How do I size an expansion tank for a closed hydronic heating system?

Q: 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.

Q: 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.

Question

How do I size an expansion tank for a closed hydronic heating system?

Accepted Answer

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.

How do I size an expansion tank for a closed hydronic heating system?

Answers