A heating unit for heating fluid is described having a first manifold having at least one inlet, a second manifold connected to the first manifold and having at least one outlet, and a third manifold. The heating unit also includes one or more heating systems which extend from the third manifold to the first manifold via the second manifold, where the one or more heating systems have an inner tube and an outer tube. Further, the heating unit includes a fluid flow path from the at least one inlet to the at least one outlet via the first manifold, an area between the inner tube and outer tube, the interior of the inner tube and the second manifold.