3.1.3 The space heating system, like the domestic water heating system, must be backed up by an auxiliary heating system.
It is not practical to size a solar system to provide all of a home's heat requirement under the worst conditions. The system would become too large, too costly, and oversized for most of the time.
3.1.4 The storage system should be sized to approximately 1.5 gallons of storage for each square foot of collector area. The fluid that is heated and stored (typically water) can be distributed into the house heating system in the following ways:Air distribution system - The heated water in the storage tank is pumped into a coil located in the return air duct whenever the thermostat calls for heat. The controller for the solar system will allow the pumping to occur if the temperature in the solar heated water is above a minimum amount needed to make a positive contribution to heating the home. An auxiliary heater can be used in two ways. It can add heat to the solar storage tank to maintain a minimum operating temperature in the storage tank at all times. In this case, the coil from the solar system will be located at the air handler supply plenum rather than in the return air duct. The auxiliary heater can also be a conventional furnace that will operate less often due to the warm air entering the air handler from the solar coil in the return duct.
Space Heating System
Hydronic system with radiators - The heated water is circulated in series with a boiler into radiators located in the living spaces. Modern baseboard radiators operate effectively at 140 degrees. Solar heating systems can very often reach that temperature. Using the solar system's heated water as the source of water for the boiler will reduce the boiler's energy use particularly if it senses the incoming temperature and will not operate when that temperature is above the required distribution temperature.Hydronic system with in-slab heat - The solar heated water is pumped through distribution piping located in the floor of the home. Lower temperatures are used in this type of system (the slab is not heated above 80 degrees in most cases). The auxiliary heat can be connected in series with the solar system's heated output water or it can be connected to the solar tank to provide a minimum temperature.