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Describe the ideal wood heater  RSS feed

 
Frank Rasmussen
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I've been thinking about using some of the principles of rocket stoves and mass heaters to design my perfect stove. Other relevant threads I've commented in to get to this point here, here, and here. Some good points that the RMH brings:

  • Ensuring that there is enough temperature and air/fuel mixture for a complete burn through adequate insulation, and separating the combustion part of the process from the heat extraction.
  • Once the exhaust mixture is ensured clean and 99% free of creosote, devote attention to serious heat exchange (extraction).
  • Smooth the highs and lows from the heat output in line with how we want our climate to be.
  • Possibly, devoting some attention to mixing of the cooled air, so that we don't have localized high temperature air at the ceiling, and hence conduction in that area. (Alternatively, this might be solved by heaping lots of insulation in this area of ceiling).
  • Ensuring by design that the wood heater user cannot run the stove in an inefficient manner through laze or incompetence (unfortunately the RMH solves this by requiring the heater to be babysat - note that babysitting is sometimes required in non-cats and cats, but the alternative there is that the CWS runs inefficiently and pollutes, whereas in the RMH case the fire goes out. The latter is the better idea, but the constant feeding is more of a "necessary evil" than a positive in my view.
  • Use as little wood as possible to heat the home acceptably (my definition of acceptable is different to others - I want the whole house heated, not just a room or an area.)
  • Enable venting of the exhaust through a wall, without much attention to heat shielding.


  • So those are the good points of the RMH, in my opinion. The next step is to define some goals. What would I want my ideal wood heater to do? In reaching those goals some compromises may need to be made, but if we never consider what we might want as a goal up front we do not have a hope of ever achieving that. So I thought I would start this thread and invite some comments. i.e. Have I left anything out? And which commercially available heaters do most of what I want? No sense in reinventing the wheel. Note that this is an "ideal", of course we will never achieve this but it's a target. So here is my list:

  • 100% efficient, in that all of the heat produced (including latent heat of vaporisation of the water) is extracted within the building to be heated, and not in the roof space either. We might measure the exhaust volume, temperature (relative to outside air temperature) and composition to estimate this, but we would need to be able to measure every necessary variable, including water content, and non-CO combustible gases, as well as CO2, CO and O2. This should also take into account the air sucked into the building, if any. In short, let's not fool ourselves.
  • Not a huge difference in temperature between the convected heat and the room temperature, so that dealing with a hot ceiling conducting more heat locally is not so much an issue.
  • Very low emissions.
  • Able to combust wet wood completely and extract the latent heat of vaporisation from the water.
  • Load it once, run it 24 hours without topping up, or at least 12.
  • Make it impossible to run it inefficiently or in a polluting manner (other than startup for a limited time, say 15 minutes).
  • No babysitting required, other than to change the heat output. This might be automatically managed with a thermostat though.
  • Passive. Will work without electrical power.
  • Extremely reliable. No moving parts to replace. Materials chosen to last given the likely local operating temperatures.
  • Can be easily cleaned.
  • Does not generate much cleaning maintenance workload. Will not generate creosote in appreciable quantities, and minimally clogs hard to clean areas.
  • Will heat the whole house at the level required (i.e. capable of being damped and running efficiently). Alternatively, the heater might employ a thermal battery to manage the heat outflow.
  • Will be capable of generating a high maximum heat output that is capable of quickly making a cold house warm.
  • Capable of being damped to a low or very low level, running in a near dormant fashion but efficiently, suitable for overnight or when out of the house. Alternatively, this is managed by employing a thermal battery.
  • May be vented without worry as to the temperature of the exhaust, possibly horizontally like a pellet stove or RMH. This lowers cost and increases practicality.
  • Inexpensive.
  • Relatively light. Does not require extensive reinforcements of floor or foundation.
  • Safe. Nothing that could explode. Preferably not a burn hazard either. No way for embers to be ejected.
  • Be able to see visible flames from at least primary if not secondary combustion.
  • Does not look hideous. Preferably looks like an appliance, or attractive. Does not look overly like a science experiment (I don't care about this factor, but my wife does).
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    Rion Mather
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    I'm on board with hybrids. I'm obviously no expert but there are big differences between rocket stoves and Rocket Mass Heaters, including the vertical and horizontal feeders/burn chambers. As far as masonry stoves go...there are numerous styles to choose from. You should check out youtube. I have found it to be the best place to learn about stoves, in general.
     
