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My adjustable wood pellet rocket stove space heater and hot tub heater  RSS feed

 
Alex Colchester
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Hi

I am new to this site and have never posted. I live in the Uk and have been building and refining rocket stoves for a couple of years.

For a while i had a large tent that i used as my workshop and I successfully heated it with a rocket stove for over two years- running the rocket stove for 8hrs per day 5 days a week. I kept a constant temperature of around 65F inside the tent (and it gets cold here in the winter!). The tent was about 15feet square and 15feet high. I didn't go down the mass route simply because we needed instant heat and there was no insulation to the tent. I did however use radiant heat very successfully by tracing a black chimney route along the length of one side of the tent. The external top of my burn barrel reached temperatures of @ 1200F (red hot glow). The end of the chimney run was @120F. So we effectively harvested most of the available heat. Here is a link to a video if the attached one doesn't work- https://www.dropbox.com/s/pptqz7sitx87ho3/rocket%20stove%20tent%20heating.mov?dl=0



The main advantage of my design was threefold

1: we could use pellets in a hopper that provided an 8hr burn. At no point was the fire attended to after initial lighting. The pellet hopper i designed maintained a constant flow of fuel.

2: I designed a movable grate that allowed us to adjust temperature of the burn, whilst always maintaining a completely clean burn (tested with a gas detector at end of chimney and visually- i.e no smoke at all). This is something that a lot of people online have said is not possible. I.e they say that unless the rocket is burning at full tilt then it will not produce a clean burn. we have disproved this (at least as far as wood pellets are concerned). We could achieve a burn temperature range (measured at external at top of burn drum) of between 450F and 1200F and hold any temp in this range consistently for as long as there is fuel in the hopper. The lower temperature setting also needed no attending to once the fire was burning. It also produced an equally clean burn as the higher temperatures (with a proportionally reduced fuel consumption, proving that the reduced temperature was result of less volume of burn and not from artificial cooling by increasing draft). The ability to be able to adjust the output temperature consistently and without affecting the efficiency is a big leap. It means that using the rocket principle- on demand adjustable radiant heating is now possible as well as mass heating. The stove would react to the changed setting in about 20mins so very useable as conditions change.

I suppose my rocket is a new genre... the Adjustable Rocket Radiant Heater- ARRH

3: I installed a refractory grade glass viewing window at the top of the burn drum. This allowed a clear view of the rocket burn and allowed us to tune the stove and make adjustments to airflow etc with a clearly observable result. It also allowed us to reflect the viewing window in a polished steel sheet which gave great aesthetic (something often lacking in Rockets as the burn is usually hidden.)

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I used vermiculate sheet to create the burn chimney and this has had over 4000hrs of burn without any disintegration. Vermiculate is a great insulator and so also meant the burn chimney needed no additional insulation. Perlite was used to surround the stainless steel burn tunnel and this has had the same 4000hrs of use, but has had to be re-welded a few times! For my next version i will be using refractory for the burn tunnel.

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I have also made a hot tub, and have been heating that for 1 year very successfully using a different burn barrel (the burn barrel simply slots on, allowing quick change from radiant heater to water heater).The tub heater barrel has a large bore (1inch) internal copper spiral and an outside clad in ceramic fibre blanket. The circulation is done with a spa pump. I can heat the 1000litres of water from tap cold to 104F in 4hrs. Again the stove needs no attendance when this is happening as the pellet system is 'fire and forget'. (please don't attempt to heat water with a rocket unless you really know what you are doing!)

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very interested by peoples reactions to this- thanks!
 
Stuart Reed
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This is pretty cool! I like being able to see the fire. Wish it was on the front of the stove rather than the top. But I guess I understand the placement after all.

Really nice job. Are you supposing to sell these? or are you intending to post build plans?
 
Alex Colchester
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Hi Stuart

Thanks- glad you like it! To be honest the version you see here is a working prototype that has been refined over the years (mostly internal tinkering). The aim was always to build a second version- much more compact, utilising advanced materials throughout (very little metal in flame path, due to longevity issues), and visually far more pleasing. I admit this prototype is not the prettiest thing around!

I am more than happy to share much of what i have learned to anyone building one (PM me).

