Hydronic flooring heating is where a network of water carrying pipes are embedded into the floor to transfer heat from a heat source (say solar hot water) to the floor - thereby providing a comfortable living environment. The only downside with such systems is that they depend upon the Sun to provide the warmth, this is often out of cycle with the demand for heat - i.e. you need to be warm in the evening and during cloudy days. Thermal mass can help phase shift the heating but you will often require additional back up heating as well.
This is often something which can only be done on a new build as you need to build the pipes into the structure of the floor. An option after build is to use a roof space heat recovery system to transfer the hot air from the roof into the living space by way of a filter and fan set up. The advantage with this is that such a system can also provide cooling air in Winter (as the roof space cools quicker than the living space below at night).