Passive solar house design is an excellent way to heat a home. Generally it's designed into the home from the ground up, with south-facing windows, thermal mass to store heat, lots of insulation, and thermal blankets for the windows at night. DanF's home was designed this way. Until we do get a page up here about passive solar heating, you can search the internet for the term and get a wide variety of books and articles on the topic.
Solar hot water heating panels became very popular in the USA in the late 1970s and early 1980s for preheating a homes hot water. Government tax credits were offered by the Carter administration, and many companies sprang up designing and installing systems. When the Reagan administration cancelled the tax credits, many of these companies went out of business, and there were very few people left to maintain and repair these systems. Many of them are still sitting on roofs today, broken. HOWEVER, it became obvious very quickly that some of these companies were 'fly-by-night' and their products were poorly designed in the first place:
In general, we advise against purchasing these old systems used. It's actually easy to find one for free, which might be OK if you are experimentor and enthusiast. But these older systems used numerous active controls, sensors and pumps, and were very unreliable compared to the excellent, low-maintainance and reliable systems available today.
Solar water heating panels are NOT a new idea! This article about the history of solar water heating is an excellent read. Thousands of systems were installed and were very effective in sunny parts of the USA around 1900.
On the other hand, modern solar water heating panels are an excellent investment in most parts of the county! The technology has changed so much that these systems are now completely different in design than those from the Carter area that caused so many problems. Some industry analysts say that the loss of the tax credits, while it had a large negative impact on the industry at the time, ended up being the best thing that ever happened to the industry in the long run.
These systems are now often incorporated along with radiant floor heating pipes. The sun heats the fluid, which circulates inside the high thermal mass floor of the home. This is a VERY efficient way to heat a home!
Solar air heating panels are another new technology that's catching on quickly. It allows the homeowner to retrofit a house that was not orginally designed for efficient passive solar heating, and reap the benefits and energy savings of heating with the sun.