Your coal may melt and run down on the bars, but if the cold air can get to the grates, the only damage this will do is to form a clinker on the top of grates, and shut off your draught.

What would you think of an engineer who fed his engine dirt with his coal, or let his draughts and flues clog with soot, or failed to remove the clinkers, or let his engine get dusty and rusty?