Solar thermal can provide up to half of your space heating if designed correctly. I have a simpler set-up at my house, where I am using solar water heating with a fan coil heater. A thermostat is connected to the second relay on my differential temperature controller (Heliodyne Delta-T Pro), and if the storage tank is hot enough, and the thermostat is calling for heat, the relay turns on the fan and pumps to blow hot air into my living room. It works amazingly well! I'm putting data together later today showing the results.
A bit more design work is needed to integrate it into an existing heating system like yours, be it forced hot air or radiant floors. You need to get the timing right to have the original heat source turn off when you are sourcing from the solar, to make sure you don't inadvertently heat the solar with your furnace. You would set up a primary and secondary loop, where the solar would provide the heat until it runs out, then your backup heat would take over.
Sizing the solar system depends on a lot of variables, like how tight of an envelope you have in the house (is it well insulated, good windows, etc?)and where you are located. My house is in New England, so we get long, cold winters. The house is 760 sqft, one story, fairly well insulated with new windows. I used four 4'x8' collectors, 128sqft, or about 20% of the footprint. I used a 120 gallon storage tank (1:1 ratio sqft of collectors to gallons of water), but 2 would be better (unless it was being used for concrete radiant floors, in which case the floor can act as storage). I haven't received my latest electric bill since turning it on last month to supplement my electric heat, but I suspect it is easily providing half my heat. If you want to see details and pix of my system, you can check it out at
http://www.altestore.com/gallery/Amy-Beaudet-gallery/v/9358727169/amys_water/.