    allen lumley
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    Frank R. : Outside of dealing with the quibble that as part of your requirement that the Ideal wood stove / I.W.S. 'heat the whole house', that job Must Be
    shared by the whole houses air handling equipment, I believe that you have a very complete list of what an I.W.S. should be/do !

    So my comments should be 'mostly' judged as a prequel to your I.W.S. list !

    I often speak about the rocket stove as having a 'built-in heat pump' effect. One of the things it does very well is to move heat energy from its internal
    combustion chamber to the Thermal Bench! This can be located at some distance removed, with one likely report of Thermal Benches located on two
    separate floors.

    However, we need to consider using air handling/ heat exchanger equipment to carry much of the heating and cooling loads at multiple locations and/or
    consider using (a minimum number of) additional Rocket Stoves the way multiple Masonry Stoves (with a single central chimney stack) heat individual
    rooms in many Northern European Country Houses.

    We also need to consider heat exchangers in their role in dealing with the need for 'make up air' created by air used-up by the fire burning in the Rocket
    Stoves !

    In the stuffy, over heated tropical bird houses we presently call homes, water vapor moves from the insides of the house through holes and breaks in the
    vapor barriers where they find a dew point within the insulation in the exterior walls and condense, and freeze, removing the insulation value within
    that area.

    Two things/techniques can improve on this all to common insulation failure condition. A properly balanced air to air heat exchanger replaces air lost to com
    bustion and replaces a certain percentage of the houses internal air, warming/conditioning it on the way from out to in ! Two, we can allow for a certain low
    percentage of the houses make-up air to come from outside through these same all too common breaks in the vapor barriers removing the trapped water
    vapors restoring most of the insulation value the home owner thinks he has, this to is called - 'lets not fool ourselves'!

    When we consider the need for make-up air we must also remember that the heat energy units found in the living space air that goes 'up the chimney' does
    not in fact go 'up the chimney' but is added to our heat energy stored in our thermal battery for latter use back in the living space. This and the energy saved
    as noted in the paragraph above need not be counted in the heat load of the building as they are not in effect part of the 'heat load' !

    Heat exchangers can also be coupled to an 'internal to the house' job, pre air drying the wood to be burned next/soon!

    Stratification of the air near the ceiling must be handled with additional equipment though this could be as simple as a ceiling fan ! A layer of mere aluminum foil
    with a proper air gap can reflect back over 85 % of radiated heat that otherwise would be lost through the ceilings, and into roof spaces !

    For the good of the Craft ! Be safe, keep warm ! PYRO Logical Big AL - As always, your questions and comments are solicited and Welcome ! A. L.
     
    Frank Rasmussen
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    Rion Mather wrote:I'm on board with hybrids. I'm obviously no expert but there are big differences between rocket stoves and Rocket Mass Heaters, including the vertical and horizontal feeders/burn chambers. As far as masonry stoves go...there are numerous styles to choose from. You should check out youtube. I have found it to be the best place to learn about stoves, in general.


    I have done a lot of googling on the subject. I have looked a lot at what Peter Berg has done on donkey's proboard's, after first coming here. I've seen his prefabricated J tubes - the Dragon Burner. And the batch box ideas. I was motivated to learn Sketchup after looking at the work he has done. I've looked at masonry heaters. I've read a lot of what Erica and Ernie have written. I've also looked a lot on youtube. I like this heater in particular. I've built my own little rocket mass heater J section. Actually two of them. And I spent two whole weeks concentrating on understanding how the RMH and wood stoves in general operate well enough to start doing some spreadsheet simulation work. At that point, I realized that one thing was being glossed over, and that was the whole house heating capability of a RMH, which is only possible with extensive insulation. So much insulation that it is probably half way to being a Passivhaus, which is an excellent concept that really works, but obviously not something you can do unless you build your home from scratch.

