The Aquahot actually uses a non-toxic "boiler type" anti-freeze, which is I different than the I glycol antifreeze used in our engines. This is to insure that if there is a leak between the boiler and the domestic water heating coils, we won't be poisoned. The engine coolant. Does not mix with the boiler fluid, but is heated in coils just like the domestic water. The boiler fluid also circulates thru the heat exchanges to make the furnaces work. I'm another one who would not buy another coach without Aquahot. We almost always use the electric element to heat ours, and have only serviced mine once in six years, and then it was still working fine before the service. I actually got a bad nozzle when I serviced it and had to replace it the first time we used it.