terry jones

Lawrence Wood wrote:  With an RMH you have a mass of heated material that slowly radiates the heat into the space in a much more localized manner. I don't doubt that you can blow a fan across it to move the air around and heat things more uniformly.  If you are one who likes a uniform heat throughout your house I doubt an RMH would fit your desires. if however you can adapt to having heat in one or two localized places then this is a potentially a good option.

I may have misunderstood what you meant here, but as I read it it does not give quite the correct picture.

IF you meant a RMH may not heat the entire house (due to layout) then ok, yes the heating is certainly not uniform throughout the house. But it seems to read 'there are hot and cold spots within the area the RMH heats' which is not the same as the first interpretation. If you did mean the 'second' interpretation then that I feel is incorrect. Within the area heated by a RMH I think the temperatures are quite uniform which is part of the heating appeal. That is due directly to the radiant heating mechanism, the radiant heat not hitting our bodies and warming us hit the opposite wall, which in turn heats up. IT then becomes a secondary source of radiant heat within the room as it too will then radiate heat to any surface in the room colder than it is. And so on and so on.

The net result is quite the opposite of the 'cold spots and hot spots' that it seems you think. Even the floors are a part of this absorb/emit radiation cycle.

If I did misinterpret your words then apologies.

I think people often overlook how much heat will be lost if the heater is NOT where the heat is ultimately wanted. 'Put it in my basement' sounds good, how are you going to get the heat from there to the living space? That's a whole can of worms right there. Even if you did solve that problem efficiently, there will be an awful lot of heat still down in the basement that you will never be able to get to your living space. The transfer of heat from the firebox is not the primary way of getting heat from a RMH (indeed that is why we insulate it as much as we can) but notwithstanding, it WILL and does get hot/warm. That will be lost in the basement, whether it is a batch heater or j tube etc. (if it is in the basement you'd be mad to go j tube anyway, but that is not the main point here)

For the amount of work/effort needed to figure out a way to transfer the heat from the basement to the living space, I personally think putting that work into figuring out how to strengthen the above floors so you can have the RMH in the living space would be more useful.

R Ranson wrote:

So radiant heat makes you feel more comfortable at a lower air temperature - which uses less fuel/resources to heat an area?

That sounds like another reason why RMH would be more efficient than a wood stove.

Here is a little investigation I did into the radiant heat side of RMH's. https://permies.com/t/55993/rocket-stoves/Bell-hidden-advantage

I was compelled to investigate this side after living with my (first and so far only) RMH built I guess six months ago now. The most immediate difference we all noticed was the enveloping sense of warmth that was never present with the (hot water) radiators, which work by heating circulating air. One thing we all noticed was how warm our feet were in comparison. When the heat is delivered by heating the air, well your feet are always cold!

So any of the illustrations already given about how you can 'stand outside in the snow on a sunny day with only a t-shirt and feel warm' are very true, the radiant heat is what is warming you. Once the sun goes behind the cloud you suddenly feel cold again, the ambient air temperature at work.

I found that the temp of the radiant heat source has a lot to do with it. For sure, a hot metal body of a standard heater radiates heat, but perhaps NOT at the right temperature (tho to be honest I did not run those calcs, but a perusal of my 'thesis' will show what I mean). The mass in a RMH is no way the same temperature of the metal body of a standard heater, it is much lower....but far more in tune with our bodies. (see the data given in the link) The fact that that temp is much lower is then coupled with another point, the surface area of that blackbody radiant source. It is vastly larger than the surface area of the metal heater. That natural consequence of how the RMHs are built perfectly fits into how blackbodies radiate heat, as the temp of the radiating body is much lower, to get the amount of power into the room requires a much larger surface area.

It all somehow 'magically' all fits together! The metal heater has a small surface area at much higher temps (so the power radiated to the room is the 'same' in this argument), but we find the frequency of that higher temp is not correct to heat our bodies) and the surface area of the RMH is much larger but lower temp (ie same power to the room) yet falls exactly into the bandwidth that heats water (ie our bodies) most efficiently.

Again, quite magical.

When it is the air that is heated and then heats us, apart from 'hot air rises' (explaining the cold feet) it was always a mad scramble to 'shut that bloody door quickly!' because every time you opened the door, the warmth (ie the heated air) was lost immediately. It was 'just gone, lost totally' and we then had to heat all of that cold air that rushed into the room before you felt warm again. When a lot of the heating value comes from radiant heat (sure, of course the air is heated too but it is far from the primary heating mechanism) then not only is less heat lost from air exchange but the radiant warmth just carries on without a beat if the door is opened.

