Water Heaters and Thermal Expansion: Does Your Water Heater Need A Thermal Expansion Tank?

Water Heaters: Friend or Foe?

Your Water Heater is one of the most expensive and important parts of your plumbing system. Without a water heater life just wouldn’t be the same. Try to imagine cold showers every day! But did you know that your water heater could be causing a problem in your pipes?

Hot Water Expands

Hot water heaters do their job by using either an electric heating element or a gas burner to heat cold water and make it hot. The problem is that water expands when it is heated. This is called “Thermal Expansion”.

A 40 Gallon Water Heater Can Cause An Extra Half Gallon

Due to Thermal Expansion, a regular 40 gallon hot water heater creates about an extra half gallon of water in the pipes while heating water from 70 degrees to 120 degrees. If the extra water doesn’t have anywhere to go it causes the pressure in the pipes to go up, sometimes way up.

Water Heater Plus Closed System Equals A Problem

If you have a “Closed System”, meaning there is a check valve or “Backflow Preventer” on the incoming water line, the extra water volume caused by thermal expansion has nowhere to go. This means that the pressure in your pipes can get too high, possibly causing problems.

Too Much Pressure Is Bad

If your pressure gets too high because of thermal expansion it can cause several problems. One of the main issues is that your T and P Relief Valve on the water heater can start to drip or run. This valve is designed to release pressure if it gets up to 150 psi. The problem is that it’s really a safety valve and isn’t designed to open and close a lot. Too much could cause it to fail.

Thermal Expansion Can Cause Running Toilets

Another symptom of thermal expansion problems is a toilet that keeps running or kicking on from time to time. This is caused by the excess pressure overcoming the float on the fill valve, allowing water to run into the tank. Over time this can cause the fill valve to stop working and make it run constantly.

A Thermal Expansion Tank At The Water Heater Will Solve These Problems

If you are experiencing either of these problems, or if you are just replacing your water heater, it is a good idea to install a Thermal Expansion Tank. This is a small tank with an air bladder inside that is installed in the cold water pipe near the water heater. When the water heater comes on and starts heating, the extra water that is created is forced into the tank and compresses the air bladder. As the water cools, or a faucet is opened, the bladder pushes the extra water back out of the tank,

A Fairly Simple Job

Installing a Thermal Expansion Tank at your water heater is a fairly simple job. The first thing you need to do is to make sure the tank is pressurized to the same pressure as your normal system pressure.  This will usually be about 60 psi.  There should be a “schrader” type valve (just like a tire has) on the opposite end of the tank from the connection fitting.  It may be under a cover or plug.   Just pump the tank up to about 60 psi.  Once this is done you are ready to install the tank.

You just need to turn the main water off, install a tee and female adapter in the cold water pipe near the water heater and then screw the Expansion Tank into the adapter. A few basic tools and a little knowledge and you can eliminate thermal expansion from your piping system.

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13 comments to Water Heaters and Thermal Expansion: Does Your Water Heater Need A Thermal Expansion Tank?

  • Dan Lisowe

    My electric water heater is installed in a rather inaccessible area in a mobile home. Can the thermal expansion tank be installed in another area as long as it’s installed on the cold side?

  • You, my pal, ROCK! I found exactly the info I already searched everywhere and just couldn’t find it. What an ideal web site.

  • Laura


    I have a hot water radiator system that is not heating, After trouble shooting various parts of the system I have narrowed it down a little bit.

    It looks like there is no water pressure in the system. I removed the expansion tank and there is pressure on the water line into the system. I reconnected the tank to the supply side only and opened the valve on the line, no water flows through the fill valve. I have a Amtrol fill-trol 109.

    Is there an other way to test the fill valve?

    I see they are replaceable. There was a small amount of crud on the screen I cleaned that off.

    The pump ( a Grundfos circulating pump) seams to be working it makes noise, the pipes vibrate, I took the cover off of the center shaft it is spinning, perhaps the impellers are bad?