The next version is actually nearly completed and subject to testing over the next 6 months I hope to do a Kickstarter campaign. I think there is room for a compact, easy to use rocket stove/heater with multiple uses. I will be using the removable drum principle i have used on this first version but with more options- i.e a drum water jacket for water heating (thermosiphon this time, no need for spa pump). A heavy wall black iron drum for space heating (with top viewing window and reflector so burn can be seen when sitting down). A cooking drum with top grill plate and internal pizza oven/oven (ultimate pizzas at 1100F anyone?!). This allows much more flexibility from the buyer POV. they can buy the basic unit with one drum, and then as their needs change, they can simply buy additional drums to use with the same unit. Equally the fuel can be changed by simply purchasing a different feed attachment (batch box, pellet feed, coal feed etc.)

The key innovation is the adjustable burn with proportional fuel consumption whilst maintaining near-perfect efficiency. This adjustable burn will work with all fuels, not just pellets.

Here is a render of what the next version is likely to look like (external cladding either aluminium or stainless steel). All internals refractory etc.

As always- due to the unique properties of the rocket mechanism the hot exhaust can easily be sent horizontally through mass if the user requires.

with-pellet-hopper.jpg
[Thumbnail for with-pellet-hopper.jpg]
internal-view-(perlite-burn-chimney).jpg
[Thumbnail for internal-view-(perlite-burn-chimney).jpg]
full-view-iso.jpg
[Thumbnail for full-view-iso.jpg]
 
Anthony Donner
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Alex,
great job on this I have been using your similar concept also for  heating my home with out mass, I have been tinkering with adjusting fuel rate to give more or less heat... I would like to know what you talk about adjusting the burn grate to get different heat settings? from my experiments I have determined that the Btu's a person can get is by the amount of fuel you can burn in a specific time, more fuel in a hour the more btus.... I have made 2 feed tubes for my stove 1 tube allows it to burn 5lbs of pellets a hour about= 40,000 BTUS, and the bigger feed tube allows it to burn 10lb a hour about= AROUND 80,000 BTU'S,
how does your adjustable grate allow this?
thanks
 
Alex Colchester
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Hi Anthony

Your stove sounds like it puts out a lot of heat- 10lbs an hour is a swift burn rate. At that burn rate does it maintain a clean burn? Also how much ash does it produce as compared to the 5lbs hour?

The adjustable grate works on the concept that the main burning mass is in a pile below the grate. The grate itself primary role is to act as a feed mechanism to the burning pile below, and should more accurately be described as a 'burn basket'. As the pellets burn they reduce in size until they can fit through the burn basket. The basket gaps are sized to make sure that the pellets only need to burn a little before they fall through and into the burn pile below. Once in the burn pile they still have the majority of their mass left to burn. The burn basket is vertically above the burn pile and the closer it is moved downwards toward the burn pile, the more the heat of the burn pile causes the pellets in the burn basket to burn. This increases the feed rate. If the basket is moved upwards away from the burn pile then the heat is less and the basket burns more slowly, releasing less fuel to the burn pile below. At every height setting the fire automatically achieves equilibrium between the basket and burn pile, which in turn ensures a clean burn that is fully adjustable.

You will notice on my videos a threaded bolt below the pellet hopper. By screwing or unscrewing the bolt the hopper is raised or lowered as is the basket attached to it below.

Hope this helps
 
Anthony Donner
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Alex,
ok thanks I think I get the concept yours works on,
as far as your question of clean burn, mine has a very very clean burn, very great draft and rockety sound , my system is a 6 inch system, the burn tunnel and heat riser are made of high temp refractory, and the pellet basket is made of high temp 309 STAINLESS STEEL, the reason I need those burn rates is because I need those Btu's to heat my large house, I need to heat 2,500 sf, I live in Minnesota,USA which is a cold winter State, when the temps are not to cold I can use the 5lb burn rate and when cold the 10lb rate, and I have discovered the fuel feed rate is what can be adjusted to get high or low Btu's,the more fuel you can feed it the more btu's it will put out... it virtually leaves no ashes under the grate either way I burn it, most are consumed or sucked out....  and it can consume as much fuel as you feed it  and cleanly, one of these days I am going to do a combustion analysis on it to see its efficiencies soon, but I am guessing by seeing the exhaust temps that it probably is in the 90's % efficacies , very low exhaust temps .... so I am not really losing much out the exhaust.
so far I have been quite impressed with it, will see how everything goes this winter...this will be the first full winter using it.
 
Alex Colchester
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Hi Anthony

would be great if you could post a short video of your pellet burner grate. would love to see how your grate works. Thanks Alex
 
Anthony Donner
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yes I will try and do that in the next few days (get video), I posted a reply to you a few days ago about the ash build up being non existence, well I need to slightly alter that because that was only on 2-3 hr burns there is none, but over the weekend I ran it 6-8 hrs and found out that it does gradually build up over the night in the basket and slows the heat and burn down...so it needs to be tapped to get ash out ...
I am experimenting with different designs to get this from not happening.

