An Expansion Tank is a sealed vessel that accommodates water volume increase in closed-loop hydronic systems, maintaining safe operating pressure between 12-30 PSIG. Internal diaphragms or bladders separate pressurized air from system water. Expansion tanks prevent relief valve opening, system leaks, and equipment damage from overpressure.
Sizing and Pre-charge
Tank volume equals system water volume multiplied by 0.04, plus 10 percent safety factor. A 60-gallon system requires approximately 3-4 gallon expansion tank. Pre-charge pressure should equal hydrostatic head pressure, typically 12-15 PSIG for residential applications. Nitrogen gas charge maintains pressure during thermal cycling and compensates for water absorption into bladder material.
System Requirements
Expansion tanks connect to the system’s lowest point upstream of the circulator pump. Annual inspection verifies diaphragm integrity and nitrogen charge retention. Proper sizing and charging extend tank life to 15-20 years. Undersized expansion tanks cause frequent relief valve discharge and water loss, reducing system efficiency and reliability.