I always loved this idea -- heat the floor and distribute warm air throughout the ground floor at the same time. It is a practice that is legal under certain limited conditions, but the problem is that it dries out the wood in the floor space so much that, when a high-temperature furnace is in operation, the floor joists can burst spontaneously into flames. I was disappointed to learn that too. If you are 15 to 20 feet away from the furnace itself (cooler air), and it is an owner occupied home (you pay attention to things more), you can use this to successfully heat the floor of a bay window.
It is an excellent means of heat distribution for low temperature space heaters such as heat pumps and solar heating.
If you do use this technique, make sure you have a well sealed plastic sheet on the soil to prevent this heating air from picking up soil gasses and conveying them into the house.