An essential part of any coal-fire furnace is the metal grate on which the fuel is burned. The grate should be designed to allow sufficient primary air to pass upward through the fuel bed for the combustion process. The ashes will drop to the ashpit below through the same openings when the grate is shaken. There should be a metal heat transfer surface (heat exchanger) of sufficient size above the fire to transfer the heat from the combustion process to the water or air in the heating system.

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