Figure 15-12. - Space heating system with a closed-loop collector.

solar-heated water in standard baseboard heaters is impractical. Only modified baseboard heaters of adequate size or radiant panels are suitable for use in hydronic systems that use solar-heated water.

One economical means of auxiliary heat supply and heat distribution for liquid types of solar systems involves the use of a warm-air system. A typical system is shown in figure 15-15. In this system, the warm-air furnace is located downstream from a liquid-to-air heat exchanger supplied with solar-heated water. The furnace can then serve to boost air temperature when insufficient heat is available from the solar-heated water, or it can meet the full heat load when no heat is available in solar storage. Auxiliary heat can be supplied by a gas, oil, or electric furnace, or by the condenser of an air-to-air heat pump.

Another method of heat distribution is to use a water-to-air heat pump that draws heat from the solar storage tank and pumps it to a condenser coil placed in a central air duct. The advantage of this system is that it can effectively use heat from solar storage at temperatures down to 45F; thus more of the stored heat is available. Also, average storage temperatures are lower, resulting in significantly increased collector efficiency. Some manufacturers are combining solar systems with heat pumps to reduce auxiliary energy costs. When a heat pump and solar system are combined

Figure 15-13. - Space heating and domestic hot-water systems.