Abstract: A heater control system is operable to modulate a heater of a building or warehouse or the like, in order to maintain the temperature rise of air passing through the heater at or below a maximum allowable temperature rise value. The maximum allowable temperature rise value is determined by a percentage or proportion of outside air being inducted into the heater and the type of heater, in order to limit the accumulation of carbon dioxide within the building. The control system is operable to modulate the heater in response to an outside air temperature, an inside or return air temperature, and the proportion of outside air to return air being inducted into the heater. Preferably, the proportion of outside air is determined by a position of a damper, which is operable to vary the relative flows of outdoor air and return air which are inducted into the heater.