It IS the radiant heat that is giving warmth 'x' hours after the fire has gone out.

I also very strongly feel that the radiant side of how RMHs work has been overlooked a lot, and is far more important than previously suspected.

why not just skip firebricks and go to castable refractory?

Donald Kenning wrote:

Multiplicity: When you look at the individual efficiencies (i.e. design and transport efficiency) in any "system", usually the efficiencies multiply (not add). So let's pretend you have a "design efficiency" of 60% and a "transport efficiency" of 50%. The "system efficiency" would be 30% (0.6 X 0.5 = 0.3).

At this point it is looking like heat transport is a bigger contributor to inefficiency than the inefficiency of the design. let us say we calculate the wood burning stove to be 80% efficiently. If the transfer efficiency is 25% , that would mean the "heating system" is only 20% efficient at actually heating the room. Wow, no way!! is it really as simple as that?

Summary in Chapter 6

http://heatkit.com/docs/ec_report-verification_of_emission_factors.pdf

Hi donald

not sure quite where you are going with this. Originally I thought you were doing rough back of the envelope calcs to get an idea of the efficiency of the transfer of heat to the room, but using (as it turned out) the wrong figures. The idea was sound enough. But this section did not change so am left a bit unsure now. I presumed the rough figure of 20% was that 625 is roughly 80% of 850...excepting now we can't use those figures...in order to multiply 20% by 60%.

As they are in chimney figures, well I think about the best you could use those figures to tell us anything would be that that is the heat, or efficiencies if you want to look at it from that angle, of the flue itself. Apart from making it easier, and cheaper to install and keep clean, I would imagine a lot of the reason for exposed flues in the space is to gain extra heating?? I am not sure if it was stated in the paper, but where was that temp gathered?? Ie, was it the surface temp oif the flue at those places? I ask, because (tho I could be completely wrong) I find it a little hard to believe/grasp that if it was measured in the centre of the gas stream it could lose that much heat in eight feet. One would presume there is quite a bit of flow, how long would the gas take to move eight feet I wonder. Anyway, just sounds implausible but then again I am certainly not an expert.

Maybe one way you could use those figures (flue temps)...it seems you like a bit of thinking and number crunching...we are told how much fuel is burned and in what time. That will give us a mass of air flowing thru the system in unit time. We already have the temps, you could then calculate the mass of air at those temps leaving the system and come up with a loss figure of the heat via the exhaust.

Donald Kenning wrote:

Transport efficiency This is the efficiency of getting the heat generated into the room being heated before it leaves the house in the form of exhaust. Now, I commented before on this. I do not want to get into it too much. There are 3 mechanisms for heat transfer, conduction, convection, and radiation.

For a wood stove there is little surface area devoted to these 3 forms of heat transfer. The "conduction" is the fire warming up the stove and part of the pipe. Infrared radiation will "radiate" from all (very limited) surfaces of the stove and pipe. Convection will only generally occur above the stove, not the chimney.

For a rocket-mass stove: The "rocket" and the"mass" has a lot of surface area and a lot of volume (compared a wood stove). Therefore, there is a very high potential for storage and transport of the heat in to the room.

What I noticed in the report was, for the stove testing at 825F at 1 foot, it was 650F at 8 feet. What that implies to me is the transport efficiency might be much lower than the 70% I stated in the first post. Subtracting room temperature (70 F) from both and dividing suggests only about 25% to 30% efficiency. Ok, there is something I am surely missing.

Multiplicity: When you look at the individual efficiencies (i.e. design and transport efficiency) in any "system", usually the efficiencies multiply (not add). So let's pretend you have a "design efficiency" of 60% and a "transport efficiency" of 50%. The "system efficiency" would be 30% (0.6 X 0.5 = 0.3).

At this point it is looking like heat transport is a bigger contributor to inefficiency than the inefficiency of the design. let us say we calculate the wood burning stove to be 80% efficiently. If the transfer efficiency is 25% , that would mean the "heating system" is only 20% efficient at actually heating the room. Wow, no way!! is it really as simple as that?

Summary in Chapter 5

http://heatkit.com/docs/ec_report-verification_of_emission_factors.pdf

I only glanced at the report really, so I could have very easily missed the detail. Those figures of 825 at one foot, I vaguely recall that it was specified as 'within the chimney'?? Heck, I should check again, might save us both time! Luckily the download was still in the toolbar, so can keep this reply open.

Table 2 specifically mentions 'low chimney' and 'high chimney', and the preceding blurb says 'In general the higher emissions heater had lower flue temperatures than the lower emissions heater.', again specifically mentioning flue temp.

