The Keystone Series boiler is a medium to large capacity, 3-pass, dry back, Scotch marine design, packaged boiler. The three-pass boiler design delivers the best in boiler fuel efficiencies. Keystone boilers come with many standard features which make them easy to monitor and service for decades of reliable steam or hot water. Many optional features are available for Keystone boilers, making them easy to customize to fit your unique needs.