A gas radiant tube heater and method of operating the heater. The heater has a housing and a radiant tube mounted within the housing. The heater further has a modular control box mounted within the housing in which serviceable parts can be easily removed as a unit and replaced. The housing and control box are sealed against the elements for outdoor as well as indoor use. The control box is divided into two chambers, with the inlet air flowing from a first chamber into a second chamber. The first chamber is subjected to a vacuum pressure, in which the heat sensitive equipment is mounted, and the second chamber is subjected to a positive pressure, in which the combustible gas and air mix prior to combustion. The heater further has a pressure switch to shut off the gas supply if the air flow through the heater is blocked, and a flame sensor to shut off the gas supply if the ignition element fails to ignite the air/gas mixture.