Table 8 has explanatory 'One foot (30 cm) above heater in chimney, average', again in chimney.

I scrolled quickly thru again so could certainly have missed it, but using those figures from 'in chimney' to have a stab at the heating transfer efficiency could lead to skewed results. Which flatter the stove I might add!

You probably read it more deeply than I did (yawn haha) so apologies if I missed the part and have the wrong end of the stick.

I posted this thread on donkeys, but thought it might be of interest here as well. Thoughts, corrections or additions are most welcome.




Most of us here use and are probably in love with our rocket heaters. We would also be aware of the many benefits, smoke free combustion, the lovely long term warmth (for those installations that incorporate mass storage) and of course, the vastly decreased fuel usage.

Almost all of us are able to give reasons for this vastly decreased fuel usage, clean burn means that all of the fuel is burned, the very low flue temperatures mean that we have trapped and stored the heat for long term usage, that is mostly achieved by the use of mass. Some use a long flue buried within a cob bench of some sort, others use the concept of a bell. Whatever the mechanism, the point is we gain extra efficiency by lowering the flue temperature.

Those who have lived with a mass heater (which naturally includes masonry heaters) will most likely attest to the gentle warmth and comfort they provide.

In this essay (?) I want to examine this from an aspect that I personally have not seen before. I am sure that there is more than a little measure of truth in it, and is perhaps a hitherto not recognised contributor to WHY rocket mass heaters are so effective and why we can achieve such seemingly low fuel usage.

By default I might use the term bell often, maybe only because that is what I use myself. It should be known that in this discussion it is a generic term for 'mass', however it is realised in a particular situation.



There would be many places to start from, but let us start from 'how do we get warm?'. Well, obviously we have a heat source in the room. This can be from a radiator fed with hot water, a normal wood heater in a fireplace, a standalone wood burner, a rocket heater. However that heat gets there is not the concern right now, simply that we have a heat source. What we are going to examine is the transfer of the heat from that heat source to us, the occupants. And this is where I have not seen any examination of this aspect regarding the effectiveness of the bell.

There are 3 forms of heat transfer: Conduction, Convection and Radiant heat. that is well known, but I will be using this reference with comments as applicable http://www.herschel-infrared.com/heater-fundamentals/radiant-versus-convection-heat/

This one is interesting.....'In a Comfort Heating situation, Conduction (physical transfer of heat from source to target by direct contact) is not an option, so whilst it is the most efficient method of the three (presuming a suitable medium to conduct of course), we’re left with Convection or Radiant heat.'

Well there ya go, those of you that do use a bench warmed by the exhaust DO have the most efficient form of heat transfer. I don't personally as I have a large brick bell (tho there are ways to have a bench with a bell, see this site http://batchrocket.eu/en/)

Conduction of course is the transfer of heat within a solid substance, heat one end of the spoon and after time you will feel it get hotter were you are holding it, the heat has moved from one end to the other within the material.

Convection is the transfer of heat by the movement of the material itself, gases and liquids. As the gas/liquid heats, its density falls and so rises to the top and is replaced by colder gases/liquids that, in turn, heat up and rise and get replaced by colder gases and liquids. This process continues in a never ending loop until all of the material reaches the same temperature, equilibrium.

The last heat transfer mechanism is radiation, where heat is given off as an electromagnetic wave. As such it transfers over a distance and once it hits an object, ourselves, the far wall, the kitchen bench, that electromagnetic wave induces the material to warm up. Once that kitchen bench warms up, it too then becomes an emitter (in this case of lower temperature) electromagnetic waves which will hit yet other objects in the room. Air is essentially transparent to these IR waves (infrared) so they pass through the air unaltered until they hit a solid object, think of a cold day on the ski fields when the air temperature itself is very cold, but you feel warm from the IR/light waves from the sun. If a cloud passes in front of the sun, you will suddenly feel cold again due to the low ambient air temperature.

So there is our basic background information to delve a little deeper into 'why the bell contributes to the efficiency of these heaters'.

Again, I think it worthwhile that you read the links provided.

It turns out that, just like light, IR is a spectrum, that is a range of wavelengths. It follows from that that these differing wavelengths have differing properties, ie not all IR rays are 'created equal'. It is clear that *our* interest lies solely in space heating, if I were an industrial manufacturer I would have a very different set of criteria. And those two different objectives would be realised by different IR radiation. 'It is IR' is insufficient to understand what is happening.

