If the firebricks are in good shape it should burn bit coal with minimal if any soot. The hollow corners of the firebrick provide
heated combustion air to the fire, burning off the volitiles..
I don't believe the DS heats the secondary, over the fire air, and this prevents the complete burning of the volitiles.
Unless the price on the 818 is very high, I think you wouild always be able to get your money back if it doesn't work well with your