Polytunnel rocket stove - finished!
Posted: Fri Dec 02, 2011 9:55 pm
Well, I said I was going to try and build one. I finished it last week, and have fired it up a couple of times, and I'm pretty pleased with it. For those who haven't met one yet - it's a wood stove where the fire burns along a tunnel, then up an insulated chimney inside a barrel. The exhaust then runs through a metal flue covered with cob, which warms up and slowly gives off heat. More here if you're interested: http://en.wikipedia.org/wiki/Rocket_stove
I decided to build the whole thing underground in my 20x10 foot polytunnel. I used to use electric heaters to keep it frost free, but that's way too expensive First I mocked the whole thing up on the drive:
It's built out of bog standard engineering bricks, the wood goes in the hole at the front, the fire burns inside the middle bit, and burns up the chimney (a stainless pipe). It worked well on the drive, so I dug a huge hole in the polytunnel, and rebuilt the whole thing 2 foot underground, sticking the bricks together with a clay:sand mix. A barrel goes over the chimney - I used a recycled copper water tank. The fifteen foot long horizontal chimney (6" diameter) was built from clay drainage pipe, and is about a foot under the main poly tunnel bed. Here's some pics of the finished thing:
The only bits visible above ground are a 4" hole to stick the wood in, and the copper barrel. The fire is about 18" underground, and burns very strongly, but sideways!
The clay pipe is buried a foot under the middle of polytunnel. This pic's taken from the other end - a couple of CIDPs keeping warm in the middle A big mistake the first time I fired it up was not to water the bed first - the dry soil acted as a good insulator, so nothing really got warm... Watering it a lot helps spread the heat from the hot clay pipe to the soil.
The exhaust pops up just outside the polytunnel, with an old aluminium flower pot to put over it when I'm not using to keep rain and wildlife out. The exhaust is only just warm to the touch even when it's burning away at full tilt - all the heat goes from the clay pipe into the soil.
I'm using waste wood from my workshop to fire it at the moment, then will move onto the big stack of sticks from cutting the hedges. It burns very hot - difficult to get a good picture, but strange watching the flames burn sideways! I put a big brick over the hole to adjust the draft, and to close it off when I'm done.
A short fire of about an hour kept the polytunnel frost free during the week when it was -2C outside. I don't know how warm the soil gets (certainly doesn't feel hot), but the barrel is too hot to touch within a few minutes. I'm happy with it, and the best news - it's now free to heat the polytunnel!
I decided to build the whole thing underground in my 20x10 foot polytunnel. I used to use electric heaters to keep it frost free, but that's way too expensive First I mocked the whole thing up on the drive:
It's built out of bog standard engineering bricks, the wood goes in the hole at the front, the fire burns inside the middle bit, and burns up the chimney (a stainless pipe). It worked well on the drive, so I dug a huge hole in the polytunnel, and rebuilt the whole thing 2 foot underground, sticking the bricks together with a clay:sand mix. A barrel goes over the chimney - I used a recycled copper water tank. The fifteen foot long horizontal chimney (6" diameter) was built from clay drainage pipe, and is about a foot under the main poly tunnel bed. Here's some pics of the finished thing:
The only bits visible above ground are a 4" hole to stick the wood in, and the copper barrel. The fire is about 18" underground, and burns very strongly, but sideways!
The clay pipe is buried a foot under the middle of polytunnel. This pic's taken from the other end - a couple of CIDPs keeping warm in the middle A big mistake the first time I fired it up was not to water the bed first - the dry soil acted as a good insulator, so nothing really got warm... Watering it a lot helps spread the heat from the hot clay pipe to the soil.
The exhaust pops up just outside the polytunnel, with an old aluminium flower pot to put over it when I'm not using to keep rain and wildlife out. The exhaust is only just warm to the touch even when it's burning away at full tilt - all the heat goes from the clay pipe into the soil.
I'm using waste wood from my workshop to fire it at the moment, then will move onto the big stack of sticks from cutting the hedges. It burns very hot - difficult to get a good picture, but strange watching the flames burn sideways! I put a big brick over the hole to adjust the draft, and to close it off when I'm done.
A short fire of about an hour kept the polytunnel frost free during the week when it was -2C outside. I don't know how warm the soil gets (certainly doesn't feel hot), but the barrel is too hot to touch within a few minutes. I'm happy with it, and the best news - it's now free to heat the polytunnel!