Expansion Tank Sizing Calculator
Size a diaphragm or bladder expansion tank for a closed-loop hydronic heating system from the system water volume, cold-fill and hot operating temperatures, and the fill and relief pressures.
- Enter system volume Enter System volume in gallons, including all piping, the boiler jacket, indirect tank coil, and air separator volume — not just the boiler.
- Enter the design temperatures Set Cold-fill temperature and Hot operating temp in degrees Fahrenheit (e.g. 180 for baseboard, 140 for radiant). The acceptance volume comes from the difference in water density between these two temperatures.
- Enter the pressures Enter Cold-fill pressure (the PRV setting, often 12 psig) and Max pressure (the relief valve setting, often 25–30 psig). Max must be greater than the cold-fill pressure.
- Calculate the tank volume Press Calculate to get the minimum required tank volume plus the water expansion (acceptance) volume and acceptance ratio. Round up to the next standard catalog tank size.
How do I size an expansion tank for a closed hydronic heating system?
First compute the water expansion volume from the change in water density between the cold-fill and hot operating temperatures, then apply the diaphragm-tank formula using the cold-fill (acceptance) and maximum (relief) pressures expressed in absolute terms. This calculator follows the ASHRAE Handbook (HVAC Systems and Equipment, Hydronic Heating) method with NIST water densities. Always round the result up to the next catalog tank size.
Why does the expansion tank depend on temperature?
Water expands as it heats, and that expansion volume is what the tank must absorb. The tool uses the density of water at your cold-fill temperature versus your hot operating temperature, so a larger temperature swing produces a larger acceptance volume and a bigger required tank. This is why baseboard systems at 180 F need more tank than radiant systems at 140 F.
Does the expansion tank replace the relief valve?
No. A separate ASME-rated pressure relief valve is always required regardless of tank size, typically set at 30 psig for residential boilers. The expansion tank only absorbs thermal expansion to keep system pressure below that relief setting under normal operation.