When you look a water heater in a large building or even a home, you may notice a much smaller tank close by. That small tank is a thermal expansion tank. An expansion tank in this application is a vessel design to accept additional system volume resulting from heating water. Water is incompressible and, when it is heated, it expands. If this happens in a system with a fixed volume of pipes and equipment, you will quickly discover the weakest point in the system as water begins to spray through it. (Hopefully, it is the pressure relief valve releasing water!)
To protect the water heater, pipes, and other system components from excessive pressure when heating water, an expansion tank is added. A simple device, it is nothing more than a tank with a bladder (thick balloon) inside. The space between the bladder and the inner wall of the tank is pre-charged with air to closely match the neutral pressure of the system BEFORE the water is heated. Then, when heat is applied and the pressure goes up, the expanded water volume flows inside the bladder. The air outside the bladder compresses against the tank walls, providing the cushion needed to protect the rest of the system.
As a rule of thumb, a thermal expansion tank should be about 10% of the total water heater volume. Let’s take some examples:
Example 1:
Water heater tank capacity is 100 gallons
100 gallons x 10% = 10 gallon expansion tank.
Example 2:
(2) 250-Gallon Water Heaters = 500-Gallons Total
500-Gallons x 10% = 50-Gallon Expansion Tank
Remember, this is just a rule of thumb. In addition to water heater volume, an engineer will consider other factors when sizing and selecting a thermal expansion tank, such as entering and leaving water temperatures and minimum and maximum pressures. The expansion volume may then be calculated and a tank selected based on acceptance volume and overall volume. In most cases, the engineered expansion tank size will be smaller than the 10% of the water heater volume, but, for budgeting/planning purposes, this estimate will get you in the right ballpark.
If you have questions about equipment size, water heaters or expansion tanks, you should talk to your manufacturing representative to ensure that you’re selecting the best equipment sized correctly for your application.
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