'IR radiation' is broken down further into three broad categories of wavelengths. See this link http://www.herschel-infrared.com/heater-fundamentals/preferred-wavelengths-comfort-heating/

These are:"Infrared -A , classified as the “hottest” Infrared with temperatures up to 2,700C and wavelengths of 0.7 – 1.4 microns and is also called “Short-wave” or “Near” Infrared;- Infrared – B is infrared with temperatures of 500 – 800C and wavelengths of 1.4 – 3 microns and is also called middlewave or “Medium” Infrared;- Infrared – C is infrared with temperatures of less than 500C and is the final and broadest waveband of 3 microns – 1mm and is also called “Longwave” or Far Infrared."

This is shown in the following graph (from the same link)








What is to be taken from graphs like this are a few things, that a 'blackbody' (as these emitters are known as-no need to delve into that) radiates some energy at all wavelengths, as clearly seen above. However, that does not mean they radiate equally at all wavelengths. We can clearly see that as the temp rises the peak energy emitted goes to shorter and shorter wavelengths, and that peak rises in intensity dramatically. The converse-and where we are heading in this little 'essay'-is also true, as the temperature of the emitter falls, the peak shifts to the right (longer wavelengths) and the emission becomes more and more consistent over those longer wavelengths.

This graph ends arbitrarily at 10 microns, that is not necessarily the 'end of the IR spectrum'. From http://www.noritake.co.jp/eng/products/eeg/support/heat/far_infrared_character.html I found the following data.."Wavelengths of 3 μm to 1,000 μm within the infrared range of electromagnetic radiation (light) are known more specifically as far-infrared. Because many materials, including water, plastic, paint, and foods have an absorption range between 2 μm and 20 μm, far-infrared radiation is effective at penetrating and transferring heat." (my emphasis)

So this introduces two things, that IR is 'extended' to 20 microns, and more importantly it introduces the WHY of choosing different wavelengths. Water absorbs certain wavelengths far better than others.

this is a graph of the absorption factor of water at varying IR wavelengths







Note how the absorption of water shown here relates to the breakdown of IR into IR A(near IR), IR B (mid IR) and IRC (far IR). The absorption of water is very high indeed across the IR B and IR C wavelengths.

The human body is roughly 80% water, and so we can see that for the most effective heating of the body it would make sense to use the most appropriate IR wavelengths, namely IR B and IR C.

This leads us back to the term used before, blackbody. It turns out that we can calculate the wavelengths emitted by a body, yes even the human body tho of course we are referring to a bell here, and that is dependent on the temperature of the body emitting the radiation. (Don't get hung up on, or assume 'body' here means a human or animal body, it is just a 'thing')

From wikipedia "Black-body radiation is the type of electromagnetic radiation within or surrounding a body in thermodynamic equilibrium with its environment, or emitted by a black body (an opaque and non-reflective body) held at constant, uniform temperature. The radiation has a specific spectrum and intensity that depends only on the temperature of the body."

Working in reverse as it were, and knowing the wavelengths best absorbed by the body for warmth, it is clear that we can use that knowledge to determine the optimum temperature of the blackbody, or in the real world, our bell.

From the above graph and earlier data, we know we want IR B and IR C, or wavelengths between 3 and 20 microns. And that we see very high and almost constant absorption of those wavelengths by water (ie the human body).

There is a nifty little interactive graph on this page http://www.pveducation.org/pvcdrom/properties-of-sunlight/blackbody-radiation which although it loses all detail at the frequencies we are interested in DOES show the points raised above, that as the temp rises the peak sharpens dramatically at higher temps and the amount of radiation in the wavelengths we are interested falls greatly. But just above that interactive graph is a calculator that we can plug numbers into.

So let's make a few assumptions about the temperature of the bell. We could go max and min, and plug them in. I'll let you play with that. I will simply, and quite arbitrarily assume we have a 'constant temperature of 60 degrees celsius'. Yes, that will go up and down, but the whole idea of using mass is to keep a relatively constant temperature. You can plug different values into the calculator and see for yourself.

So 60 degrees celsius is about 325 degrees kelvin if my memory serves. (degrees celsius plus 273 gives degrees kelvin-without checking)

The value given for that temperature is '8.9231 um'.

Slap bang right in the middle of the peak absorption range for water.

Plugging in temperatures (ie max and min) for the bell either side of that arbitrary value does nothing, they all fall within the optimum range for absorption by water, that is they all emit IR radiation of type B and C.

You can now browse the links by Herschel (given above), they go into why radiant heating is better than convection heater, and how much energy is saved by using heat that falls into the most optimum wavelengths. And all of those arguments explain why the bell, or mass, is such a perfect way to provide warmth.

