Build your own solar water heater to provide hot water for household use as well as space heating within your home. The main challenge when building a solar water heater that will be used in a cold climate is the threat of water freezing within the system. The most common way of overcoming this problem is to install a glycol loop, with all of the exterior components of the system circulating a glycol antifreeze solution, which transfers heat to water inside the house through a heat exchanger.
Construct an insulated box with a glass or plexiglass top. Commercially made solar hot water panels feature circulation pipes that are set into a copper sheet, but this is beyond the technical abilities of most DIYers. Create a similar effect by arranging coils of flexible black PVC hose inside the box. Paint the inside surfaces of the box with flat black paint to maximize heat retention.
Drill two holes in the side of the box that are the same diameter as the PVC hose. Run both ends of the hose out the side of the box and into the house. Insulate the hose well with foam pipe insulation.
Attach the ends of the hose to the inlet and outlet attachments of an in-floor radiant heating system, if you are creating space heating. Plumb the hose into a self-contained heat exchanger if you are creating hot water. A heat exchanger is an insulated manifold in which the domestic water pipes and the pipes from the rooftop samples are sandwiched together. The heat from the glycol solution that circulates through the rooftop system is transferred to the water by convection. Install the heat exchanger in the water line just prior to its entry into your conventional gas or electric hot water heater; it will function as a pre-heater for the other heater.