Gleaned from an idea in one of the eco friendly books Here, the concept is to use the power of the sun to heat up rocks which in turn warm the air around them. Then use convection to duct the air into a building for free underfloor heating. It requires absolutely no input of energy apart from the sun so it is completely free and eco friendly. I think the French know it as a "duct canadien".
We had an ideal situation in the outhouse as there was no floor so putting in a duct was not a problem. The area infront of the building slopes away and faces directly south.
- Find a way of getting a 2"-4" duct into the building under where the floor is gonna go and make sure that it is inclined up into the building from where the box will be situated. We found the duct in a skip. We routed the duct under the door step and dug a shallow trench to the box site which meant that we could easily alter the gradient of the duct.
- Find a sheet of plate or toughened glass (coffee table top or a big glass shelf would do).
- Make a wooden box to fit so that the glass forms the "lid" of the box. On the basis that it is easier to make a box to fit the glass than cut the glass to fit the box. We used off cuts from planking the roof.
- Make a hole in the box to accept the end of the duct. This should be pretty near the top i.e. at one end near the glass lid.
- Make other holes in the other end of the box to let air in.
- Line the box with tin. We used corrogated iron as we had some and didn't want to spend money.
- Put the box in position on an incline (exit duct higher than the inlet holes).
- Fill with rocks so that air can still circulate. We grabbed rounded rocks out of the stream bed.
- Fix on the glass lid.
- Wait for the sun to shine.
We laid the concrete floor in the building such that the duct came up through it just inside the door. This could then be fitted with a closable grill if required. We battened the concrete and laid a pine floor so that the warm air then flows under the wooden floor.
Another idea is to fit the box with a wooden cover. If you want to turn the heating off just close the cover.
- Paint the rocks black to absorb more solar heat
- Using lots of small rocks is good if you want then to heat up quickly
- Using larger rocks acts a bit like a storage heater – they still heat the air after the sun has gone
- Using a double layer of glass makes the box more efficient, similar to the concept of double glasing