Selection and Installation of Thermal Expansion Tanks

See Part 2 for the calculation of the expansion tank volumes. Now, using the calculated acceptance volume and the tank’s total volume, select an expansion tank from the manufacturer’s data that meets these requirements.

Be aware that in most states, unfired pressure vessels larger than a certain volume must have an ASME stamp. Check your state’s requirement for this limit. If the expansion tank you need exceeds this limitation, it must be constructed in accordance with the requirements of ASME Section VIII. ASME-rated vessels are much higher in cost than non-ASME vessels. Installing multiple smaller tanks, which are below the volume limitation for ASME tanks, is also an option. Most tank manufacturers offer both ASME and non-ASME thermal expansion tanks.

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How To Size Thermal Expansion Tanks For Hot Water Systems – Part 2 of a 3-part series.

In Part 1 of this series, we looked at where thermal expansion tanks are needed. When it is determined that a tank should be included in the plumbing system, the next task is to select the correct tank size. Referring to sizing tables in an expansion tank manufacturer’s catalog is the easiest method, although not the best method, for sizing thermal expansion tanks. Sizing tables provided by the manufacturers give the tank model number and size based on the water heater’s volume and the system supply pressure. These tables, however, are based on three important assumptions that designers need to be aware of.

First, for most manufacturers, the tables are based on a maximum allowable line pressure of 150 psi. This is the maximum allowable working pressure of most water heaters and thermal expansion tanks. It is also the setting of the water heater relief valve. In other words, the thermal expansion tank, if selected by the tables, could accommodate the thermal expansion up to a system pressure of 150 psi, which is the relieving point for the water heater relief valve. Remember that the purpose of the thermal expansion tank is to prevent the relief valve from relieving. Sizing the thermal expansion tank for a maximum allowable pressure of 135 psi, which is 10% below the relief valve set point, is a better alternative.

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