Frank Rasmussen

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since May 02, 2013
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Recent posts by Frank Rasmussen


Thanks for your comment. I've read your web page and a few of your posts and it seems that you have had a similar idea to me. One of the first things that occurred to me when thinking about RMH technology was to create a water heater, storage and radiator system as you have done. Well done on your implementation.

I keep thinking of how to make the system safer, e.g. by making an insulated fire box rather than using a feed tube. Obviously if you do that, you have to incorporate some secondary air into the system for secondary combustion. This system then starts to look a lot like the EPA non-catalytic stoves, since they have somewhat insulated fireboxes and preheated secondary air.

Another thing I was considering was to have a 180 degree turn at the top of the heat riser for the heat exchanger to follow, so that you could have a counterflow heat exchange (ideally you want the cold water contacting the coolest part of the exhaust where it can extract the most heat), which may also permit a thermosyphon effect as the water is travelling in at the bottom, cold, and out of the top, hot. This might also help with pumping the exhaust, as it is doing what the barrel does in the conventional RMH. In effect a reverse thermosyphon I believe, as the hot air goes in at the top and exits as (relatively) cool.

If this concept was executed as a 2 tonne mass heater (the mass is vertical within a 2 metre tall, 1mx1m footprint), I was thinking that you could just route a bunch of vertical pipes straight down through the liquid you are trying to warm, which carries exhaust. Added up, the CSA of these tubes would be the same as your flue diameter. One would have to have a way of accessing these tubes from the manifold so as to be able to clean these with a pipe cleaner. The walls of the mass heater would do the radiate the heat and convect it. The mass container might be filled with either paraffin wax or water. Water is much cheaper, and the high initial heat transfer and exponential lowering over time would work quite well with typical heating needs.

There are some issues that would make this impractical for me though - I don't think my landlord would approve. Also you need a floor capable of holding 2 tonnes of water. Perhaps outside. Nice work though. Consider making plans and selling them.
7 years ago
Paul, I'm not calling you a liar. There is a third possibility, that the parameters of a system in which someone burns 1/5 to 1/10 the wood of a CWS are not nailed down properly, that it is not an apples to apples comparison.

I'm open to the possibility that my math, physics or chemistry is incorrect, but that would require some discussion. Physically, a RMH is not magical. It can only convert the chemical energy in the wood to heat energy, and since there is no fission or fusion going on, this is all the energy that is available. Now, it is possible to pump the heat from outside the building to inside the building with a heat pump (this is how the electric heat pump is able to do 4 or so times as much heating as the electricity going in, through the Coefficient Of Performance - COP), but to do this with a RMH would require situating something like a Stirling engine on the RMH barrel and hooking it up to a conventional heat pump with compressor situated outside.

Because none of that is going on, there is an upper limit to the energy that is available from any wood heater, and that is the amount of chemical energy liberated from a given quantity of oven dried wood (i.e. 0% Moisture Content) when it is burned completely. We are looking at something like 20,000kJ/kg or 8600 BTU/lbs. I have seen 35lbs-45lbs/day as a worst case, coldest day figure used for the 8 inch RMH systems. If the 45lbs figure is used, that equates to 38,7000 BTU/day, or 16,125 BTU/hour. Alternatively, that equates to 409,000kJ/day, or 4.73kW average. In reality few people are burning oven dried wood or the highest energy density wood, the exhaust temperature of the RMH will be significantly above the outside temperature, and this will have risen by the end of the burn. As a result, the heat available to heat your house will be somewhat less than this ideal figure.

