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New rocket mass heater and condensation ?

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Hello we just took the plunge and built our first RMH we went with a 8" system in our basement through an existing fireplace so we have 25' vertical chimney to vent and all of the runs are downhill from the chimney. the bench has 18' of pipe with 3 turns.  i am on my 5th fire trying to dry the bench out  and we are having condensation drain from all our clean outs is this normal in a new system (lots of wet cob still )? will this likely go away ? or did i plan all my ducting wrong? she drafts well with a good woosh and no smoke from the chimney once she gets going but i worry about the abundance of liquid smoke (4-6 ounces per burn) when i am not making jerky. oh we are burning 2 year old seasoned ash that has been in a open wood shed.

Any Help would be welcome?


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Location: Southern alps, on the French side of the french /italian border 5000ft elevation
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Jenny, this is normal. Might last up to two months. Covering your cleanouts with mineral wool might lessen the tendency. Tho, just humidity in the air swallowed by the stove, and in the wood will always produce a little steam. Which might still condense in the bench when this one is dry. But nothing to worry about in a new build.
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Location: +52° 1' 47.40", +4° 22' 57.80"
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Not only the moist in the fuel and the air is causing condensation. The process of burning wood itself produces water vapour, it's a by-product besides heat and CO². Assuming 15% of the weight of the fuel is water to begin with, combined with the produced H²O it could add up to 60% of the original fuel weight. Normally, you won't notice this amount of moisture, unless you route it through a stone cold duct where all the heat is used to evaporate the water in the cob bench. When that's the case, all the water vapour in the duct will condensate to its liquid phase.

This is quite normal, not even possible to avoid unless you dry the cob by other means. And then, every fall you'll see some condensation until the bench get warm and it will evaporate again. So please keep running the thing, you might need to mop up the water but the effect will go away by itself in due time.
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Location: Chillan, Chile, zone 9b, 475m above the sea
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Heya, as Mr van der Berg just stated, most of the water will be formed when combusting.

This will happen because the wood is mainly composed of carbohydrates, who are themselves composed of monosaccharides or bigger polymers conformed by modification of mono or oligo saccharides, called polysaccharides. These structures generate a "chain" formed by molecules of carbohydrates (in this case, modified monosaccharides) and its proportional chemical composition is basically "C x n"+"H x 2n"+"O x n", being n a real entire positive number, as 1,5,107. There are cycloamyloses that have a formula of C48H96O48, with n as 48. The heaviest molecule i know is the Botulinum toxin (BTX), formula: C6760 H10447 N1743 O2010 S32 and weights 150 kg/mol, water weights 0.018 kg/mol, holy hens.

For example, your saliva contains an enzime capable of denaturalizing (modifying/destroying) the polysaccharide chain that holds together amylose, known as starch. Worth to note that starch has the exact same chemical composition than cellulose, C6H10O5, but its spatial-configuration will arrange the atoms in different places, thus, grating a whole new set of properties.

The beetroot sugar, which is most consumed sugar, is a disaccharide combination of the monosaccharides glucose and fructose with the formula C12H22O11. The glucose is a type hexose from the family of monosaccharides, conformed by 6 atoms of carbon. Its chemical formula is C6H12O6, just like our proportinal base multiplied 6x. The cellulose, in the other hand, is a polymer composed by the repetition of modified glucose and is the most abundant carbohydrate in nature. Has a formula of C6H10O5, which highly resembles the components of glucose. The chitine, a carbohydrate molecule that mainly composes the shell of the arthropods, named exoesqueleton; aswell as the external membrane of the fungi realm. Chitine has a quemical composition of C8H13O5N, forming a long-chain polymer of N-acetylglucosamine, who too is, drums, a derivative of glucose.

Once we realize that all these stuff are basically made with the same elements, in the same proportions and very similar same spatial-configuration; the following example will be made with glucose, instead of cellulose (as a scientificaly accepted equivalence).

The reaction in which the carbohydrates reacts with molecular oxigen, O2, releasing energy from the atomic bonds that held the former molecules together is:

C6H12O6 + 6 O2 = 6 CO2 + 6 H2O

The resulting products will be a mix conformed strictly by gases, one of this gases behaves extremely different than the regular gases, and this is of course, the H2O in gas state, known as water vapor.

This water not only yields the internal elemental energy (nuclear energy, given at the bigbang), but also the energy thats grants its kinetic energy. The kinectic energy is, in overly simplified words, the amount of energy related to the movements and speed of the molecules. When the water molecules gain so much speed, they are able to "un-hold hands with the other water molecules and run away", each molecule indivudually. For them to un-hold hands an amount of energy is needed to change its physical state of matter. For the same molecule to run faster will require incrementaly more energy, being this energy its temperature. Therefore, the heated wated not only posses the energy to become hotter, but also a hidden energy that allowed it to leave the liquid lands and become a gas.

