Polytunnel rocket stove - finished!

Nigel

Re: Polytunnel rocket stove - finished!

Post by Nigel »

why does the hot air go backwards down the pipe instead of rising ?
Carl

Re: Polytunnel rocket stove - finished!

Post by Carl »

Based on what I've read, it's all about the insulated chimney inside the barrel. As the cold incoming air hits the fire it expands, and wants to travel up the insulated chimney, as the insulation keeps the inside of the chimney really hot, so you get a very strong draft. As it gets to the top of the chimney it goes into the barrel, and the only way out is through the underground flue. The hot air is pushed out by the expanding air coming in. Either that, or its some some of pyrotechnic magic that I don't understand :?

Here's the book what I read: http://www.rocketstoves.com/. It explains it all in loads of detail - I recommend it to anyone wanting to build one.
Nigel

Re: Polytunnel rocket stove - finished!

Post by Nigel »

It obviously works.... maybe a good way to heat a larger tunnel in winter too.
Kristen

Re: Polytunnel rocket stove - finished!

Post by Kristen »

Very impressive! How long does it burn? (or more correctly: how long does it provide heat?) i.e. will it provide heat for all night long if you light a fire at a reasonable time of the evening?
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Arlon Tishmarsh
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Re: Polytunnel rocket stove - finished!

Post by Arlon Tishmarsh »

Kristen wrote:Very impressive! How long does it burn? (or more correctly: how long does it provide heat?) i.e. will it provide heat for all night long if you light a fire at a reasonable time of the evening?
Kristen,
I looked into this last year with regards to greenhouse useage and it seems the idea is to build up as big a heat store as possible and let it release back slowly . I 've read the heat can be released over a period of a couple of days depending on build quality and method, thus negating the need to light a fire every day.

It's given me an idea in how to incorporate it into my greenhouse project, either with the heat store under a central insulated walkway or raised beds with the heat store banked inside. With the greenhouse insulated i think this could work extremely well.

I like it. Best of luck Carl. Look forward to hearing how successful it is. icon_thumleft
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Re: Polytunnel rocket stove - finished!

Post by Dave Brown »

Arlon Tishmarsh wrote:
It's given me an idea in how to incorporate it into my greenhouse project, either with the heat store under a central insulated walkway or raised beds with the heat store banked inside. With the greenhouse insulated i think this could work extremely well.

I like it. Best of luck Carl. Look forward to hearing how successful it is. icon_thumleft
Just one comment Arlon, raised beds reduces height, most people talk about excavating the floor to add height rather than reduce it :wink:
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khaskings

Re: Polytunnel rocket stove - finished!

Post by khaskings »

Well done!

I take it that the copper cylinder radiates heat into the air from the combustion (but isn't dangerously hot) as well as the radiated heat from the underground clay pipe as the exhaust gases travel to the external vent?

Did your research reveal what dictates the maximum length of the underground exhaust pipe and whether you could add a bend or two to maximise heat transfer from the pipe to the earth?

I'm guessing that you can't have it cool totally underground otherwise the wouldn't be any movement of gas?
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Re: Polytunnel rocket stove - finished!

Post by Arlon Tishmarsh »

Dave Brown wrote:
Arlon Tishmarsh wrote:
It's given me an idea in how to incorporate it into my greenhouse project, either with the heat store under a central insulated walkway or raised beds with the heat store banked inside. With the greenhouse insulated i think this could work extremely well.

I like it. Best of luck Carl. Look forward to hearing how successful it is. icon_thumleft
Just one comment Arlon, raised beds reduces height, most people talk about excavating the floor to add height rather than reduce it :wink:
Dave
It would only be at the one side, if i did it in a raised bed and I have 11ft to the ridge to play with anyway. Probably would go with a big insulated trench under a central walkway tho. icon_thumleft
Carl

Re: Polytunnel rocket stove - finished!