Even the best case level of heat with the specified 45lbs of wood for 24 hours is insufficient to heat my home adequately. I know this because my heat pump will output 8.5kW and it barely keeps up at maximum output. I live in a temperate climate where snow might happen once in ten years. i.e. it rarely gets colder than freezing temperature overnight, and usually not to that level.
7 years ago
I had some more thoughts, Marc. I was thinking of straight paraffin wax for a thermal mass in use with wood heat, so that it is at a temperature that will be able to bring down the temperature of the exhaust gas to a point where most of the energy is sucked out of the exhaust stream (for efficiency), while still being able to radiate/convect the heat effectively to a room. This is not what you want for walls. If you just want to capture the energy from sunlight on a warm day, I would look for something that changes phase at room temperature, possibly at the minimum (20C). That way it is absorbing heat from the moment the wall gets hotter than is necessary, and the low temperature inclines it to suck in more heat from the environment.

Paraffin wax on its own is too high melting point for this application. I would look further down in melting points, such as petrolatum (petroleum jelly) and mineral oil (baby oil). Mineral oil is pretty cheap. Maybe they can be used straight or in combination? I don't know. I note that DuPont Energain (TM) has a melting point of 22C and is composed of a paraffin wax (not sure exact chemical composition, but certainly petroleum based).

Some experimenting is probably required, let us know how you go.
7 years ago

paul wheaton wrote:A rocket stove is for cooking outdoors.

A rocket mass heater is used for heating your home.

Sure. I'll accept your nomenclature. I suppose that we might call the rocket mass heaters some have made without the mass "rocket heaters", even though that is probably more confusing. If that's the case, the CWS is better named the CWH.

paul wheaton wrote:The math on how they are so much more efficient is spelled out at

The great thing about a rocket mass heater is that not only does it heat your home with 1/10th of the wood, but it requires far less fooling with than a conventional wood stove. If nothing else, you have 1/10th the wood to fool with. But even better is that you can burn a fast hot fire in the evening and then you're done. The house stays warm through the night with no further fire. You wake up in the morning and the house is warm - no need to start a morning fire.

Keep in mind that a rocket mass heater leans heavily on radiant heat and conductive heat - which are both far more efficient than convective heat. Also keep in mind that most people run their conventional wood stoves in a way to try to make the fire last longer. So while their stove might be rated as 75% efficient, they might be getting only 10% efficiency. Or worse.

I've read your page several times already, before I even registered for Permies. It's part of what got me so interested in these rocket heaters to begin with. And what I want to reiterate is that the secondary burn concept, using insulation to get the gases to combustion temperature before attempting to extract the heat, is an excellent one.

I've also posted here on the subject.

As an engineer, like professor Rich I think it's better to attempt to quantify what the stove is doing thermodynamically. I haven't noticed that conductive or radiant heat is any better than convective heat. I don't get any hotter from a 1000W radiator than I do from an electric 1000W bar radiator vs a 1000W fan radiator vs a 1000W oil filled radiator (which is primarily a convector I believe, which is why it is designed like a passive heat sink with widely spaced fins). I will get as warm from a 70W electric blanket if I stay in bed but that's a big proviso. And the only reason that works is because both above and below the electric blanket there are several inches of insulation (blankets) to trap the heat in, where my body is. It's not because conduction is more efficient. If I took any of the other heaters under the covers I would soon be scorched because it is the insulation that is providing the "force multiplier" so to speak. (I am not referring "force" in "force multiplier" as in physics, I'm using the term as is typically used, to get more from your investment.)

From what I can see, there are some unstated provisos that are used to get the 1/5 or 1/8 or 1/10 the wood use required that I have seen. Namely, that in order to achieve those benefits you either need some combination of:
a) Have some serious insulation in your house (e.g. R30 or more)
b) Have a small house
c) Not live in a very cold climate
d) Are willing to put up with a (much) colder house except the room you choose to heat
e) Are willing to constrain yourself to being on/near the rocket mass in much the same way as you need to be proximate to an electric blanket or water bottle in order for it to be useful.

What could also be made explicit is the amount of wood that is deemed necessary to heat a house of particular specification (size and insulation level, outside air temperature) and for how long in order to heat it effectively so that the claims might be evaluated. From what I can see from the figures typically bandied about, 35-45lbs of wood worst case, we are not looking at big power levels even if the heat is liberated with 100% efficiency. I don't think that the comparison is apples to apples with the CWS in order to get the 1/5-1/10 figures of wood used.
7 years ago

Marc West wrote:Hi all,

Firstly, to answer the original poster: there is an encapsulated PCM material called "Micronal" made by the German company BASF:
but I appreciate that that post dates from a couple of years ago...

