Many legacy hydronic heating systems use a fossil fuel boiler to supply fin-tube baseboard heat emitters in some areas of a building and lower-temperature radiant panels in other areas. The boilers in these systems are typically set up to supply high water temperatures in the range of 180° to 200° F for the baseboard. One or more mixing valves are used to create lower water temperatures for the radiant panel circuits. Figure 1 shows an example of such a system with two high-temperature and three lower-temperature zones.