What is the difference between an open and closed system in hydronic heating?

Closed system implies that the system is under some pressure and the fluid is not exposed to the ambient air. Home systems are usually set at 12psi. An open system is exposed to the air with a holding or storage tank at the highest point. An open system needs to be built with parts and materials that will not corode ie; copper, brass, pex pipe, etc. ( do not use cast iron pumps, steel tanks or fittings) lc