To circulate combustion air through the burners and remove combustion by products from the furnace, ID systems use a fan that is housed in the exhaust. The term "pull" system is occasionally used to describe this kind of air supply. The fan must be built to withstand high gas temperatures, but these temperatures shouldn't be too high because the flue gases often pass through an upstream convection section that eliminates a substantial amount of energy before they reach the ID fan. By producing a vacuum of negative air pressure, induced draught fans, also known as ID fans, are used in systems like steam boilers and thermal oil heaters to pull out and remove exhaust gases from combustion chambers.