A duct heater is a heavy duty modular unit installed in the ventilating ductwork of commercial, residential, and institutional HVAC systems, although smaller "duct" heating elements are used in the forced air heating systems of vehicles. Duct heaters can be complementary to a central heating system that utilizes forced air heating, also known as forced circulation heating or convection heating, in which air is the medium that transfers heat. Fans and blowers are integral to the transfer of heat; their motion conveys the air through the duct heater.
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Duct Heater Design and Function
Duct heaters work with existing central forced-air heating systems to preheat fresh air or returned air. The central air handler filters the air, re-heats it, and blows it through the ductwork. The conditioned air is distributed throughout the building and is then returned to the central air handling unit (AHU). Electric heater manufacturers generally construct duct heaters in rectangular or square box configurations, but they can create rounded ones for the best fit in certain sheet metal ductwork. Consisting of two major components, the heating elements and the electric terminal within the boxed framework, duct heaters can be bottom mounted, flange mounted, round collar mounted, or slip-in and are easily installed in existing or new HVAC filter systems, with the heater control panel mounted in any convenient location.
The two types of heating elements used in duct heaters are open coil and tubular (standard and finned), each offering certain advantages. Open coil elements are coils of nickel chrome wire that are popular due to excellent heat dissipation. However, tubular elements are more suited to demanding environments, as they are sheathed with protective covers of metals, such as aluminum, iron, stainless steel, or steel. Because of their sheathes, tubular elements are slower to respond but can hold up to humid and dusty atmospheres, and fins are used to help more quickly distribute the heat. Sheathed elements, especially finned varieties, offer a long life with less maintenance and lower risk of fire and electric shock. Choosing the right heating element, electrical components, and controls is essential to achieving the highest economy and efficiency from a duct heater. Temperature sensing duct heaters can be encoded to regulate electric heat output based on a desired temperature, and modulating controllers can allow successful duct heater operation in even extremely low air flow. Plus, electronic enclosures act as moisture/debris-resistant and explosion-proof housings.