Thermal mass is any dense material used to store heat—water and masonry are the most common. In passive solar houses, for example, interior masonry walls and floors store solar heat gained through south-facing windows.
This solar heat-storage can also use active solar hydronic collectors to feed heat directly into hydronic tubing embedded in masonry floors. Under proper control, the floors warm by day and discharge heat by night—to keep the home within the comfort range, thereby delaying or preventing the backup boiler from operating. It helps when the floors are well-insulated, both around the perimeter and underneath, to slow heat loss into the ground.
Common slab-on-grade radiant floor construction practices will work quite well as solar heat-storage floors. But they should be insulated underneath, between the warm concrete and the ground. Two to 3 inches of waterproof rigid insulation (“blue board”) is reasonable for improved heat storage, but more than 3 inches is probably overkill. Common slab thickness of around 4 inches works very well, and up to 8 inches is reasonable for extra heat storage. While thicker slabs will store more heat, they will operate at lower temperatures (possibly below the range for human comfort) and have a longer lag time.
Placing the tubing in the center of the slab usually works well. In Southwestern climates, that means spacing the PEX tubing typically 8 to 12 inches apart, and locating it near the center or below the center of the concrete with an approximate 4-inch thickness.
This is also used for solar heating in concrete swimming pools and spa tubs. When hydronic tubing is embedded in the floors and walls of a concrete pool, solar heat can be delivered in a controlled way, independent of the filter pump system.
In the Southwest, large heat-storage water tanks are only necessary when hot water baseboards or fan coils require it—but not when the house has hydronically heated masonry floors. Typical slab floors contain a tremendous amount of heat storage capacity—up to five times as much as a properly sized water storage tank. When the solar heat is delivered directly to the floor, the heat loss associated with the water storage tank and its pipes and heat exchangers is eliminated. This results in about 25% more solar heating available on a typical winter day. This makes the most efficient use of the heat provided by the solar collectors, which can be sized slightly smaller.