Thermal Expansion Tank

What Is A Thermal Expansion Tank?

A question I get asked very often is, "What is a thermal expansion tank for?" We get this question many times when we are installing, replacing, or repairing a tank-style water heater. A thermal expansion tank sits on the incoming water line of the water heater. It is typically about the size of a basketball. What it does is allow for the thermal expansion of the heated water inside the tank. This is called a closed system.

When you have a closed system with a tank-style water heater where you are heating water in a closed vessel like a pressure cooker, heating the water will create pressure, and that pressure will continue to build unless it is released. On a pressure cooker, this is typically controlled through a weighted rocking cap that sits on top of the pot and rocks back and forth, releasing pressure within the pot to protect it from catastrophic failure.

One of the first devices we had for this safety was, and still is, the temperature and pressure relief valve, otherwise known as the T&P valve. This safety device, created by the Watts Corporation, was introduced in the late 1930s as a result of many catastrophic failures and injuries. This one device has virtually eliminated tank ruptures and made these appliances safe for use in today’s homes. The T&P valve is designed more as a last-ditch effort to protect the heater from dangerous pressures, which could result in the heater failing catastrophically or rupturing and causing significant damage.

The T&P valve is designed to open and release some of the pressure when the temperature or the pressure gets too high for the tank. Or, if the combination of the temperature and pressure gets too high for the tank. As you increase the temperature, the pressure needed to open the T&P is reduced. So when you have high water temperatures, the pressure to open the T&P can be as low as the 50 or 60 psi range, causing the T&P valve to open up and release, thereby relieving the dangerous pressure off of the tank. However, the T&P releasing to save the heater still results in property damage caused by the dripping or running of the water from the T&P.

To ensure further safety of this appliance, the need for the thermal expansion tank was introduced in 1991 by Amtrol. Amtrol created the first expansion tanks for boilers, then pressure tanks for wells, and finally the thermal expansion tank for water heater safety. The requirement of a thermal expansion tank on tank-style water heaters has been in place since 2006 when plumbing codes around the country started adopting it as a result of heater manufacturers recommending it as part of the installation of a new water heater.

The thermal expansion tank is similar to a pressure tank on a well. It's typically a steel tank with a rubber bladder, and that rubber bladder is filled with air equal to the home's normal incoming water pressure. That way, when the incoming water pressure in the tank is being put in and the heater is being heated, the pressure inside that vessel, once it exceeds that 55 or 60 psi, will start pushing against the bladder inside the tank, compressing it and therefore opening up room within the small tank to allow the water heater room to expand. As its name implies, it allows for thermal expansion to occur.

Many people witness this every day in their own homes; they just don't realize it. When you're in the kitchen and you need to boil a pot of water, you will put a lid on the pot and turn the heat on. We do this when we're trying to cook pasta, rice, or maybe boil eggs or something of this nature. What will happen is you will put the lid on, turn the heat on, and go about doing something else while you're waiting for the water to boil. The next thing you will hear or see is the water boiling over on the stove. You run over, grab the lid of the pot, and raise the lid. When you do this, you'll notice as soon as you raise the lid that the boiling will subside and it will stop the pot from boiling over.

By raising that lid on the pot, you just allowed for thermal expansion to occur. Think of it as when you raise that lid, it's the same as that rubber bladder inside that tank compressing, therefore opening up and providing room within the tank for the water within the heater to expand. Just like you did on the stove when you raised that lid, you allowed the increase of the thermal energy inside that pot to expand.

Thermal expansion tanks do fail and must be checked on a regular basis, typically annually, to ensure proper safety for the water heater and the home. This can be done relatively easily by checking the valve stem, Schrader valve, on the tank. This is the valve that is used to put air into the tank in order to set it at the proper pressure. By removing the protective cap on the Schrader valve and depressing the small plunger inside of it, there should be air that comes out. If there is moisture or water that comes out, then the rubber bladder inside the tank has failed and the expansion tank must be replaced. Another method sometimes used is by tapping on the expansion tank. When you tap the tank, it should create a thud sound indicating water in a portion of the tank and then a ting sound indicating the presence of air. This signifies the tank is still probably good, but the air check method is still the best.

Expansion tanks must be properly supported by either a mount or a strap that holds the weight of the tank. This way, when the bladder inside the tank fails and the tank fills with water, it becomes very heavy, and if the tank is not properly supported, it can break off and cause significant and possibly major damage. There are multiple methods to support the expansion tank, such as a strap around the tank secured to the structure overhead, a mounting bracket secured to the structure for the expansion tank, a threaded connection that can support the weight of the tank when completely filled with water, or even resting on top of the water heater or other structure.

Placement of the thermal expansion tank must be on the cold water line before it enters the water heater. It must not have any valves or stops or other obstructions between the water heater and the thermal expansion tank that would impede the flow of water back into the thermal expansion tank. This can be virtually anywhere along the cold water line prior to entering the heater. However, most of the time you will see the expansion tank located right at the heater itself. Although not code in many jurisdictions, that is the easiest place to position the thermal expansion tank.

Now that you are familiar with what a thermal expansion tank is and what function it performs, you may want to check if you have an expansion tank and see if it is in good working condition. You may also find that you do not have one, or that maybe you just cannot access it because it is in the crawlspace or way high in the ceiling. If this is the case, then you should contact a professional to come out and replace or install one.


- Don Hilderbrand
PlumbSmart Plumbing and Drains

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