Expansion Tank Sizing Calculator
Size a diaphragm-type expansion tank for closed hydronic heating or cooling systems based on system volume, temperature range, and system pressures.
Results
What is it?
As water is heated, it expands � about 4% from 50�F to 200�F. In a closed hydronic system, this expansion must go somewhere or the pressure relief valve will open repeatedly, wasting water and causing problems. A diaphragm expansion tank accepts this extra volume during heating and releases it during cooling, keeping system pressure within bounds.
How to use
Enter the total system water volume (all pipes, coils, boiler, buffer tanks), the cold fill temperature and maximum operating temperature, and the fill and maximum pressures. The ASHRAE-based formula calculates minimum tank size. Always select the next available standard commercial tank size above the calculated value.
Example scenario
A 200-gallon hydronic heating system fills at 50�F and heats to 200�F. Fill pressure 12 psig, max 30 psig. Expansion factor � 0.0296 � 200 = 5.9 gallons of expansion. Tank size = 5.9 � 44.7/(44.7-26.7) = 14.7 gallons ? select a 15-gallon Amtrol WX-251.
Pro tip
Always set the expansion tank pre-charge pressure equal to the system static fill pressure (measured cold at tank location) BEFORE connecting to the system. A waterlogged tank (failed diaphragm) causes repeated pressure relief valve discharge � the most common expansion tank failure symptom. Size tanks generously � undersized tanks require frequent valve replacement.