 
Alex Colchester
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I have found, with a pellet burn system and auto feed wood system, dealing with the ash build up (without human interaction such as riddling a grate) is the most difficult issue to resolve. As my aim for my rocket stoves is to replace more modern heating alternatives, one of my primary aims is to make the design as user friendly as possible and create a design that needs no tending once lit. "fire-and-forget" as i call it. This convenience is key to getting these more eco-friendly alternatives to be generally accepted. The average person, especially in Europe, has very little knowledge of firemaking and certainly does not want to 'tend' a fire. No one has to 'tend' a modern gas/electric boiler.

The dream is to create eco friendly designs, such as a 'fire-and-forget" rocket stove, that has both the eco credentials and efficiency inherent in its design, whilst RETAINING modern convenience.

I have spent more time on attempting to make my stove run for 8hr+ without the need for tending, than i have on any other aspect of its design. It is the holy grail, IMHO, and it must be achieved by using off-grid, natural forces- gravity, draught, burn chambers, grate design etc. .
I.e an off grid solution with all the convenience of modern alternatives.

Also, as you found, running for a few hours is possible on most pellet stove designs. Running for more than 8hrs separates the true 'fire-and-forget' designs, from those that are on a downward curve of performance (caused by ash build-up) as burn time progresses.


For the above reasons i am very interested to see other grate designs, as anyone who is interested in achieving long burn times from a rocket stove, is helping to solve the biggest obstacle to their widespread acceptance. I would add, that those who say that a RMH is meant to only be run for short periods of time- within that short burn period (which is still going to be an hour or so) the fact the fire still has to be tended, and can be difficult to light (especially by a non-skilled operator), is a big problem, that will stop widespread acceptance (which surely is our ultimate goal, right?).
 
Anthony Donner
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Alex,
yes I agree with you, mine runs great for 2-3 hours with no tending but after that the grate builds up with ash, I will post pictures and video once I get a chance,
my next thought is to redesighn a burn basket.
my next theory is to build one that uses the wt. of the pellets and the aerodynamics of basket to get ashes to fall through and blow away, because the one I use now any ashes that do fall through do get sucked up the system, I really don't have anything accumulating under the basket ,the draft is enough to suck it down the system....
 
Anthony Donner
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Alex,
I am working on getting ashes to not accumulate on grate, I believe I may have found some success, I built a basket/grate out of 309 ss 1/8th inch rods, spaced them 1/4th inch apart and angled the grate to slightly less than 45 degrees I ran it for 4-5 hours the other night with no accumulation on grate, only accumulation was what fell through to floor of burn tunnel, and this thing was running so scorching hot I had to shut it down because it was getting to hot in my house, I started it and my house was 64 degrees F and after about 4 hours it was 74 degrees F the outside temp was about 40-45, and my sq ft of house is 2500,
so I am working on making the basket grate to hold a little less pellets.. I will keep you informed as my findings progress.
 
Alex Colchester
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Hi Anthony

How is the grate R&D going? Would love to see a photo of your new grate design. Also 8hrs is the time i feel a grate should be able to run for without getting clogged- have you had a chance to do a continous burn for that time period?

Thanks

Alex
 
Anthony Donner
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Alex,
sorry I haven't got a picture and video yet...been busy, but I have found that by widening the gaps in the grate to 1/4 in they do not build up with ash... but they do burn faster and hotter I use 1/8 in 309 SS for grate/basket material, the only problem that I have found this way is that the ash does fall through but buids up on bottom of burn chamber... it doesn't cause a problem with running the stove but it does want to burn the grates out,cause when the hot ash sits there and touches the grates it is like glowing red and deteriorating the grates....so far the longest I have ran it is maybe 5-6 hours...the weather here isn't cold enough to burn all night yet, but I think this is the principle I will use for my unit because I can scoop out ashes from underneath the grate...I am trying to figue out how I can eliminate this build up...under the grate but haven't come up with a plan yet. also before I forget between burns you need to keep the grate/basket clean because if it has old residue on it from previous burns it will build up on it and cause a poor burn..what I do every night before I start a new fire is take a steel brush to it and brush it clean real good and I have no problem. I will get you some video of my basket and my setup so you can take a look.
 
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