To get hot water for washing the dishes, I fill two big kettles with water. One goes on top of the vertical flue, and the other on top of the thin flat stone ceiling of the horizontal flue. But I have never tried if the heat of the stones would be enough to bring the water to boil. Then, on the other hand, not all the cooking methods require actual boiling temperatures, so I'd guess the horizontal flue would work for slow cooking, like simmering a stew. I have also noticed that the stones stay hot for couple of hours after all the fire is out - that's a proper improvement compared to plain fire.