One of the oldest known man-made solar thermal space heating systems was built by the Romans, who constructed radiant floors for their bathhouses over 2,000 years ago. The systems heated the bathing water and the buildings by circulating hot air under the floors, then up through multiple chimneys. Today, radiant floors are not only the most comfortable heating system available, but the most economical – especially when tied into solar thermal heat generation!
The main difference between a high-mass and a "normal" solar thermal space heating installation is the medium used to store heat. In the standard setup, a closed solar loop carries heat from a solar collector array to a hot water tank; in a high mass system, the heat is carried to and stored in sand underneath the home, by means of Pex tubing. That heat slowly rises, warming the entire building. High-mass installations can also heat a domestic hot water supply.
The first thing most people notice is that sand is a poor heat conductor and a poor heat storage medium - the huge sand pit underneath the building will take a very long time to warm up completely and will never get very hot. So why do we use it? Because we can turn those traits to our advantage and make them work for us.
A high-mass system can begin charging in summer, so it is vitally important that you complete your installation in spring. Depending on the size of the storage pit and your local climate, you may never need to turn the system off – though in many locations, it is switched on in late summer and harvests heat all the way through fall and winter, into spring. The collectors are angled at maximum tilt, to optimize the winter sun's energy and minimize gathering in summer, and the installations do not use a thermostat (they are "unregulated systems" in that sense).
Buildings with a high-mass installation rarely (if ever) overheat because the sand isn't much hotter than the air in the house, at least compared to fluid-based systems. They self-regulate because the heat is released slowly but constantly. However, this does mean that you will definitely need a backup system to top off the heat without using the radiant floor. The good news is that the backup doesn't need to be very big, as it will only be needed in very cold weather.
As an added benefit, you should no longer have any condensation problems. The high-mass sand bed will warm the basement enough to eliminate the problem, though if the bed is on a grade, you should allow the system to always heat your floors a little, even in spring and summer, to keep humidity under control.
High-mass systems do not work well in houses with carpets and other floor insulation. They can work with bare hardwood floors (parquet and so on) but they are much better if there is nothing between the radiating slab and the room. Appropriate tiles can be used if the slab is unattractive, as they pass heat very well.