I recently inspected a very intricately designed 1980 home with Geo Thermal. The house has a main floor and a full basement built with engineered floor trusses. (high tec for 1980?) I pulled up a floor register on a hardwood floor to discover no duct work. I at first I thought it was a cold air return. I found out that this is the heat source. The owner/designer has charged the entire floor cavity with warm air. (Keep in mind this is geo thermal, so no risk of overheating, fire or even carbon monoxide) Where ever he wanted heat, he simply cut a register into the floor or ceiling. He says this works well, and keeps his floors warm too.
This is the first time running into this configuration, and it took me a minute to wrap my head around it. Have anyone else seen this or have anything to add?