The main function of closed expansion tanks with replaceable membranes in heating systems is to balance the changing water volume depending on the change in temperature in heating systems. For example, the density of water at 0°C is 0.9998 kg/dm3 while its density at 100°C is 0.9583 kg/dm3. This means that when the temperature of any amount of water is heated from 0°C to 100°C, its volume increases by about 4%. For this reason, a system should be allocated to compensate for this change in the heating system. This system is called controlled expansion system. In the absence of an expansion tank, a few °C increase in water temperature causes a huge increase in water pressure. Since the compressibility of water is low, this increase in pressure may cause damage to the weakest points of the system.