Combustion gases are circulated through the heater and oven chamber by a fan. An overflow or vent removes part of the spent combustion gases to compensate for fresh combustion gases added by the burner. Flue-type ovens are ovens which burn fuel in a furnace which is connected through flues which carry the combustion gases to a stack. Steam-tube ovens are ovens which are heated by a group of tubes which are partially filled with liquid and sealed at both ends. A small part of each tube is exposed to the heat of a furnace and the larger part placed inside the baking chamber.
Heat is transmitted by evaporating liquid in the furnace and of the tube.
Steam thus formed travels to the other end of the tube where the steam condenses and returns to the furnace by gravity. Gases of combustion are circulated through this enclosed system and mixed with fresh combustion gases generated by the burner in the combustion chamber. A vent or overflow removes a portion of the gases to compensate for the fresh gases added by the burner. No unburned gases or products of combustion have access to the baking chamber. Electric ovens are ovens which are heated entirely by passing an electric current through resistance elements.