Good question. What I am about to write is not condoned by any building code that I know of. Any public health officials skip this comment. Same for any water treatment operators. The floor loop is tapped directly into the bottom and top of the water heater. The floor loop water is pumped through the water heater as needed. The water that heats the floor could be giving a shower in the next instant. The hot water tank temperature is set to medium temperature on the dial. Nothing extremely hot or anything out of the ordinary. Do not do this at home unless you are comfortable with the methods to keep pathogens from growing in fresh water. Not to recommend anyone do this, but the key is to keep the water in the floor loop circulating at all times. My judgment was this would prevent health hazard from the water. We have been doing it since 1990/91 and no trouble to date. This includes when the floor is drawing no water from the tank. There are a lot of smart people on this forum and I know the arguments against this. I agree with those and do not recommend this type of system for use in any application.