    Thanks for any insight

    • BTS

      Hey Laura,

      I’m sorry, but as a lifelong Georgia boy, I know very little about hydronic heating. We just don’t do that here. I can, however, point you in the right direction. Here are a couple of resources you should definitely check out:

      The first is a website run by a guy named Dan Holohan. Dan has forgotten more about steam and hydronics than most people know and he likes helping people with their problems. His site is:


      You probably won’t need these other sites but here are a couple more:



      I really hope this helps and I would definitely check out the first link first.



  • Hugh

    I am not sure why anyone would want pressure effects from the water heater in the cold water line. Don’t water heaters have a built in check on the cold water side? I have seen relief valves on the cold water line. In my opinion there should be (if there is not already) a check – built in or external – on the cold water side with an expansion tank or relief valve only downstream of the water heater. This would avoid contamination of th domestic cold water supply if there is any built up contamination in the water heater.

    • BTS

      Hi Hugh,

      Water Heaters do not have a built in check valve. You could certainly install a check valve on the cold water line and then a thermal expansion tank on the hot side only. You are right that there might be a slight bleed back of hot water into the cold water side without a check valve but that water would probably never make it to a cold water outlet since the tank is so close to the water heater and the pressure on the cold side would prevent any significant backflow.

      The reason the Thermal Expansion tank is usually installed on the cold side is because the rubber membrane inside might have a shorter lifespan on the hot side.

      Thanks for the comment.


  • jeff

    High water pressure coming in from the service side and heaters turned up to the highest setting is often the problem to start with.

  • Greg

    Hi Bryan,just wondering how the toilet valve is put under extra cold water pressure if there is a non return (check valve) installed on the cold water inlet to the water heater.

    • Bryan

      Hey Greg,

      Well, actually if there is a check valve in the cold water inlet pipe to the water heater it won’t, it will just cause an increase in pressure in the in the system downstream from the check valve. It is not very common to have a check valve in this location, at least in the USA.

      But…the check valve I am referring to is a check valve in the water service inlet pipe to the house. This is very common in the US to protect the municipal water system from possible contamination. It is usually at the “house side” of the water meter. This effectively makes all the water pipe in the house a “closed system” and any thermal expansion that occurs affects both the hot and cold piping.

      I hope that makes sense. I would be interested in how they do it down under.



  • Pat

    I’m wondering why having an expansion tank rust through, explode and potentially flood your basement (if you have a basement) is a better choice than replacing a cheap float/fill valve every few years?

    I’ve never heard of anyone having issues without an expansion tank but I can name a dozen people who have had an expansion tank burst and flood their homes.

    • Bryan

      Hey Pat,

      I guess it depends on what your goal is. In many areas it is a Code requirement to have some type of Thermal Expansion protection. I wrote this article in response to several questions I received on the subject.

      While I understand your concern, I have never seen a Thermal Expansion tank rust through and “explode”. I guess it makes sense that a tank might not last as long as a Water Heater because the inside not coated the same way a Water Heater. I can say I have never seen that happen in over 25 years of plumbing, but I am in the Southeast and water quality varies in different locations so maybe that is why.

      The bottom line is Thermal Expansion is one of those things that may not cause a noticeable problem (or it might) but it can definitely cause you to waste water and probably shortens the life of many of your plumbing system parts and components.

      So, you are right that it is probably a decision involving risk vs reward, like so many things. Thanks for pointing that out and thanks for reading my blog.


  • Bryan,

    I am a consulting engineer and i had to comment on your expansion tank article. I do a lot of renovations to buildings and find amazing things installed by some contractors. I have found many domestic hot water systems without an expansion tank and the owner wondering why the relief valve continues to lift for apparently no reason. After a bit of explaining, he gets the idea and has an expansion tank installed.

    Good article!

  • Eric Sartin

    This is fairly inexpensive and really makes a difference. I installed one myself in a couple of hours. Now when I monitor my pressure, it never gets too high after a long hot shower. My pressure regulator will backflow but not until the system gets up to street pressure which is 130psi in my area!

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