This Trane air handling unit photo shows a Trane AHU with electric duct heater above the Trane air handler in the supply duct. In places where natural gas is not available and propane LP gas is impractical or not wanted for use electric heat makes a good source for heating. This electric duct heater is is situated in the supply side of the air handler instead of being inside the air handler. In all situations when duct heaters are used the duct heater will not energize until air flow is proven. An airflow proving switch is usually used to prove there is air flow through the duct before the electric duct heaters can energize. Air flow proving switches can be used for either electric duct heaters or gas fired duct heaters. The air flow proving switch in this case is nothing more than a pressure switch across the duct heater so the pressure switch reads a negative on one side of the duct heater and a positive on the other side of the duct heater. That pressure causes a bellows switch inside the pressure switch to close and allows the duct heater to energize.