I want to put a toilet right where there is a heating duct under the floor. Moving the heating duct is not an option because of cost and the amount of labor required. This is the main furnace duct serving the entire home. But I have an idea. Why not use that heating duct for both supplying heated air, and toilet drainage. After all, a pipe is a pipe. The toilet water will flow across the bottom of the duct, and the air (being lighter), will flow in the top portion of the duct. There's no reason this duct can't be used as dual purpose, since it's fairly large.
Just before the duct enters the furnace, there is a main sewer line, so if I install a baffle at that point, I can divert the toilet water into the sewer and the upper portion of the duct will still serve as the main heating duct for the home.
Not only will this solve the problem of the duct under the toilet, but will save a large amount of money on that costly 4 inch sewer pipe.