Hot water boilers should be provided with expansion tanks. There are two types of expansion tanks designated for use with a residential boiler system. A non-pressurized tank is simply a cylinder filled with air set at atmosphere pressure. A pressurized tank is a sealed cylinder divided by a flexible diaphragm. An expansion tank provides space for the water to expand as it is heated, and it keeps the water pressure within the normal operating range while the boiler is working. Thank you www.nachi.org