A flow though heater device comprising a fluid tube carrying a fluid to be heated, a tubular heating element extending at least partially parallel and in thermal contact with the fluid tube, wherein the tubular heating element comprises a profiled casing tube and at least one heating coil mounted in a tubular portion of said profiled casing tube. The profiled casing tube further comprises at least one tab portion laterally protruding from the tubular portion and being wrapped around the fluid tube, wherein the assembly of the fluid tube and the tubular heating element has an approximately helical shape so that the tubular portion of the casing tube is positioned with respect to the helix radially inside of the fluid tube.

A heat exchanger including a header having a plurality of header openings with rigid tubes that may be made of plastic are inserted in the openings. The tubes are sealed to the header to prevent leakage between the header and the tubes to prevent water and air leakage between the wet, scavenger air stream flowing through the tubes and a dry air stream flowing around the tubes. A method of making the heat exchanger includes providing the openings with a flange and uses an interference fit between the rigid heat exchange tubes and the header openings. A self-leveling sealant may be used to seal the heat exchanger tubes to the header using, for example, a paint roller and/or a paint sprayer.