system. The ability to circulate air to the building directly through the collectors is one of the major advantages of this system. The rock-bed storage also works best with a warm-air system.

Although warm air as low as 100F can be used to heat an occupied building, most existing warm-air systems are sized assuming warm-air temperatures of 120F to 150F. Typical midday collection temperatures usually range from 130F to 170F. Maximum storage temperatures are typically around 140F at the end of the collection period. Thus the heating load can be met by the temperature of the solar-heated air for a large portion of the day. When storage temperatures are insufficient to maintain the desired temperature of the building heat from an auxiliary source must be added to supplement the solar-heated air. The auxiliary furnace is located downstream from the rock bed, so the rock bed serves as a preheater for the furnace. This arrangement allows the rock bed to deliver useful heat until all of the rocks are at room temperature.

An air handler unit provides the dampers and blowers necessary to direct air circulation between the solar collectors, rock bed, and building. An air handler unit may be more expensive than the combined cost of individual dampers and blowers, but it is probably less expensive to install. It is also more compact.