We are experimenting with a hypocaust system in our Roman dining room. We have 8 flues which take hot air from under the floor, up the wall and out under the eaves. To draw the hot air under the floor, we need to be able to shut down the flues near the furnace, but leave the furthest ones open. Also a baffle is needed to prevent the wind pushing the air back down the flue. The answer is these metal plates with a closed and open setting.