Expansion Vessel

An expansion vessel (also called an expansion tank) is a sealed, pressurized container in a hydronic heating system that accommodates the volume changes of the heating water as it heats and cools. Water expands by approximately 4 percent when heated from 10 to 80 degrees Celsius, and without an expansion vessel, this volume increase would cause dangerous pressure spikes that could damage pipes, fittings, and the boiler.

The expansion vessel contains a flexible rubber diaphragm or bladder that separates the water side from a pre-charged air or nitrogen cushion. As the heating water expands, it pushes against the diaphragm, compressing the gas cushion and absorbing the volume change without significant pressure increase. When the water cools and contracts, the gas cushion pushes the water back into the system, maintaining stable pressure.

The most common expansion vessel problem is loss of the gas pre-charge, which causes the vessel to become waterlogged. When this happens, the system has no room to accommodate water expansion, and pressure rises excessively when the heating fires. Symptoms include the pressure relief valve dripping (the safety valve on the boiler), frequent need to bleed radiators, and pressure gauge readings that swing widely between cold and hot conditions.

The expansion vessel's pre-charge pressure should be checked annually — typically during the autumn heating system startup — using a tire pressure gauge on the vessel's air valve. The pre-charge should match the system's static pressure (usually 0.5 to 1.5 bar depending on the building height). If the vessel cannot hold its pre-charge, the diaphragm has likely failed and the vessel needs replacement. This is a job for a heating professional.