What kind of rocket stove fits best, advice needed

[wurzeli]

Dear experts and visitors of this very informative forum; since many years I am a silent reader of this forum and now I would like to get active myself.

I would like to replace an existing wood stove with a cleaner one, but there are several factors which make the choice of which build is best a challenge...

First I discribe the circumstances:

We need to heat our workshop / bicycle shop entirely with this stove.

The current stove is a "la Nordica Gemma forno", a lightweight (121 kg) stove with permanent flame and an output of about 7kw, efficiency in theory is 85% and wood consumption for it 2kg/h.
The stove is powerful enough, on a very cold day after 40 hours without heating (start room temperature is 8 degrees Celsius) I need about 12 kg of wood during the whole day for having comfortable working temperature (15 degrees after 3 hours of medium fire, end temperature 19 degrees after 11 hours of heating.) I could heat much faster and to much warmer temperatures, if needed with this stove. Problem: Often, we don't need so much heat and run the stove with only little wood continuously for not having to start a new fire several times a day and then the burn is not very clean (much soot formed in the stove pipes)

What are the circumstances/ important points for a new rocket stove:

- only little space available for stove (height is good, 2.4 meters till ceiling , max depth 75cm, max width 70cm.

- current joint for chimney is only 100mm diameter, chimney itself is much larger, if necessary we make a bigger joint.

- draft of the chimney is very very good, current stove is made for 120mm pipe, but works perfect with 100mm pipe, even on cold start with wind

- we would like to have a small oven

- stove should not have barrel(s)

- It should be possible to heat pretty fast (some surfaces which get hot fast), but after a certain temperature is reached, it should hold the temperature (with or without continuing of firing) for a long time without having to fire with a dirty burn.

- Maybe I could build some kind of heavy doors / windows which can be moved against the thinnest parts of the oven when no more "fast heat" is needed

- I do not mind if we have to buy some more expensive materials for the build to create a long lasting, clean burning stove

- we have access to high quality, high temperature firebricks with very good durability in surroundings with big, frequent heat changings.


- It's okay if we have to start two fires a day (from beginning without ember left)

- it would be good if we could vary the wood quantity we use for one load without having to compromise much with a much dirtier burn


Thank you very much in advance for your suggestions.

Daniel

[fuegos]

Hello Daniel and welcome to the community .I'm a beginner with RMHs but i'm sure some of our more experienced members will be along soon to help out .One thing i can comment on is "stove should not have barrel(s)- It should be possible to heat pretty fast " any reason why you don't want a barrel ? It's going to be very hard to warm up your shop without some radiant surface on the stove .
"Maybe I could build some kind of heavy doors / windows which can be moved against the thinnest parts of the oven when no more "fast heat" is needed" sound interesting , maybe like an insulating lid on a cooktop ?

[belgiangulch]

Sounds like maybe one of Matt Walkers designs might fit the bill perfectly!
I would think either one of his riserless cook stoves would do the job you are wanting.
Matt sells complete plans for these stoves along with unlimited tech help.
here is a link to his shop walkerstoves.com/index.html

[wurzeli]

Feb 17, 2021 4:12:05 GMT -8 fuegos said:

Hello Daniel and welcome to the community .I'm a beginner with RMHs but i'm sure some of our more experienced members will be along soon to help out .One thing i can comment on is "stove should not have barrel(s)- It should be possible to heat pretty fast " any reason why you don't want a barrel ? It's going to be very hard to warm up your shop without some radiant surface on the stove .
"Maybe I could build some kind of heavy doors / windows which can be moved against the thinnest parts of the oven when no more "fast heat" is needed" sound interesting , maybe like an insulating lid on a cooktop ?

Hi fuegos, thank you for your reply, 

Why I don't want to use barrels:

- I don't like the look of them
- Due to the thin, big surface of barrels they will put out heat very fast, this is good when the room is pretty cold but turns into a problem when the room is already at a comfortable temperature: Just for maintaining the room temperature the barrels put out too much instant heat when the wood is burning inside the stove. 
thicker materials in the bell with much more mass will get less hot and radiate less, but during a much longer time - the problem of overheating is much smaller. We had to insulate our workshop inside the walls and inside the insulation it has not big mass which can store heat, we put about 5 tons of fist sized stones on some parts of the floor for some mass inside, but other than that there is no big mass which keeps the store warm for long when the oven does not radiate anymore - overheating can occur very fast. 

Our current stove has iron "walls" which get hot after about 20 to 30 minutes from the start of the fire, this is fast enough. I think, when we use thin bricks in some places of the upper part of the stove bell (where the temperature is highest) then they will get hot fast enough and start heating pretty fast. The advantage of using thin bricks instead of barrels is the possibility to use thin bricks at the sides of the bell which point to the inside of the room in a defined size and the relation between fast heating and slow (long) heating surfaces can be chosen in any desired proportion. 
It should also be easier to cover these flat areas of fast heating thinner bricks with "windows" made of thicker bricks to make the heat output smaller. These windows will store the heat and can be opened later to relase it faster, if needed. These "windows" would make sense on vertical surfaces pointing to the inside of the workshop, there the effect of open / closed window would be bigger.

[wurzeli]

Feb 17, 2021 5:11:09 GMT -8 belgiangulch said:

Sounds like maybe one of Matt Walkers designs might fit the bill perfectly!
I would think either one of his riserless cook stoves would do the job you are wanting.
Matt sells complete plans for these stoves along with unlimited tech help.
here is a link to his shop walkerstoves.com/index.html

Thank you for your suggestion, but Matt Walker's riserless cook stoves do not fit in out workshop, we have space in height and could make the stove even 2 meters high, but the base area cannot be big... There is for sure space for a riser and the oven can be very high in the stove...