A duct heater has a unique and symmetric arrangement of an auto limit and a pair of back up limits to permit the duct heater to be mounted in a duct in a number of different orientations. The duct heater can include a diffuser screen and have a particular size with respect to an upstream piece of equipment such as a single duct variable air volume damper box so that it can be used in these types of applications as well as stand alone applications in a duct.