Abstract: An apparatus for heating a hydronic heating fluid comprising a casing body having an inlet and an outlet adjacent to the inlet and a fluid flow path between the inlet and outlets. The flow path extends along first and second substantially straight sections with a return bend therebetween. The apparatus further includes a heating element located within one of the first or second sections of the flow path. The second end may include a threaded heater bore aligned with the second section wherein the heating element is threadably insertable into the second section through the heater bore. The first end may include a pump in fluidic communication with the inlet, the pump being operable to draw the heating fluid in through the inlet and to discharge the heating fluid through the first section of the casing body.