Heating - without electric

I have been surprised by how much heat they do give out.

Its still only 40W-80W - naked flame feels hot of course!

Newton is so old hat, I much prefer jrko's 1st law of thermodynamics

Newton is so old hat, I much prefer jrko's 1st law of thermodynamics

Me too, although I've enjoyed reading all of the thread! 

This is what we did to my friends greenhouse. Over a few weeks we went skip diving and collected old disused plastic down pipes and some dumped polycarbonate twin wall roof sheeting. He then dug out his growing areas on each side to a depth of about 450mm.
We then laid the roof sheet in the bottom and around the sides for insulation. We then laid the plastic down pipes on top of the roofing sheet and connected some bends to both ends of the pipes.
The in-take pipe is at high level near the roof and the exit is at ground level.
 We then back-filled the growing trenches with new poo and organic matter and soil.
He purchased 4 number 12v 5watt 100mm diameter fans. Two of the fans were blowing downwards into the pipes at high level, and the other two were drawing the air through at low level and expelling it at low level. These were then connected to a 12v 25watt solar pv panel that was pointed south at 22 degres.
The greenhouse was then insulated with bubble wrap and everything in it was pianted matt black.
When the sun comes out the greenhouse heats up very quickly, The brighter the sun the faster the fans run. When there sun goes in the fans slowdown or stop. All the hot air pumped through the burried plastic pipe is transferred into the soil and used as the thermal store.
In the summer the fans are moved to a high level vent and are used to pump the excess hot air out of the greenhouse.