If a site has sun exposure, Solar Thermal Collectors can work well with a hydronic heating system to supply a percentage of your heating requirements, in both space heating and domestic water heating. While a 100% solar heating is generally not practical, a system can be sized to handle basically all of domestic hot water needs in summer and a large part of the space heating into the fall and early spring.
Solar Collectors are installed in an area of direct sun exposure, usually the roof for space and sun exposure considerations. Water is pumped through the collectors and back directly into the hydronic system or into common heat storage. One good way of doing this is by using solar powered pumps that are powered by photovoltaic (PV) panels on the roof with the Solar Collectors. When the sun is shining on the collectors, the sun is also powering the pumps for an effective control system.
Solar Collectors are capable of 80% heat recovery. As such, consideration must be given to heat disposable in the warmer months or limit sizing of the system. Another technique is to use a drain-down system that drains the collector when the heat is not required. Solar Collectors are built and tested to handle the high temperatures that can result with no flow.
A drain-down system requires a bit more attention to detail, both in design and installation. The collector-circulating pump has to be sized to provide the additional lift head until flow is established, and the lines have to slope to totally drain on shutdown. A major advantage of the drain-down system is that it does not require antifreeze. Antifreeze adds its own complexity. When glycol antifreeze overheats it can get acidic which is not good for the copper in the collectors or the rest of the system. Has been said, the only good thing about antifreeze is that is doesn’t freeze
Hydronic Heating Design can analyze your site and recommend solar applicability.
A point of note: Why not remove the complexity of the Solar Thermal Collectors and use straight Photovoltaic Solar Collectors and generate electricity that would be used to heat the water? Not to downplay the use of photovoltaic, but it comes down to efficiency. PV panels are approximately 10% efficient, meaning that they convert 10% of the available solar energy that strikes them into electricity. This compares to 80% efficiency for water solar collectors, which means that 80% of the solar energy is converted to heat. If heating (space heating or domestic hot water) is the ultimate goal, then thermal collectors have the advantage.
For information, there are combo systems being developed that can do both PV and Thermo. These might be outside most of our budgets but perhaps a combination of existing PV and Solar panels can work. Locally produced energy is a good idea no matter the form.