Radiant floor heat systems use solar power to heat water, which is then pumped through your home's floor. You can use solar-heated water to heat your home off-grid. Radiant heat, without solar, costs much less than forced-air heating for efficiency reasons.
Installing a radiant heat floor system is almost surely not a do-it-yourself project, but it merits elaboration because it's such a complete and effective way to use solar energy water heating. A snaking closed loop of metal or plastic tubing runs beneath your floor. When hot water flows through the tubing, the heat radiates upward through the floor and into the room.
When you supplement your radiant heat system with a solar water heater, you can drive your heating costs down to nearly nothing. Of course, sunshine isn't very reliable, particularly on the coldest nights when you most need heat, so you can't use solar heating exclusively; it can only be a supplement. But it can be very effective. Plus, it's also the most comfortable way to heat a home.
You can use any type of water heating system with a radiant floor system, but the capacity of hot water that you use goes up dramatically with a radiant heat floor system. So the attraction of a solar system also goes up dramatically, because you get much cheaper hot water per BTU.
At the very least, radiant heating reduces your carbon footprint. When you combine it with solar, the pollution savings can be impressive.
The engineering is complex, and the installation is clearly not for the faint of heart. There are technical problems, of course, but the systems have been in use for a long, long time. New technologies are making these the system of choice for a lot of homes.
Here's why: With conventional forced-air systems, hot air comes in through the vents and immediately rises to the ceiling. That's not where you want it, so you need to either pump in more heat than you really need (inefficiency) or use overhead fans to move the air back down (inefficiency). Moving air makes you feel colder, and you get stuck listening to blower noise as a big machine goes on and off all night. Furthermore, heated air dries out very fast, so your lips dry up, and your skin gets tight.
With radiant floor systems, there's no blower noise, wind chill is nonexistent, and you don't have to mess with HVAC filters. The big benefit is that the heat is in the room — the floor and furniture — not just the air. You can adjust your thermostat to a lower temperature in a radiant house and achieve the same comfort level because the floor and furniture are where the heat is. Where you set the thermostat is a question of comfort, not numerical temperature.
If you're planning on a room addition to your house, consider using a radiant floor in that room. Your existing HVAC system likely won't have enough capacity to heat an additional room. Adding a solar water heater to your house and using your domestic heater to heat the radiant floor in the addition works wonders, and it's usually cheaper than adding another small HVAC system.
You can also cool your house with radiant flooring. It doesn't work quite as well as heating, but if you have solar panels, you can use these at night to cool the water that's already in the closed loop of the radiant floor system. The reason the collectors will cool is simply because they have so much area, and the heat will escape into the cool, nighttime air. This is especially true if a breeze is blowing.