Now that our roof has been fixed, we can fix our damaged ceilings, and remove popcorn throughout the house. To enhance the look of our ceilings, we'd like to cover them with panels similar to wainscoting panels. Looking through the different options, I had a new idea for heat distribution. Wainscoting can be installed in walls of any kind of room, there are styles available for kitchens, living rooms, bathrooms or bedrooms, so we can conceivably install wainscoting panels throughout the house. We could use those panels as heaters, if we install a pex loop with heat spreaders behind the panels. That will give a significant surface, and will completely hide the heating loops. Some precautions would have to be taken during installation, but that should be a lot simpler than radiant floors, and more efficient too, since the wainscoting panels are rarely covered, unlike a floor. The cost of material for one room is below $100, so this is also a cost efficient option.
The heating system may have just 4 loops: 1 loop for each living room (because they have a wood stove), and one zone for all the other rooms, upstairs, and downstairs. The thermostat for the other rooms may be installed in each master bedroom (upstairs and downstairs).
So this idea put the focus back on radiant heating, since it becomes cost competitive with forced air.
The electric furnace would stay as the backup heat. No modification of the forced air system needed. I may upgrade the forced air furnace with a sequencer and a static pressure sensor controlling the blower speed, to allow for better zoning.
It seems the key to the efficiency of the solar system will be the size of the storage tank, which will likely be integrated inside the wall that will be built to separate the downstairs living room. The available tank size, removing the volume taken by insulation, is 2'x4'x12' = 1440 gallons. The tank will use a 12'x24' liner, and a heat exchanger made of 10' long copper pipes, for hot water, unless a separate solar hot water system is installed.