coming in to the house.  The municipal water supply for the house acts as a gigantic expansion tank... and nobody notices.  This is illustrated in the diagram below.

Natural expansion

What would happen if a one-way valve, or check valve, was installed on the water supply piping for the house?  The water wouldn't have anywhere to go.  As the water heater heats the water, it expands, which builds up pressure in what is now essentially a closed system.  When the pressure builds up enough, the T&P relief valve on the water heater just does it's job and relieves the excess pressure by leaking a little water.

Pressure regulator prevents expansion

In Minnesota, it's rare for a check valve to be installed on the water supply line for the house, but it's fairly common to have a pressure regulator installed.  When the pressure from the water supply coming in to a house is too high, a pressure regulator needs to be installed on the water main, to prevent damage to the plumbing components in the house.  The problem that these regulators can create is that they will act as a check valve; they'll allow water in to the home, but they won't allow water back out.  This creates what is called a 'closed system'.

When this happens, the T&P relief valve for the water heater can leak.  This doesn't happen every time a pressure reducing valve is installed, but there may be other problems that show up in the house, such as the toilet fill valves randomly re-filling toilets, or faucets chronically dripping.

The Fix  When a closed system exists on the water distribution piping in a home, an expansion tank needs to be installed somewhere on the plumbing system.  This is a fairly simple and straightforward fix; an expansion tank will give the water somewhere to go when it expands, and the T&P relief valve on the water heater will stop causing problems.

Expansion tank installed

This rule also applies to hot water heating systems; when a boiler heats the water in a hydronic heating system, the expansion tank allows for the water to expand without the pressure relief valve leaking.  If the pressure relief valve on a boiler system chronically leaks, even after replacement, it probably means there is a problem with the expansion tank.

Reuben Saltzman, Structure Tech Home Inspections


Comment balloon 35 commentsReuben Saltzman • January 24 2012 05:54AM


Reuben. good info on the relief valve, I had to replace mine that was just worn out from old age.

Posted by David Popoff, Realtor®,SRS, Green ~ Fairfield County, Ct (DMK Real Estate ) over 7 years ago

Very rarely do I see expansion tanks installed on water heaters. 

Posted by James Quarello, Connecticut Home Inspector (JRV Home Inspection Services, LLC) over 7 years ago

David - hopefully it didn't start leaking again, right?

James - me neither, although I do find quite a few leaking relief valves when pressure regulators are installed.  In those cases, I usually have a long paragraph in my report trying to briefly explain a 'closed system'.  I plan to start linking to this blog post from now on :)

Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) over 7 years ago

Reuben: I knew the expansion tank was important, but couldn't explain it. I have seen a lot of tanks in older homes in Bucks and Philadelphia. Thanks for a great explanation. 

Posted by Anne M. Costello (Weidel Realtors) over 7 years ago

You mean I've been heating my yard!?  Who knew hot water expands!

Actually, expansion tanks are required here now.  And it's not a surprise.  And for what they are used, they should last a good long time.

Posted by Jay Markanich, Home Inspector - servicing all Northern Virginia (Jay Markanich Real Estate Inspections, LLC) over 7 years ago

Reuben good explanation, this is a situation I could run into, and now I feel more informed.

Posted by Jennifer Fivelsdal, Mid Hudson Valley real estate connection ( JFIVE Home Realty LLC | 845-758-6842|162 Deer Run Rd Red Hook NY 12571) over 7 years ago

Thank you for the information. I will bookmark this post and share with others.

Posted by Gita Bantwal, REALTOR,ABR,CRS,SRES,GRI - Bucks County & Philadel (RE/MAX Centre Realtors) over 7 years ago

This was a great set of info, the pictures definitely helped me to get an understanding of what is going on in with your explanation.

Posted by Morgan Evans, LICENSED REAL ESTATE SALESPERSON (Douglas Elliman Real Estate) over 7 years ago

The illustration and explanation helps. Not sure how many homeowner knows about this information but certainly is useful.

