Stainless steel boilers do not have any internal coating; it is plain marine grade stainless steel which will not rust in contact with water.

Glass lined boilers are made of steel or other materials and have a “coating” of glass. Because these expands and contracts at different ratios during heating and cooling, sooner or later the glass will crack and water will touch the metal, rusting it and further cracking the structure. Stainless steel boilers last longer.