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Greenhouse install: Exhaust pipe gauge etc.

Anthony Mecca


Joined: Feb 12, 2012
Posts: 5
About to get started on a Greenhouse Rocketstove install with the mass being a 15' long bench to germinate seedlings on. While shopping around for exhaust pipe I came across some in 8" x 60" sections but it is 28 ga metal. Is this too thin to hold a whole lot of cob? How much cob could I feel safe about piling on?

Related, is it necessary that the chimney in the heat riser be thick metal (I think it was suggested in the book to be at least 1/4") ? What gauge can this piece be?

It also seems that many folks are forgoing the metal drum with top and bung for the wood feed area in favor of one built of bricks. What's the reasoning here? Are small drums difficult to find? Is one preferable?

I'll try to do some documentation along the way for those interested as I've learned a lot from what has already been posted. Thanks!

-Anthony
Erica Wisner
volunteer

Joined: Feb 10, 2009
Posts: 720
Location: Okanogan Highlands, Washington
    
  85

Post Today 02:01:18 PM Subject: Greenhouse install: Exhaust pipe gauge etc.
About to get started on a Greenhouse Rocketstove install with the mass being a 15' long bench to germinate seedlings on. While shopping around for exhaust pipe I came across some in 8" x 60" sections but it is 28 ga metal. Is this too thin to hold a whole lot of cob? How much cob could I feel safe about piling on?

Related, is it necessary that the chimney in the heat riser be thick metal (I think it was suggested in the book to be at least 1/4") ? What gauge can this piece be?


28 ga should be fine. Think of the pipe as the form, and the cob as the material that will actually be supporting the weight. If you mix a proper cob, with a lot of sand and a little decent clay, and pack it tightly under and beside the ducting as well as above, you can have an arch-like structure that supports a huge amount of weight.
For a greenhouse, my main concerns would be: 1) including some kind of barrier like a tile or brick layer, so you don't pierce the ducting with a garden fork, and 2) providing a way for your plant-water to drain away outside the bench, if you want to avoid rusting the ducting. Galvanized has some ability to withstand rusting, but clay is very good at holding water, so it's a battle of the nano-materials.

For the heat riser, thicker metal or brick helps with drafting. Also, the temperatures in here can get very high, and we've seen a number of thin steel or duct materials that warped here, or just oxidized and flaked away. Thin firebrick or ceramic chimney liner with a good insulation would be ideal; a cardboard form for a castable material (clay-perlite cob insulation, or castable refractory for over 2800 degrees, 3000 would be better) would be good because the cardboard would burn out; the thicker options in high-temp steel are a third alternative because they are too stiff to warp. They do sometimes break seals from thermal expansion; make sure there is good support and insulation around it.


It also seems that many folks are forgoing the metal drum with top and bung for the wood feed area in favor of one built of bricks. What's the reasoning here? Are small drums difficult to find? Is one preferable?

I'll try to do some documentation along the way for those interested as I've learned a lot from what has already been posted. Thanks!

-Anthony


The small drum around the fuel feed, with a bung hole 'vent' in its shut-down lid, allows you to do a couple of foolish things: 1) pile in wood that doesn't fit the stove, and hope it will burn, and 2) respond to any problems you do have with the stove not drafting properly, smoking back, etc. by putting a lid on it instead of fixing the problem. Putting a lid on it traps the smoke under the lid, but you still can't get to it to make the fire work properly without releasing the smoke into the room. This option is popular to appease a person in the house who is scared of fire, or smoke, or responds to physical challenges by wanting them to go away. It does not fix any of the problems it is designed to address; it only gives an intimidated person a way to 'make it stop' until the main operator can be called to fix the problem.

The plain brick alternative does not trap smoke in a chamber that will eventually be released into the house, does not concentrate heat above the firebox making counter-drafts more likely, and provides more incentive to operate and maintain the stove properly: in order to control shut-down by sliding bricks across it, you have to have your wood cut to a size that actually fits your firebox. You can go up by a half-course of bricks, or a few inches of ceramic tile collar, without creating a big enough void to encourage trapped smoke.

If you have draft problems, you check the ducting for blockages: check that the screen is still over your exhaust and that pack-rats haven't nested in the manifold; take your shop-vac or gnarled little hands and pull the fly ash out of the first section of duct; and make sure there isn't a drift of fly ash on top of the heat riser while you're at it.

If the draft is sluggish because it's cold inside and the weather is warm, you open the doors and let some sunshine in, and put a candle under the heat riser to 'prime' the draft until you can see by the flickering flame that the draft is pulling in the right direction.

It's not a bad idea to have some kind of emergency shut-down option, I don't know, your roofer could hammer the cap onto your chimney while the stove is running and you need to drop a lid over the firebox and go yell at him. Or at the end of the night, you want to close off the draw to keep more heat in the channels once the fire is out.
But adding a taller box to hold this lid encourages putting longer wood in the stove, which leads to exactly the kind of awkward flare-ups that make people want to put a lid on it.

Glad you're having fun, look forward to seeing your project!

-Erica


Play with nature, make nifty stuff:
www.ErnieAndErica.info
Ernie Wisner
volunteer

Joined: Oct 16, 2009
Posts: 788
Location: Tonasket washington
    
  23
Erica gives her reasons for it now by request i will give mine.
that little bucket thing is dangerous, have you ever seen the movie back-draft? you take hot flammable gas and contain it with little O2 and when someone opens the lid Whoosh!!! your kid is trying to explain why daddy/mommy looks like an alien.


Need more info?
Ernie and Erica
Wood burning stoves, Rocket Mass Heaters, DIY,
Stove plans, Boat plans, General permiculture information, Arts and crafts, Fire science, Find it at www.ernieanderica.info


Anthony Mecca


Joined: Feb 12, 2012
Posts: 5
Thanks for the replies and great explanations, I feel a bit more confident going forward.

I'm planning to mitigate the seedling bench being constantly wet by sloping it slightly to help the water drain rather than puddle and applying a few coats of whitewash. I was thinking maybe a little lime plaster layer on top of the cob just to be safe.
Ernie Wisner
volunteer

Joined: Oct 16, 2009
Posts: 788
Location: Tonasket washington
    
  23
That works use hydraulic lime and it will stay dry inside. the stove wont care and the seedlings would be pretty happy on the wet cob cause its going to steam. nut if you want to keep it for longer than a small time yesw white wash and lime plaster.
Randy Paul


Joined: Feb 02, 2012
Posts: 2
Erica
in you response you said "make sure there isn't a drift of fly ash on top of the heat riser" how do you do that, do you have to take the Barrel off and check it ? or do you have a way to take the top off the barrel?
Thanks
allen lumley
pollinator

Joined: Mar 16, 2012
Posts: 2245
Location: Northern New York Zone4-5 the OUTER 'RONDACs percip 36''
    
  37
- W.o.w., i get to make a comment after E And E Wisner, the pipe you are describing is probably cold air return pipe, and will be fine.
Like Erica said this should be considered as a form around which you will pack your cob, as the Arch is very strong she used that as an example,
the tighter, more carefully you pack your cob the more surface contact area you have to absorb the heat energy through the ductwork.

There is an old joke about getting rich 'pick your parents carefully' - there is a 55 gal drum* out there with a removable top secured by a clamping band
You cut out the other end to slip down over the Heat Riser and mate with the Rocket stove brick base. and 'Bob's your Uncle' you have a removable top
to make inspection/cleaning inside the barrel easy ! Be safe , Stay warm , - PYRO-Magicly yours , - Allen L. - * and some 30 gal drums !


Success has a Thousand Fathers , Failure is an Orphan

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