An immersion heater is a heating element, typically electric, that can be installed in a pipe or tank to heat fluids or air. To control temperatures and prevent overheating, an immersion heater thermostat is used to maintain desired temperature ranges. Some thermostats are included in the heater assembly, and some can be installed in a separate location if needed.
Electrical heaters are common for home or business hot water production, or heating air for room temperature control. The term "immersion" often refers to putting something in liquid, but can refer to air heaters if the unit is installed in the air stream. An immersion heater thermostat is used to turn the heater on and off as needed, or in some cases can be used as a safety control to shut the unit off if it overheats.
Many immersion heaters are devices that look like tubes with a threaded fitting and a control box on one end. The heater element, which may look like a tube in a U-shape, contains electrical wire that becomes hot when electrical current passes through it. Heating occurs because the wire material has high resistance, and changes the electrical current to heat, like in a toaster.
The threaded fitting allows the heater tube to be screwed into a pipe or the side of a tank. When the tank is filled, the heater is immersed in the tank contents and will heat the contents of the tank. The immersion heater thermostat can be preset by the manufacturer, or some units can be controlled by the user, a common feature in hot water tanks.
Steam can be used to heat materials, with the steam immersion coils installed inside the tank. A control valve can be used to adjust steam flow, and is controlled by the immersion heater thermostat installed somewhere in the tank. As heat is required, the thermostat sends a signal to the electrical control valve, which opens and allows more steam into the heater coils.
The immersion heater thermostat can be manufactured in different ways. Before digital electronics, bi-metallic strips were common for temperature control. Two different metals were fused side-by-side in a strip, and as temperatures changed the two metals moved at different rates and could be connected to thermostats or gauges. If one end of the bi-metallic strip was installed next to a switch, the coil would move and activate the switch when desired temperatures were reached.
Digital electronics became more common in the late 20th century as solid-state electronics prices fell. A digital thermostat uses an electronic switch that is temperature-sensitive, and will open and close as needed. There are no moving parts, and the devices can be produced with sensitive temperature control.
Replacing an immersion heater thermostat may be possible in some units. Some units have the thermostat built into the heater connection box, in which case the entire heater would be replaced if the thermostat failed. In other units, the thermostat may be installed as a separate part of the heater unit, or at a different location in the tank, in which case it may be replaced separately from the heater unit.