Thermal Expansion Tanks for Water Heaters

With a water heater when it is heating the water there will be some expansion, which in a closed system will increase the pressure and cause the T&P Valve on the water heater to discharge. One of the functions of the T&P Valve is to open as a safety mechanism when the pressure in the water heater tank reaches 150-psi. The amount of expansion that occurs is determined by how much water is in the tank and the difference in the temperature of the water at the start of the heating process and at the finish of the heating process. The larger the volume of water is being heated and the larger the temperature differential the more expansion you will have. As an example if a new 40-gallon water heater was installed and filled with 40-degree F water, which was then heated to 125-degrees F the amount of expansion would equal about .44-gallon. If we increased the temperature to 140-degrees F the amount of expansion would equal .6-gallon. This scenario would result in the expansion tank holding from 1 to 1.5 gallons of water when properly installed. While the actual volume change is very small it is easily masked by dripping faucets and leaking toilets often causing that you fixed my toilet the other day and now my water heater leaks type of call. The effects of thermal expansion can have a detrimental effect on the service life expectancy of a water heater. The water heater is a pressure vessel made in most cases from steel with a glass lining applied to the inside of the tank to prevent corrosion. As the pressure increases in the tank from the supplied water pressure to the 150-psi where the relief valve opens the tank will start to bulge from the added pressure. Then when the added pressure is released the tank will spring back to its original shape. These two events combine to become a pressure cycle and contribute to cracking of the glass lining exposing the steel to water causing corrosion and also metal fatigue which will eventually cause the metal to crack at points such a seams where the top and bottom of the tank are welded on. In a worst-case scenario with a failed T&P valve not opening the pressure increase may cause the tank to permanently bulge, which would result in the tank having to be replaced. Installing a thermal expansion tank will eliminate pressure cycling greatly increasing the service life expectancy of your water heater.

The thermal expansion tanks used on water heaters should be listed as “Potable Water Expansion Tanks” which means they are constructed from materials that are safe to use on drinking water systems. Expansion tanks are also used on hot water boilers but the water in those systems are never consumed so the materials used are different. Make sure the right type of expansion tank is used on your potable water system. The expansion tank should be connected on the inlet side of the water heater and connected by a tee between the inlet to the water heater and the shut off valve on the inlet of the water heater. There should be no valve between the water heater and the expansion tank, which would defeat the protection provided by the expansion tank. The water in the tank will add considerable weight to the expansion tank and it should be adequately supported to avoid placing stress on the pipes. The tank may have a longer line plumbed to it allowing the tank to be remotely mounted. it does not have to be directly on top of the water heater.

Potable Water Thermal Expansion Tanks are made by a few different companies, Watts with their model PLT-5, PLT-12, PLT-20, and PLT-35 tanks used in residential and light commercial, and Amtrol with their Therm-X-Trol model ST-5, ST-8, ST-12, and ST-25V tanks also used in residential and light commercial are very popular. Larger commercial water heating systems would require using an ASME Rated expansion tank. The tanks are made of drawn steel that has a rubber bladder dividing the tank into two sections. On one side there is a threaded connection that allows water to enter the tank. On the other side there is an air valve similar to the one on a tire through which the air pressure of the pre-charge on the air side of the bladder can be adjusted and checked. The pre-charge should match the typical pressure on your water supply system. The air is able to compress which allows the expanding water to enter the expansion tank with only a slight increase in system pressure preventing the water heater tank from experiencing pressure cycles and providing a substantial increase of the service life expectancy of your water heater.

Posted in How To Plumbing Tips, Water Heater Repairs, Water Heater Reviews Tagged with: , , , , , , , , ,
8 comments on “Thermal Expansion Tanks for Water Heaters
  1. donaldschwartz says:

    I have a Zurn Wilkins thermal expansion for 40 gallon electric water heater. The pressure valve is leaking about 2 oz per day into the pan. I checked pressure in expansion valve model wxpt78. I get no reading at all.I tried to use a bike pump-no results My tank is 12 years young. Do I replace or do what?
    Thank You,
    Donald Schwartz Florida

    • Redwood says:

      Hi Donald,

      It does sound like you may need a new Thermal Expansion Tank installed. The dead giveaway would be if you pressed the center of the valve and water came out. But that may not happen if there is still any air at all in the tank remaining.


  2. Chuck says:

    I replaced my old 40 gallon tank with a new 50 gallon tank and the pressure relief value on the tank is leaking after a heat cycle. I have a smaller old thermal expansion tank and was told it could need more pressure in it. When I checked the psi it reads 70 but water comes out the value. My neighbor has the same old tank with the old 40 gallon tank and his psi is 35. My question is the expansion tank bad and could it be too small? Thank you for any help. Chuck

    • Redwood says:

      Hi Chuck,
      The pressure in the expansion tank should match the water pressure that is supplied to your home. However, on an active system under pressure the system pressure will show as your reading, so to obtain a correct pressure reading, the water needs to be off, and a faucet opened to bleed off the pressure before taking the reading.

      Inside the thermal expansion tank there is a rubber bladder which separates the water from the air charge. In your case with water coming out of the air valve it is obvious that the bladder in the expansion tank has failed and the expansion tank needs replacement.


  3. dave says:

    What difference does it make if the x-pansion tank is installed on the hot side or cold side? They should see the same pressure.

    • Redwood says:

      Hi Dave,
      Good Question, initially you would think there would be no difference as the pressure would be the same. However, the temperature of the water will be higher. The higher temperature may shorten the service life of the bladder in the expansion tank. In addition the higher temperature water pushed into the expansion tank may be the ideal place for bacteria present in the water supply to multiply, and when the pressure drops, a slug of bacteria laden water will be pushed into your hot water supply.

      These waterborne bacteria in low levels are present even in municipal water supplies disinfected with chlorine and chloramines without presenting problems, unless they are allowed to multiply to higher levels. Many of these bacteria remain dormant at lower temperatures, which are typically seen in water distribution systems, but when the water reaches 80ºF (26.6ºC) to 120ºF (48.9ºC) an ideal temperature for bacteria growth is reached, and bacteria multiplies rapidly to unhealthy levels.

      These reasons are why the installation instructions for expansion tanks used for potable water systems suggest installation on the cold side of the water heater.

      Thanks for asking the question,

  4. Matt says:

    Hello Redwood,

    Just found this site, and looking forward to poking around on it some more. Regarding expansion tanks, is your explanation also the reason why the instructions with some expansion tanks require a certain minimum length (I think 18″ or so) of piping between the inlet nipple and the expansion tank tee?

    I’m curious is if there are other reasons besides the bladder’s vulnerability to hot water and the bacteria issue.



    • Redwood says:

      Hi Matt,
      I’m not sure, the thermal expansion tanks I’ve been using didn’t have that in their installation instructions, until you mentioned it I’ve never known of any that did. Thanks for the reply to my e-mail telling me that the State Water Guard thermal expansion tanks have this in their instructions. I know there are some codes that state a similar distance where plastic pipes mat not be used for protection from the heat of the vent on a gas water heater. In addition as you state the hot water in the tank has a tendency to migrate back up the cold supply despite the use of heat trap nipples for a short distance so there could be an exposure to hot water that could affect the bladder in the tank and have the water in the tank be at a temperature conducive to fast bacteria growth.

      I’m going to write to State Industries and ask them what their reasoning was behind them placing this requirement in their installation instructions for thermal expansion tanks.
      Thanks for the intriguing comment,
      I’ll post their reply when I receive one,

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