    Now, I am not going to say that the RMH doesn't serve a purpose and for those it works for, I'm sure it works very well. However, I've been thinking about the community that uses the CWS, why they use it and what do they want. One thing that motivates me is providing a solution for those people who will not accept less heat output than a CWS can provide, but who will only use it in a way that pollutes the air, gives me and my family increased emphysema and lung cancer risk, and cause/contribute to the asthma my children suffer from. I know that there are myriad CWS out there produced by different companies. Many are claimed to be advanced. I'm sure that even most of them aren't all that advanced, and if they were critically analysed against my matrix of desirable qualities, most would fall down somewhere but you would not know until you seriously trialled them. That's in part why I'm starting this thread - to see if anything is out there that is revolutionary and ticks all the boxes before I go wasting my time reinventing the wheel. If there was something that did the job as well as my Miele* vacuum cleaner cleans the floor including removing all allergens from the exhaust stream, I'd want to know about it. I wouldn't want to go into business competing with something near perfection.

    Another thing I'd like to know is if something that ticked as many boxes as possible would sell well. Would people buy them or would the idea get lost among all the marketing BS that is out there from other companies?

    *I'm a huge fan of Miele and their design principles. While the appliances they sell are not unattractive, they have a very intelligent approach to thinking up everything you really need in an appliance and making sure that it works properly. So it is not really surprising that this sort of thing is what I would want from a wood heater. The Passivhaus concept is hands down better than any wood heater because it obviates the need for space heating. However, a well designed wood heater would not go astray in the world.
     
    Frank Rasmussen
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    Thanks again for your comments Al. You introduced some knowledge I was not aware of, especially the condensing of water inside insulation material. IIRC, this is why the Passivhaus spec uses closed cell foam, so that this problem is pre-empted. Closed cell foam may introduce flammability issues though. (This is heading away from the purpose of the thread though).

    One thing I forgot that you reminded me of was the pump effect that the riser and barrel have in an RMH, using the heat from the fire to drive the exhaust but not requiring heat to be exhausted in the process. i.e rather than the chimney drafting, the riser/barrel combo seem to create a pump.
     
    Rion Mather
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    You are way into the traditional RMHs more than I am, Frank. Good luck to you on your build. Cheers!
     
    Frank Rasmussen
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    I've had a look and two of the ideas that immediately jump out at me are to make a heater, or to use the thermal mass concept but with water or paraffin wax as the storage element. Doing some googling it appears that there are already commercial units out there that achieve some impressive figures. See this list. The one that leads the list in particulates also has an impressive efficiency, 92.5%. Not sure what the standard involves, but I would not be surprised at high efficiency. It's basically similar to the RMH concept - burn something in an insulated firebox with a secondary insulated burn chamber, and then extract the heat with a heat exchanger. This water can be stored and just radiate the heat (e.g. in a basement), or routed around the house hydronically. So there is no point reinventing the wheel there, other people have had the same idea much earlier and gotten there first. The first item on the list costs $8k plus, which is quite a sum of money. It seems that the other outdoor boilers are priced similarly.

    I don't see why this sort of thing needs all the electronic monitoring to do 90% of the job that something similar could do passively if designed correctly, or why it would need to cost that much. Perhaps that's worth designing. Have the water storage, the firebox, the secondary burn and the heat exchanger all in the one unit. Use 2m or so of vertical space that is otherwise wasted above the footprint of the CWS. No need to route the heat around the house, the large surface area should radiate the heat and convect the heat effectively into the room (and surrounding rooms), much like a CWS. The "only" issue is reinforcing the floor to deal with 2 tonnes of water. This could be done in a basement though, for similar effect perhaps. The heat rises and warms the house.

    The other idea is of course to design something similar to the CWS that will not need a thermal mass.
     
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