Posted by Mike Yeo (3:16 team REALTY) over 7 years ago

Reuben, great explanation! Your visuals diagrammed it very well. Thanks.

Posted by Michael Setunsky, Your Commercial Real Estate Link to Northern VA over 7 years ago

Reuben, Your timing is perfect.  Our relief valve has been doing exactly what you describe on an intermittent basis and the recommended fix was an expansion tank.

Posted by Liz and Bill Spear, RE/MAX Elite Warren County OH (Cincinnati/Dayton) (RE/MAX Elite 513.520.5305 over 7 years ago

Hi Rueben,

I have found that if the T&P valve leaked and was replaced then it leaked again, then the water temp. is probably set way too high. Did you check this out?

You don't need this kind of trouble when they should never leak in the first place. But things happen.

Best, Clint McKie


Posted by Clint Mckie, Desert Sun Home, Comm. Inspection 1-575-706-5586 (Desert Sun Home, commercial Inspections) over 7 years ago

Great information again, Reuben! I can't wait to see how this is explained at my next home inspection--I'm sure your explanation is better!

Posted by Peggy Chirico, REALTOR® 860-748-8900, Hartford & Tolland County Real Estate (Prudential CT Realty) over 7 years ago

Thanks for the information Rueben. Always good to expand knowledge on main home components.

Posted by Surprise Arizona Realtor Jim Braun Sun City Grand Active Adult Communities, Surprise AZ real estate Phoenix West Valley (Coldwell Banker Residential Brokerage Jim Braun Sun City Grand Az ) over 7 years ago

Always use a qualified plumber for any and all repairs/alterations to the public water distribution system.

Clean drinking water is no accident. 


Posted by Jim Mushinsky (Centsable Inspection) over 7 years ago

I had a couple of thoughts visit me after reading this GOOD post. First, every new idea coming out has problems that need to correction or they wont work long term. Secondly, I looked around the room I was in and saw so many things that have been tweaked and corrected so that they work well but I couldn't tell you what was done. On your valve, I always wondered what would trigger that to go off. Many years ago, I was around an old 100 gallon heater that made so many strange noises and I thought it would explode any time. I was always trying to stay away from it in case it went off. Ignorance and fear....

Posted by Richie Alan Naggar, agent & author (people first...then business Ran Right Realty ) over 7 years ago

Thanks for sharing this easy to tip  regarding replacing the relief vaule on the water heater.  Will share it with my DIYers, this could help save alot of homeowners money instead of calling  for service. 

Posted by Michele Cadogan-Cell 917-861-9166, Licensed Real Estate Associate Broker - (Fillmore Real Estate 2990 Av U, Bklyn , NY 11229) over 7 years ago

I have seen more and more expansion tanks in newer housing, thanks for the information Reuben.

Posted by Chris Smith, South Simcoe, Caledon, King, Orangeville Real Esta (Re/Max Chay Realty Inc., Brokerage) over 7 years ago

Reuben - thanks for the explanation on hot water heating systems... and a perfect diagram of my hot water heating system too!

Posted by AJ Heidmann ~ CRS, YOUR Alexandria & Arlington, VA Real Estate Expert (McEnearney Associates, Inc.) over 7 years ago

Reuben -- What an education!  The diagrams are awesome as well. I know a lot more now..  Thanks for the well presented lesson.

Posted by Barbara Altieri, REALTOR-Fairfield County CT Homes/Condos For Sale (RealtyQuest/Kinard Realty Group, Fairfield and New Haven County CT Real Estate) over 7 years ago

Anne - I explained this to clients many times, but I've always wished I had some diagrams to refer to.

Jay - do you guys put pressure regulators or check valves on the water supply pipes coming in to your houses?

Jennifer, Gita, Morgan, Mike, & Michal - thanks!

Liz and Bill - did the expansion tank fix your issue?