All of this response come to this. The first cooling sytems used the "un-holding hands" energy by changing the state of the water. When the gassified water; for it is not dew, nor mist, nor a gas-suspended liquid, but true gas; becomes in direct contact with a colder surface, which is only colder because it has a higher value of the specific heat (the amount of energy required to raise in 1°C the weight of 1 gram of any given matter. Thats why the metalic portion of a chair feels colder than the cotton, the metal is absoring your bodys temperature at a much faster pase than the cotton), not only it will drain the temperture energy from the water molecule, but also drain its "hidden energy of state phase", changing the state of the water from gas to liquid. So you are giving the RMH another bit of extra energy. As stated before, this energy is considerable enough for the bright minds of the mankind to have implemented it in the very first cooling systems until today. I would attach data but it becomes extra-complex to explain and i aint got that much time heheheh.

So if you have a propper method for getting rid of your condensated water, with it not having damaged any internal component of the pipe nor mass system, it wont be trouble but gainance. The galvanized pipes used for the RMH should withstand with no problems the water for at least 5 years. It would be advisable to array the pipes in a degree in which the flow of water will be directed to a "withdraw" chamber, as you do with the ashtray.


  • For the water vapor generated by 1 kg of wood to lose 1°C would approximately disipate 625 calories; for the same amount of water at the same temperature to change its phase state without losing temperature would disipate 337.500 calories, 540 times more energy. In fact, boiling water to freezing temperture would disipate only 18.5% of the energy disipated by the same amount of water changing its state from vapor to liquid.
  • For the flow of gas from inside the RMH to move properly and have an optimal efficiency when it comes to drafted air (the intake), will require an minimal temperature of 80°C at the exhaust.

  • The kind of wood you will be using. Coniferous wood gives the most BTUs per kilo due to the highest lignin content and high content of extractives like fatty acids, resin acids, waxes and terpenes. However, denser wood is usually preferable despite lower BTUs per kilo. High content of extractives may cause unwanted effects and poisonous products.

  • Depending on the stage of development and ageing of the section of the plant, the amount of lignin contained in the in the different layers of the plant will greatly vary compared to those of a younger or older tree. Cell wall lignification is a complex process occurring exclusively in higher plants; its main function is to strengthen the plant vascular body. The lignification process involves the deposition of ill-defined phenolic polymers, the so-called lignins, on the extracellular polysaccharidic matrix. These polymers arise from the oxidative coupling of three cinnamyl alcohols in a nonrandom reaction, in which cell wall polysaccharides appear to influence the freedom of cinnamyl alcohol radicals, giving rise to a highly orchestrated process.

  • The amount of bark still attached to the wood will highly increase the amount of ligning, aswell as overall fatty molecules, granting several low impact desirable and underisable effects.

  • The amount of initial moisture contained in the wood will dramatically change the proportions of the calculations, for a log of water at 3% will weight 10 to 20% less than log stored at an avarage level of relative humidity percentage.

  • One cup of liquid water would be roughtly 16.000 cups of gas water.

  • Cellulose and molecular oxigen reactions products at 0°C and 1 atm, note that water is the 62,4% of the total weight:

    Carbon dioxideNitrogenWater
    1584 g4002 g624 g
    36 mol143 mol35 mol
    806 L3203 L784 L

    Rough average composition of wood:

    Cellulose 43%:
    CellobioseC12H22O11342 g/mol 

    Hemicellulose 32%:
    PentoseC5H10O5150 g/mol 

    Lignin 25%:
    CumarylalkoholC9H10O2150 g/mol
    ConiferylalkoholC10H12O3180 g/mol 
    SinapylalkohollC11H14O4210 g/mol 

    On average, lignin can be seen as formed from Coniferylalkohol:

    C10H12O3180 g/mol 

    Average wood reduced to:

    Hexose 75%C6H12O6180 g/mol
    Alcohols 25%C10H12O3180 g/mol 

    Average for calculation of resulting volume:

    C4H7O3103 g/mol9 mol/kg of wood

    C448 g/mol432 g per kilo
    H20 x 318 g/mol x 3486 g per kilo
    H2 g/mol x 1⁄29 g per kilo

    Moisture ~ 7.3%

    :D Hope it helps matey
    [Thumbnail for water.png]
    The data for the vaporization phase change presumes that the pressure is one standard atmosphere.
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