Sizing Expansion Tanks

Expansion of Water

The water used in the heating system expands with the temperature. A volume increase of 4% is experienced when the water temperature is increased from 7oC to 90oC. To compansate this expansion in the system, closed or open expansion tank should be used.

An expansion tank is required in a heating, cooling or air condition system to avoid an unacceptable increase of the system pressure during heat-up.

Expansion tanks are in general designed as:

• Open tanks
• Closed compression tanks
• Diaphragm tanks

The net expansion of water can be expressed as
Vnet = (v1 / v0) – 1 (1)
Vnet = necessary expansion volume of water (gallon, liter)
v0 = specific volume of water at initial (cold) temperature (ft3/lb, m3/kg)
v1 = specific volume of water at operating (hot) temperature (ft3/lb, m3/kg)