Among conventional (noncondensing) commercial water boilers used for space heating or process heat with input sizes of 400 MBH and up, three boiler types loom large: those with cast-iron sectional heat exchangers, firetube, and watertube.
For a variety of reasons, the latter two have dominated the commercial process heating market. Advantages include relatively compact size, lower standby heat loss, and the speed with which heat can be generated and delivered into distribution piping.