How the expansion tank works

The principle of operation of the expansion tank is considered using the example of a pressurized membrane tank connected to a closed heating system circuit.

Water expands in volume when heated. For example, at a temperature of 20°C, one kilogram of water occupies a volume of 1.002 cubic decimeters (liters), and at a temperature of 90°C, it expands in volume to 1.036 cubic decimeters (liters). That is, each kilogram of water increases its volume by 3.4% when heated from room temperature of 20°C to the maximum temperature for the heating system of 90°C.

Water is an incompressible liquid, so even a slight increase in volume in a closed heating system circuit can cause a significant increase in pressure that can damage its elements. The function of the expansion tank in the heating system is to absorb excess water volume during heating and displace the deficient volume during cooling.

The pressure in the cold heating system is equal to the pressure in the expansion tank and the initial operating pressure (see the section Calculation). At this point, most of the tank is filled with gas (air).

During heating in a closed circuit, water expands, and the pressure in the heating system increases. The volume increase goes into the tank, and the membrane stretches and reduces the share of air space, similar to a piston (air compresses). At this time, the pressure in the expansion tank, and therefore in the heating system, increases. When the water temperature reaches its maximum value, the pressure in the heating system and the expansion tank rises to the maximum allowable value.

As the temperature decreases, the water volume in the heating system decreases, and the pressure decreases, and the previously taken water is pushed out of the tank with compressed air. The share of air space in the expansion tank increases, and the air displaces the water. In the case of minor water leaks, the pressure in the system, and therefore in the expansion tank, decreases, and the compressed air displaces the reserve volume of water, thereby feeding the heating system.

It should be noted that the effective operation of the expansion tank in the heating system can only be ensured by its proper calculation and qualified adjustment. You can calculate the volume of the expansion tank, the initial pressure of the gas space, and the initial operating pressure by following the link Calculate the tank volume.