To sum up, the bell has 'only' been viewed as a way to capture heat, store it and release it later. Yes, that certainly contributes to the efficiency of the heater. I feel that the TYPE of heat released, ie the IR radiation it releases has not been previously considered. I do not know how to calculate it, but it seems to me, as fortuitous as it might be, that it releases the *perfect* heat for humans must also contribute in some way to the efficiency of these heaters. Any and all illustrations used by Herschel in their promo of the increased efficiency gained by using radiant heat apply equally well with our heaters.

I hope you found this as interesting and enlightening (?) as I did.

Peter van den Berg wrote:Terry, a man from Australia is doing the proof reading in order to spot the typos and suggest corrections. Most of the corrections he suggested are done, my writing style isn't enthousiastic, I know that. Can't be helped, that's my writing style. However, I take it as compliment that you as a non-native speaker are able to understand pretty well everything.

my writing style isn't enthousiastic, typo, enthusiastic

To be honest, these last two posts were meant to be a bit of a laugh, and Blendi replied whilst I was doing the first response. It is clear then that me mucking about might muddy the waters a bit. I cannot speak for Peter of course but essentially I agree with Blendi's viewpoint. I think what peter is doing is extremely valuable and it would be a shame for his site to not have the impact it can have due to factors such as simple spelling mistakes as a first example. It is pretty quick and easy for me to go thru and correct the most obvious ones and I gladly do it.

But, as blendi rightly points out, even after that the underlying skeleton of the language can still be awkward at times. In a couple of instances I have suggested some alternative wordings and peter had no hesitation in adopting them. What I feel very keenly when suggesting these alternatives is that, even to me, they sound like me and definitely NOT Peter. That makes it hard for me to suggest more, because it then feels like I have taken over. Yet, to be completely honest, I do feel that quite a bit more editing than mere spelling corrections is needed to aid in readability etc. But I am hesitant to suggest further, whether it will come across as an insult (hardly! Crikey, for someones second language at a minimum it is pretty damned good!) or that I feel like an invading force coming into someone's land and taking over.

So, for the moment, I am limiting myself to simply correcting typos and such when the successive pages are published. That way Peter can let his 'creative juices flow' and the data gets out there. After that, my hope is that Peter will agree and allow me to have a go at the other side (or someone else) and do any needed re-working of what was written in order to increase the reading warmth (still love it!)

If Peter agrees that that would be a worthwhile exercise it is still down the track a little. For now, none of these types of distractions until he has done HIS side, the giving of the hard data that very few others have.

In any case, I am sure peter knows what he wants and seems to me a strong enough character to be able to articulate and hold firm to those wants.

Blendi Kraja wrote:However, I would say an native English-speaking editor is needed to check the texts. I understand pretty well everything written there, but if the site is to propagate the idea among the laymen, it has to be more simplistic, the reading more logical.

In other words, there is no warmth for the reader (reading pleasure, if you will) that is reading for the fist time in his life about such a thing.

fist time in his life about such a thing. typo first

just as an aside, some of the non english when translated to english can accidently be delightful after the translation, well to me anyway. I love this bit 'there is no warmth for the reader'. What a lovely way to say it! I doubt a native english speaker would come up with a turn of phrase like that. (that is assuming that you, from albania with a surname of Kraja is in fact a non native english speaker. If that is not correct then I just made an ass of myself)

not sure I can help, but I feel your pain in not getting a response!

What would I do in your situation? Try it, outdoors if needed (ie depending on what is happening with exhaust gasses)

As far as I know, the height of the riser is not 'super important', possibly better to be longer than needed than shorter. Too long I guess would be when it no longer contributes to complete combustion yet robs heat from the exhaust. I would not be nitpicking that aspect until I found evidence to do so.

An experiment does not require the presence right now of the complete insulation, as whatever happens the heat needs to be into the bricks before insulation helps. So, for a finite amount of time at least, the performance will be the same insulated and uninsulated. How long that is I have no idea, but you can certainly test things like draw etc etc right now with a test burn.

Is it meant to be portable? I am trying to understand why the rigidity of the riser is important to you. If it is not being moved then gravity should do it's job. (I can't see the pics as I type so cannot refer back)

Anyways, as I said I personally would just fire it up using what you have and see if you need those extra bricks. Plus it is fun, even if a bust in some way. Most importantly it starts to give you the data you are trying to gain 'in written words', which is never as good as hands on experience.

good luck, hope others more knowledgeable chime in

EDIT having posted I could see the pics again. If your riser stacks as well as the bricks you have stacked, then I would think it perfectly fine rigidity wise if once built it does not move. Especially for experimenting purposes, very thin clay slip is perfectly adequate, no need just yet for refractory mortar or cement.

start burning!