Clint - this post isn't about any specific house or any specific inspection; just a concept.  You should be able to crank up the heat on your water heater as high as you want to without having the pressure relief valve go off.

Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) over 7 years ago

Peggy - thankfully, this isn't something I run across all that often.

Jim - thanks!

Jim - solid advice.

Richie - those valves are important, but you're right, they can go off at any time.

Michele - as long as they're qualified :)

Chris - me too.  It seems to be a standard item to add on new houses.

AJ - thank you.

Barbara - thanks.  Those diagrams took a lot longer to create than writing the post did.

Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) over 7 years ago

Reuben, Just had this exact same situation yesterday. 95% of all the home in a closed system do not have one. The new homes do because they now enforce the code that has been there for a while.

Posted by Anonymous over 7 years ago

I would like to take this opportunity to thank you and all the inspectors that regularly post blogs like this.  I AM NOT a handiman, actually I am all thumbs. I do, however, forward many of these to my son, who is fairly handy, and recently bought a home. He throughly enjoys reading them, and says he has learned alot about his house, especially since I couldn't teach him!

Thanks guys!!!!!!!!  Keep em' coming!

Posted by Woody Edwards, A Realtor® Who Answers His Phone! (First Choice Realty, Inc) over 7 years ago

Thank you so much, I have been having this problem and now I know what to ask about.  This was very informative.  Laura

Posted by Lori & Scott Mitchell, Breckenridge Colorado Real Estate (Slifer Smith & Frampton Real Estate - Breckenridge Colorado Luxury Homes) over 7 years ago

Thanks for the great educational post. Most of the leaking releif valves I've seen were caused by water hammer. Most of the the newer homes here in Idaho have an expansion tank installed on the hot side to prevent damage from water hammer.

Posted by Wayne Jackson, North Idaho Realtor, Serving Coeur dnullAlene and Hayden Lake (Lakeshore Realty 208-714-4109) over 7 years ago

Pressure regulator, yes, but no check valve.

Posted by Jay Markanich, Home Inspector - servicing all Northern Virginia (Jay Markanich Real Estate Inspections, LLC) over 7 years ago

Haven't had it installed yet, but the valve had been replaced and continued to "leak".  It's on the list of appointments to make.

Posted by Liz and Bill Spear, RE/MAX Elite Warren County OH (Cincinnati/Dayton) (RE/MAX Elite 513.520.5305 over 7 years ago

Woody - much appreciated, thanks!

Laura - glad to help.

Wayne - no kidding?  I haven't heard of water hammer causing that issue.  Interesting.

Jay - gotcha.  

Liz & Bill - I know how that goes.

Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) over 7 years ago

When this happens I call the Home Warranty company.  Pay my deductible and let them worry about it.  :)  My wife does not trust me with tools.

Posted by Gene Riemenschneider, Turning Houses into Homes (Home Point Real Estate) over 7 years ago

Thanks for a great post!  I know that pressure relief valves are pretty common out here but expansion tanks are few-and-far between.  Only rarely do I hear of a leaking pressure relief valve but now I know a little bit more of what to look out for in the future.  Thank you.

Posted by Michael J. O'Connor, Eastvale - 951-847-4883 (Diamond Ridge Realty) over 7 years ago

If it is leaking, don't ever take the easy route and cap it off like some do :




Posted by Joshua Frederick, Home Inspector in Defiance & all of Northwest Ohio (Home Inspector for ASPEC Residential Services, LLC) over 7 years ago

Hi Reuben, great post and explanation and excellent diagrams!  Thanks so much as I see this occasionally on on inspections.

Posted by Bob Miller, The Ocala Dream Team (Keller Williams Cornerstone Realty) over 7 years ago

Really, i am so glad to view this amazing & informative post. Thanks...

Posted by Relief Valves about 7 years ago

Thanks for this good & informative post....

Posted by Relief Valves about 7 years ago