Assuming that an air preheat system is installed, there are two control loops required. The induced draft fan should be used to control draft at the heater arch (immediately below the convection section) at ~2 mm water gage negative pressure. The forced draft fan should be linked to stack oxygen. Actually, the reading is not taken from the stack but at the arch. This is for safety reasons since any air leaking past the convection section maintenance access doors will give a false high reading in the stack and risk incomplete combustion. Peak energy efficiency is achieved by using VFD on both fans, with dampers wide open.
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