The hot air is then ducted into one or more rooms using a small fan, which is usually the only electricity required for the system to work. There can also be a temperature controller that only operates the fan when the air in the heater box is warm enough to have a reasonable heating effect. While there are many variations on design, almost any flat, sealed and glazed box will produce enough heat to have some warming effect. Variations include systems that duct heat from the solar collector to thermal mass storage below the house, for example a sealed box full of building rubble. The heat can than slowly radiate upwards into the house during the evening and on cloudy days.
Some systems use water to collect and store heat, using collectors similar to solar water heaters.
The heated water is stored in a large insulated tank, and is then pumped through hydronic radiators in the house during cooler periods. On clear, sunny days this type of heater is very effective, often producing more heat than is required to warm a home. Heat production is reduced during overcast conditions, and will cease altogether on very cold, rainy days.