Manual draft controls work on outdoor furnaces but they are not efficient. All drafts other than forced air burn cooler, which means wet wood won?t burn as well and creosote is harder to burn off. The best way to compare the difference between type of draft units available is to think of a blacksmith working with steel; without a bellows blowing air into the fire, the heat is not hot enough to bend or shape the metal. Hot fires give more choice of wood conditions, they burn creosote and above all they reach total combustion. If you don?t like to empty ashes, buy a forced air unit. Forced air units heat approximately 40% more square footage, based on size of firebox and water contact square footage. Manual draft dealers will try to con you into believing that air injection furnaces simply blow out all the hot air and are inefficient. Watch the smoke exiting from both - the velocity is about the same.