Hot Water Boiler Expansion Tanks – There are two types of expansion tanks used on hot water boiler systems. The steel expansion tank and the bladder type expansion tank. These expansion tanks are used to provide a cushion for the expansion of water when it is heated. Without this cushion of air, the pressure in the boiler system would exceed the setting on the pressure relief valve. This will cause the pressure relief valve to open and discharge water to relieve the pressure.It is necessary to for an HVAC technician to understand how expansion tanks work. This understanding is important to properly troubleshoot hot water boiler problems which may occur as a result of an issue with an expansion tank. One of the first things to check on a hot water boiler system when the pressure relief valve discharges water excessively is the expansion tank.