Liquid based solar heating systems use a liquid, usually water, which is used as a heat transfer fluid. In places where it gets cold at night, like below freezing, water is mixed with an antifreeze to keep from freezing. The liquid is pumped through a solar collector where it is heated by the sun and then pumped to a storage tank or directly to the living space. Once the liquid gets to the living space it goes to a heat exchanger, or radiator, much like the old steam radiator systems in older houses. From the radiators or heat exchangers it is radiated or blown, using fans, into the living space.
There are other ways to distribute the heat collected from a liquid
based solar system. Heat can also be distributed in a radiant floor
system where the solar heated water is circulated through pipes embedded
in a concrete slab floor. The heat then radiates upward in the room.
This type of heating system is considered one of the most comfortable
heating system since the floor is nice and warm and the heat radiates
upward instead of blowing out of vents.
Flat-plate collectors are the most common collectors used for liquid-based solar heating systems, but evacuated collectors and concentrating collectors are also available and becoming much more affordable. Flat-plate collectors are the cheapest but evacuated tube collectors and concentrating collectors can produce much higher temperatures.
Liquid systems usually store solar heat in tanks of water. The cheapest and easiest tanks to use are standard domestic water heaters, like the one you have in your home. They are designed to meet building codes for pressure vessel requirements, are lined to prevent corrosion, and designed so it is easy to attach pipes and fittings.