How does the expansion tank work

Working principle of the expansion tank is demonstrated using the example of a pressure membrane tank connected to a closed heating system circuit.

Water expands when heated. For example, at a temperature of 20°C, one kilogram of water occupies a volume of 1.002 dm³ (liters), and at a temperature of 90°C, it increases in volume to 1.036 dm³ (liters). This means that each kilogram of water, when heated from room temperature of 20°C to the maximum system temperature of 90°C, increases its volume by 3.4%.

Water is an incompressible liquid, so even a small increase in volume in a closed heating system will cause a significant rise in pressure, which can damage its components. The function of the expansion tank in the heating system is to absorb the excess water volume when heated and expel the deficient volume when cooled.

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

As the water in the closed circuit heats up, it expands, and the pressure in the heating system increases. The excess volume enters the tank, the membrane stretches, reducing the air space, similar to a piston (the air is compressed). At this time, the pressure in the expansion tank, and thus in the heating system, increases. When the water reaches its maximum temperature, the pressure in the heating system and the expansion tank rises to the maximum allowable value.

As the temperature drops, the volume of water in the heating system decreases, and the pressure drops, with the previously accepted water being expelled from the tank by the compressed air. The share of the air space in the expansion tank increases – the air pushes the water out. In case of small water leaks, the pressure in the system, and thus in the expansion tank, decreases, and the compressed air pushes out the reserve water volume, thereby replenishing the heating system.

It is important to note that the effective operation of the expansion tank in the heating system is only possible with proper calculation and qualified adjustment. You can calculate the volume of the expansion tank, the initial gas space pressure, and the initial operating pressure by following the link Calculate tank volume.