Active solar thermal installations come in two main types for space heating: liquid-based and air-based. The third type – high-mass systems – is completely different to active systems and deserves separate treatment.

Liquid-based active solar space heating systems work either with or without storage. Systems with storage have one or many insulated storage tanks, usually filled with water or sand. They use these tanks to retain the captured heat and distribute it throughout the home as required. Systems without storage dump the generated heat directly, though they still use the house itself (and the air within it) as a storage medium of sorts.

As you might suspect, liquid-based space heating systems are really just extended solar hot water installations. They gained in popularity in the 1980s and are still popular now, especially since several innovations and advances since their introduction have made them more practical.

If you've researched solar thermal hot water systems, you'll know that they are among the most cost-effective renewable energy installations possible. Solar thermal space heating systems come second. They don't get the top spot because they're not used year-round, but they tie in very well with a hot water installation and can be set up at the same time.

Liquid-based solar thermal space heating systems capture the sun's radiation in a typical solar collector array. If the system has a storage medium, the heat is passed to it; if not, the heat is sent straight to the house. Whichever way the heat is transferred, it ends up in some type of radiant floor, radiant panel or other liquid-to-air exchanger mechanism, which heats the home.

Systems without storage only heat the building when the sun is shining and the heating is turned on. Those with storage can keep the heat for later and provide it even when the sun is no longer shining.

If the house has a combination hot water and space heating setup, the excess energy from heating the domestic water supply can be directed into the space heating outlets as a shunt load.

It is important to note that the angle of the collector array is extremely important in installations which only provide space heating, especially those without storage. Since these systems do not operate at all in hot periods, the collectors are usually angled to capture as much of the low winter sun as possible.

There is also a serious risk of overheating when no storage is installed – both in the collectors and the house – so the array is often tilted to a less than optimal angle, or even at minimum output (vertical) to avoid problems during hot months.