The Complete Handbook of Duct Heaters

Chapter 1: Understanding Duct Heaters

A duct heater is a device installed inside an air handling duct that raises the temperature of the air passing over it. It is a controlled, direct source of heat that uses the existing ductwork as the delivery mechanism. The term covers a broad family of equipment, from a 1.5-kW electric coil in a residential supply run to a 500-kW hydronic unit heating a factory's outside air intake. Despite the range in size, the fundamental job is always the same: transfer heat from a medium—usually an electric element or a hot water coil—into an airstream, and do so safely and efficiently.

Consider a typical commercial scenario to ground the concept. A retail store in a northern climate has a rooftop make-up air unit (MAU) that brings in 4,000 cubic feet per minute of fresh air to satisfy ventilation requirements. In the winter, that outside air is 10°F, and the store's thermostat is set to 72°F. Without a duct heater, the incoming air would drop the space temperature and create drafts. The solution is a duct heater installed within the MAU's supply duct, sized to add 624,000 BTUs per hour (about 183 kW) to the airstream. This is not a trivial add-on; it is a critical component that makes the ventilation system usable in cold weather. The heater is also a safety device, with high-limit temperature switches that will cut power if the airflow stops or if the outlet temperature exceeds a safe threshold. This one