I have a slightly different enquiry, and I wonder if anyone can help.

I co-own some land with a small community, and we wish to build a cobb wall on it, partly to provide support and warmth for trees like apricots and peaches. (We are in Cardiff, Wales, UK). Now, it has long seemed to me that incorporating PCM in the cobb structure would be a great way to extend the warm period at the end of a hot day. I've just read this thread and realise that paraffin wax in tins embedded in the wall may be just the thing. (Thanks to Frank R for his clear posts!)
The issue for me, then, is to find a paraffin wax that melts at the right temperature, which I guess should be in the region of 22 degrees C or so, for it to be most use for extending the warmth of the day for plants. (Anyone have a better guess?). Now, as I understand it, paraffin wax has a melting point of between 46 and 68 degrees C (variable because paraffin wax is not a single substance but a mixture of compounds [called 'alkanes'] of the form CnH(n+2), for example C25H52). So using 'ordinary' paraffin wax wouldn't be any use, because its melting point would be too high.

So how to get a lower melting point paraffin wax-like substance?

2 possibilities occur to me:
1) mix in or use some shorter chain alkanes. Ought to work fine, but looks like it will be a lot more costly than standard paraffin wax is;
2) Dissolve something into standard [inexpensive!] paraffin wax to lower the melting point, in rather the way that dissolving salt in water lowers its melting point (which is what we do to ice to make it melt when on driveways etc). Does anyone know what substances can be dissolved in paraffin wax to lower the melting point? Salt? Some other "more organic" substances that will dissolve better? I'd be happy to experiment with a thermometer and a saucepan of the stuff in a water bath, but I don't really understand how to calculate the likely effect on melting point of dissolving a certain quantity of material X into paraffin wax.

Not being a chemist, I don't really know where next to go with this. I'd appreciate any thoughts you guy may have on this. Or if someone knows of affordable off-the-shelf paraffin wax substances with melting point in the right range perhaps they could let me know.

Thanks in anticipation...


Hi, thanks for the compliment. I haven't yet got to playing around with paraffin wax for this purpose yet. However, a quick googling suggests that petrolatum (petroleum jelly) may be added to reduce melting point of paraffin wax. Since that melts at 37 deg C, it might pay to add something with shorter chains, perhaps a mineral oil. Good luck.
7 years ago
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.
7 years ago
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.
7 years ago

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.
7 years ago
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).
  • 7 years ago

    allen lumley wrote: Frank R.: While there are stoves that can be run nearly as efficiently as a rocket stove, and new ones being ''rushed to market'', the new owner soon teaches themselves
    how to 'load that dirty bastard up.' ' to let it burn all the day without taking up any more of my time ' ! ! i.e. burning dirty and inefficiently, like my neighbors ! The worst
    ones are the ones that deliver domestic hot water, they smolder along 24-7, 52 weeks of the year to guarantee a hot shower at will 24-7, 52 weeks of the year !

    I will make a General statement that while the Rocket Stove has a major draw back, that it needs frequent attention to run at its most efficient, that this is the only way
    that the Rocket Stove can be run! This is also its biggest asset, forcing you to burn it in its most efficient manner, or get smoked out of house, or left with a cold house !

    Living with a Rocket Stove and its hungry Dragon is a life style commitment! If you are not willing to live this way, the Rocket Stove is not for you! If someone has to be
    'out of the house' to be 'the Bread Winner', then they most spend 6-8 hrs prepared to and attending on their Rocket Stove To get 20-25 hrs of heat stored inside their
    Thermal Battery,or have a Partner willing to assume that job ,or deal with a cold house !

    Some good points. Thanks for your comments.
    7 years ago