Induced draft fans or ID fans are used in systems such as steam boilers and thermal oil heaters to draw out and remove flue gases from combustion chambers, by creating a vacuum of negative air pressure. When mechanical ventilation is supplied to these boilers, the heat transfer rate increases; the boiler can be reduced in size to produce the same amount of energy as a natural draft. The induced draft fans used in combustion systems are normally high temperature and extra heavy-duty construction.