There are three types of solar radiant heating systems.
First is heating the water directly, this is difficult because the solar heat is not available when the heat is needed.You do not need a southern climate to take advantage of solar
Second is to store the heated water until it is needed. We have installed a few of these systems, the cost of the storage system makes this type not practical for radiant floor heat most of the time.
Third is to install a gas or propane combo heating and hot water system and supplement it with a solar panel. These systems work very well and are very cost effective.
but you do need a home with a very low heat loss and a carefully designed system.
In a solar assisted system the heat loss of the structure is the most important part to consider.
Smaller homes that are extremely well insulated and protected from solar gain in southern climates could benefit from combining solar and radiant, but they would need the structure to be designed around the heating and cooling system. In other words you design the heating and cooling system to take advantage of the available climate and geo features and then design the structure within those limits.
Solar radiant heat should be used in a slab it can not be used on a staple up system, because radiant staple-up requires at least 120° water.
See page 98 as one example for solar cooling.