Space Heating and DHW Systems

Space heating systems are a simple extension of the domestic hot-water (DHW) systems (fig. 15-10). The collectors and storage tank need to be resized to provide greater loads. A heat delivery system is added and the auxiliary heater (or existing heater) is connected into the system as backup. The design of the space heating system, if a retrofit, depends on the existing system. Water-to-air heat exchangers may be placed in existing ductwork, in which case, an unpressurized, unlined tank may be used. This represents a minimum heating system, as shown in figures 15-11 and 15-12.

Domestic hot water could be added to the system shown in figure 10-33 by adding a preheat coil in the storage tank. Figure 10-31 has the potential to provide some cooling to the building by using the collector at night to radiate heat to the sky and store cool water for use during the day. A heat pump is another option that could be used to cool the building, reject the heat to the storage tank during the day, and then, as before, cool the tank at night through the solar collectors. Unglazed collectors are superior to glazed collectors for this application. There are many variations that can be used with the configurations shown.

Air types of space heating systems are receiving increased attention, and a typical system is shown in figure 15-14. (See table 15-1 for advantages of air versus liquid.) The heat storage tank is replaced by a rock bed (nominally 1 to 3 inches in diameter). Rock provides desirable temperature stratification. Designs should emphasize the minimum pressure drop through the rock bed. The rocks can be stored in a bin, which should be insulated, or beneath the building if this is feasible. Heat collected by the collectors is blown through the rock bed from top to bottom. Heat is delivered from storage to the building by circulating air in the reverse direction, bottom to top. Note that in contrast to water storage, heat cannot be added to and removed from the rocks at the same time.

During heat collection, the rocks at the top of the bin attain a temperature almost equal to that of the incoming solar-heated air, while the air leaving storage is delivered to the collectors at the minimum temperature of the rocks. The conduction between the rocks is small; thus, with no air circulation, the rock bed remains stratified with the top of the rock bed warmer than the bottom. Also, limited conduction and convection in the rock bed significantly reduce heat loss from the rock bed.

Heat is drawn from storage by circulating air from the building directly through the rock bed from bottom to top. The air is delivered to the building at a temperature near the maximum temperature of the collectors. If additional heat is required, supplementary heat is added downstream from the storage unit. This system allows the rock bed to deliver useful heat until all of the rocks are at room temperature.

A variation is a no-storage air heating system that circulates heated air when available. Performance is limited to daytime heating because of the lack of storage, but such systems are well suited to warehouses and factories with daytime operations.

Domestic hot water is provided by pumping the water in the preheat tank through an air-to-water heat exchanger placed in the return air duct from the collectors. This is not efficient and is one of the disadvantages of the air system.

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