Abstract: A fully submersible immersion heater having reduced dimensions. The immersion heater includes a waterproof housing, an electric heating element, a thermostat, an end cap, a temperature control, a power cable and a sheath. The waterproof housing has an opening at one end. The electric heating element, which generates a heat output, is placed inside of the housing. The thermostat is also placed inside of the housing. The thermostat is electrically connected to the heating element for regulating the heat output of the heating element. The end cap sealingly engages the opening in the housing to form a watertight seal between the end cap and the opening in the housing. The end cap has a first conduit and a second conduit that extend through the end cap from a first surface of the end cap outside of the housing to a second surface of the end cap inside of the housing. The temperature control, which controls the thermostat, is seated adjacent to the second conduit.