The warmed air or liquid is then distributed to the rest of the house just as it would be with an ordinary furnace system. Solar collectors are usually placed high on roofs where they can collect the most sunlight. They are also put on the south side of the roof where no tall trees or tall buildings will shade them. Solar heating systems must have some way to store the heat that is collected on sunny days to keep people warm at night or on cloudy days. These materials absorb surplus heat and radiate it back into the room after dark.
Some passive homes have walls up to one foot thick.
Houses with active or passive solar heating systems may also have furnaces, wood-burning stoves, or another heat source to provide heat in case there is a long period of cold or cloudy weather. This is called a backup system. Solar Hot Water Heating Solar energy is also used to heat water. Depending on where you live, and how much hot water your family uses, a solar water heater can pay for itself in as little as five years. A solar water heater works in the same way as solar space heating.