What does “three doors and one panel” in a heating furnace refer to? How do the opening sizes of the flue dampers and air valves in a heating furnace affect operation?

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The “three valves and one plate” of a heating furnace refer to the throttle (including fuel gas), valve, air valve, and flue damper. When the flue damper is opened wide, the suction force increases, which leads to more heat being carried away; this results in high losses and low efficiency. At the same time, there is an excess amount of air, and this excess air enters the furnace, causing oxidation and peeling of the furnace tubes and thus reducing their service life ； If the baffle opening is too small, combustion is incomplete, smoke appears inside the furnace, and it may even lead to positive pressure backfire. If the air damper is set too low, the amount of air entering the furnace decreases, resulting in a weak and diffuse flame, incomplete combustion, high levels of carbon monoxide in the exhaust gases, increased fuel consumption, and low thermal efficiency of the furnace. Therefore, in practical operation, the dampers of the heating furnace and those of the flue must be adjusted in close coordination to ensure a sufficient draft, control the excess air coefficient, improve thermal efficiency, and extend the service life of the furnace tubes.