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How to Find the Right Size Thermal Expansion Tank for Your Hydronic HVAC System

Updated: Nov 30, 2023

A thermal expansion tank is a small tank that typically sits near a water heater in a facility or home. The expansion tank acts as a vessel designed to accept additional system volume resulting from heating water.

Water is incompressible and expands when heated. 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. (Ideally, the pressure relief valve will release the water!)

The expansion tank is added to protect the water heater, pipes, and other system components from excessive pressure from heating water. 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 – an engineer should also consider entering and leaving water temperatures and minimum and maximum pressures when sizing and selecting a thermal expansion tank, such as.

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 10% of the water heater volume, but, for budgeting and planning purposes, this estimate will get you in the right ballpark.

If you have questions about equipment size, water heaters, or expansion tanks, contact a Hoffman Hydronics Sales Engineer to ensure you’re selecting the best equipment sized correctly for your application.


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