Post by Carl »

The copper barrel is getting hotter now (180 C this evening) - I think the whole thing is drying out with each fire. I think that a 15-20 foot flue was about as long as would suit a rocket stove with a 6" chimney. You can make them push gas further with a wider chimney. I'd use a bigger chimney for a bigger polytunnel.

Some pics from today, when I arranged all my tender plants over the hot spot in the middle of the tunnel:

Image

Image

Image

Once all the tender plants are in the middle, I covered them over with fleece - trying to keep the heat from the soil in for as long as possible!

Image

Today, I have mostly been burning buckets of old raspberry canes :D You'll see I've got the fuel hole about 1/2 closed with a brick. This really increases the draft, making it roar (OK, roar quietly...).
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Re: Polytunnel rocket stove - finished!

Post by Arlon Tishmarsh »

Carl
are you aware of any reason why the copper tank could not be replaced with a brick structure with an access door at the front , to act as a heat store as well which would continue to throw out heat long after a metal cylinder has cooled. I was intending to use brick as the central chimney as well, as they recommend it be insulated . Some of the vids i've seen also use brick as the central chimney.
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Re: Polytunnel rocket stove - finished!

Post by Dave Brown »

Arlon, the bricks would make it a storage heater :wink: Particularly if you have old storage heater bricks, which I have.

Carl, with the weather being mild in the south until yesterday, what temp are you aiming to keep the poly at ? The problem I can see is if too warm it will encourage growth, but with insufficient light to grow properly.
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Carl

Re: Polytunnel rocket stove - finished!

Post by Carl »

Arlon - Dave's right, it would make it a type of storage heater. I've read about designs made of brick, and there are some good examples on the net too. If you google for 'masonry heater' you can find some good designs - a good starting place would be here: http://mha-net.org/ That's what I'd like to build inside the house - in the polytunnel I've got the advantage of a lot of wet soil to use as a heat sink, and there's no better heat storage material than water. I'm more interested in soil temperature in the polytunnel than air temp - I can use fleece to hold onto the soil warmth for longer, and keep it round the plants. Once the heat is in the air, it's soon gone, due to the lousy insulation properties of polytunnels! If I had a nicer, insulated set up that was drier, I'd build a masonry heater. Problem is, the spider mites would love it - warm and dry :shock:

Dave - I plan to keep the polytunnel frost free, but not much warmer. Polytunnels are a very inefficient way to keep plants warm - if I had something better (conservatory or keder house perhaps) I might consider keeping it warmer, but then you're right - the lack of light would make for spindly plants... So I'd have to buy lights. Then the spider mites would come back... :roll: A cool polytunnel is about my level at the moment!
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Re: Polytunnel rocket stove - finished!

Post by Arlon Tishmarsh »

Thanks Carl icon_thumleft

i might try a combo of both in that case. I read somewhere that its the insulated internal chimney in the rocket heater that produces the "super heat" and plasma is then formed in the top of the outer metal cylinder chamber. I was trying to ascertain whether changing the metal cylinder to brick would interfere with that physics.

If that were the case i could always change it back to a metal cylinder. I'm just trying to maximise the stored heat output by trying a brick outer chamber.

Thanks for the info icon_thumleft
GoggleboxUK

Re: Polytunnel rocket stove - finished!

Post by GoggleboxUK »

Arlon, it might be useful to create a heatsink below your greenhouse, a trench filled with pea gravel creating air spaces that can hold the heat and release it back into the earth and the greenhouse space above.

Running the clay pipe through a 3ft deep gravel pit will store and distribute heat much more effectively than if the pipe just ran through soil.
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Re: Polytunnel rocket stove - finished!

Post by Arlon Tishmarsh »

Exactly GB

thats why i thought about combining the two , have the horizontal chimney undergound and under a central walkway and swap the metal cylinder for a brick jobbie which will create a heat store above ground as well. icon_thumleft

Got a stack of bricks here to create an underground chimney which will make a very good heat store .
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