4.4 Gallon, Pre-Charged Water Worker Boiler Expansion Tank, For Closed Loop Hydronic Heating Systems Using Baseboards Or Radiators, A Heavy Duty Diaphragm Flexes Against An Air Cushion To Maintain A Safe Operating Pressure, 240 Degrees Fahrenheit Maximum Temperature, 100 PSI Maximum Working Pressure, Pre-Charged At 12 PSI, 1/2" NPTM.