Hydro-Plus Hydronic Expansion tanks are designed to absorb theexpanded fluid in closed-loop of residential or commercial hydronic heating and cooling systems to control pressure build up. Hydro-Plus expansion are tanks composed of a tank in sheet steel which incompliance with IAPMO and EN Standard and welded according to ASME & EN procedure by certified welder. The diaphragm is made withhigh temperature resistance EPDM rubber. All models are subject to hydraulic test with a pressure of 1.5 times higher than the design pressure. Hydro-Plus Expansion tank is the best safety choice for your Hydronic Heating system.