Apparatus, systems, and methods for a modular heating unit that may be adapted to be inline with a pipeline. The unit includes a base member having a main inlet pipe, a header, and pipes connecting the main inlet pipe with the header. A combustion chamber is positioned within the pipes. One or more heat exchangers are connected to the header. The heat exchangers each having a top surface, bottom surface, plurality of fins, inlet ring, inlet port, outlet ring, and outlet port. The modular heating unit includes external inlet and outlet pipes. A first flow path enables fluid to flow from the header into the one or more heat exchangers. An exit flow path connected to the external outlet pipe connects the one or more heat exchangers to an exit port with a portion of the exit flow path being positioned above the one or more heat exchangers.