Sometimes the space heated directly by the sun is separated from the rest of the house. In the 19th century these spaces were known as conservatories, and later as solariums. Today, they are called attached greenhouses or sunrooms. These rooms actually date back at least to the Romans and were used to grow the Emperor's vegetables during the winter.
Because an attached greenhouse is warmer than the rest of the building, convective air flows circulate the warm air to heat the house. Entry ways can be used as airlocks, with separate interior doors closing off the living areas.
In a greenhouse, operable windows or vents are necessary to exhaust excessive heat during the summer and sometimes, in the winter. Depending on the types of attached greenhouse and the amount of glass it has, some interior shading might be necessary.
Attached greenhouses can be used to supplement a home's heating needs, but most structures still require some type of small-sized backup heating system.