Circulation heaters (In line heaters) are used in water heating or oil heating systems that flow either liquids or gases. These circulation systems can be re-circulatory in design, often referred to as closed loops systems where the material being heated can flow multiple times past the heater; or in single pass systems, where the material being heated does not return to the heater.

The circulation heater is comprised of two main components, the heater and the steel vessel. The vessel can range in size form two in up to 14 inch to accommodate the appropriate sized heater. There are normally one inlet and one outlet, located at opposite ends of the vessel, but location and size are determined by the requirements at hand. NPT fittings in various sizes are typically used, but flanged fitting are also used. Various options available for the vessel include insulation, drain plug, pressure relief and mounting legs and are determined according to the final installation.

Size and heating requirements determine the type of Inline Water Heater utilized in a circulation heater. Larger applications call for flange heaters, which are bolted to the vessel and all flanges and vessel are rated to the same pressure. Smaller units use screw plug style heaters, which thread into the appropriately sized NPT female fitting, welded to one end of the vessel.

Flange and/or screw plug materials are typically the same, either steel or stainless steel in order to prevent or reduce galvanic corrosion or reactions between unlike materials and to reduce reactions with the material being heated.

Terminal enclosures can be constructed for various needs including NEMA 1 (standard housing), NEMA 4 (moisture resistant) and NEMA 7 (explosion proof typically used in waste heating applications). Regular thermostats or pilot duty thermostats can also be fitted within the terminal enclosure