The controls for this system turn on the electric boiler and its circulator at the start of each off-peak period. The boiler and its circulator remain on until the thermal storage tank temperature reaches 180ºF. Space heating can take place during this time, with excess boiler output routed into thermal storage. In this system, the panel radiators are sized to deliver design load output at a relatively low supply water temperature of 110ºF. Doing so allows for the thermal storage tank to be “discharged” to a relatively low temperature while still supplying adequate heating capacity to the radiators.
The amount of heat stored in the tank is 8.33 Btu/gallon/ºF. Thus, a 119-gallon tank undergoing a temperature discharge from 180 to 110ºF would release 8.33 x 119 x (180-110) = 69,390 Btus. This could represent several hours of heating in a low-energy or net-zero home, especially during partial load conditions.
The 3-way motorized mixing valve blends hot water from the thermal storage tank, or coming directly from the electric boiler, with cooler water returning from the radiators to achieve a target supply water temperature to the radiators. That target temperature is based on outdoor reset control. The latter allows the tank to supply the radiators down to the minimum water temperature that can still maintain comfort in the building. On partial load days, that temperature could be in the range of 80ºF. The lower this temperature can be, the greater the amount of heat the tank can deliver to the load on each discharge cycle.
If the tank cools to the point where the radiators can no longer maintain comfort, as determined by some minimum tank temperature, the electric boiler is enabled to operate regardless of which electrical rate is in effect. Boiler operation during on-peak periods should be minimized to keep overall operating cost low. This situation would typically occur during early evening hours, when the output of a solar electric system is low or zero, and internal loads in the building tend to diminish.
The alternative concrete slab floor-heating system adds substantial thermal mass to the system, extending its ability to maintain comfort in the building for longer periods when on-peak electrical rates are in effect. For example, a 4-inch-thick concrete slab can store 9.8 Btu/square foot/ºF of temperature change. A 6-inch-thick slab can store 14.7 Btu/square foot/ºF of temperature change. Thus, a 1,500-square-foot slab, 4 inches thick, and undergoing an average temperature drop of only 3ºF, can release 44,100 Btus.
A word of caution is appropriate. High-thermal mass distribution systems, although good at storing heat, and thus allowing buildings to “coast” through several hours without heat input from source equipment, are not well-suited to buildings where significant and unanticipated internal heat gains might occur. This type of system should not be used in buildings that are designed for significant solar heat gain, or where a large number of occupants or other heat-generating activities may be present at times.
It is also critically important to insulate all piping and components in areas of the system that are subject to high water temperatures. Be sure that all components used are rated to handle the highest water temperatures that might be present. The system shown in figure 5-6a also uses several spring-loaded check valves to limit heat migration within the system.
The thermal storage tank, piped as shown in Figure 5-6a allows the electric boiler to operate at or above its minimum flow rate, even when only a “trickle” of hot water is passing into the 3-way mixing valve. It also provides hydraulic separation between the boiler circulator and the distribution circulator. Another variation on the system is shown in Figure 5-6b.