Stupid pseudo-rocket questions

Hi,

I'm very fascinated by the RMH videos and am currently watching the Freaky Cheap Heat video. I don't think I'm quite ready to build my own RMH for various reasons, one being space/weight constraints in a small cabin.

I was wondering if there are some principles one could take away from the RMH idea and make a regular box wood stove slightly better? I'm not under the illusion that I could achieve anything near the performance, but could at least some of the heat be stored in a little bit of mass?

Some people put firebricks in their wood stove box to store heat. But could more of that be done? Could I just stack a ton of bricks around/on top of my wood stove to have them soak up the heat? What about the exhaust, traditionally very hot in a wood stove? Could I have some of that waste heat siphon off into bricks? I suppose there would be an issue with the conductivity between the chimney pipe and the bricks..

And, in a traditional wood stove that doesn't burn at 9999°F, would taking heat out of the chimney mess with the air flow/burn dynamics?

Has anybody tried taking a few of the RMH principles and making a regular stove keep heat a little bit longer or be more efficient?

High efficiency of masonry heaters is affected by ability to extract the heat from the exhaust. Even if you put bricks on the cast iron stove, it will not extract more. Of course they will get warm being heated by radiation and conduction of the iron touching them. The iron will be touching internal bricks that already extracted as much heat as they could in the firebox. The heat will just get transferred to the stacked bricks for a illusion of improvement. Without them, the heat would be radiated towards the room. They may change the dynamics of this process, but will not extract more.

I already wrote it in some post, but will re-paste it:

Efficiency of the heater can be expressed by an equation:

Ef (firebox efficiency) * Ea (thermal mass absorption efficiency) * Ee (mass emitting efficiency)

Assuming that you have a perfect firebox with imaginary Ef=1 and Ee is also 1 then all loses are in thermal mass ability to accumulate exhaust heat which is expressed as:
Ea = (Tf - Te) / (Tf - Tr)
Tf = firebox gases temperature
Te = chimney exhaust temperature
Tr = room temperature

When you use values: Tf as 1200 C, Tr as 20 C and Te as 100 C you get 93%, but if you allow gases of, let's say 500 C to escape to the chimney then you get only 59%. That's why mass extracting heat from the exhaust is the key.

I you can not build heavy mass, you can always use light barrel with some bricks/cob to extract exhaust heat. This will help a lot.

I see, that makes sense. So "increased efficiency" is out of the window without extracting from the exhaust. Do you think attempting this on a regular wood stove would ruin the burn dynamics? I assume these stoves are somewhat designed for a certain exhaust heat to create the draft?

Another aspect would just be retaining the heat via some mass, even without improved efficiency. It's a common "wood stove in small cabin" experience that the stove burns way too hot, and people open the window to cool it down. Then the fire goes out and 2h later it's cold again.

Even without improved efficiency, could this unfortunate sequence of events somehow be averaged out by stacking a bunch of mass around the firebox?

To determine if you could add mass to exhaust of the existing wood stove would need some experimenting as each one is different. You would be measuring the exhaust temperature before and after extraction and observe the behavior of the stove.
Stacking bricks will surely change the dynamics of heat radiation. For example, maybe you don't want to have too much heat during the night and it would be accumulated in stacked bricks to be radiated during the day.

Experimenting is the key and good people here will help you with details.

M Pearson wrote:Another aspect would just be retaining the heat via some mass, even without improved efficiency. It's a common "wood stove in small cabin" experience that the stove burns way too hot, and people open the window to cool it down. Then the fire goes out and 2h later it's cold again.

Even without improved efficiency, could this unfortunate sequence of events somehow be averaged out by stacking a bunch of mass around the firebox?

This is something we're (still) experimenting with, though not with any actual testing/ info recording, only playing around with it. While our house is not small, the woodstove we have is rated for about 400sf more than our house - but, we have cathedral ceilings, making it more difficult to regulate heating and cooling, even with the high ceiling fans (which are a cleaning/maintenance nightmare).

Three years ago, we bought this woodstove, and began the process of learning it's quirks, and how it works so very differently from what I grew up with, as well as how it plays with our living space. It was nice to be warm all winter, for a change - so very unlike relying on the heat-pump, when the temps were too low for there to be any heat to pump! Bit, we were most definitely shivering hard in the mornings, and opening windows, within an hour or so of starting the stove.  

Two years ago, in an attempt to regulate the heat better, we started putting fire bricks around the stove - about a foot away. The sides facing the stove warmed, but were comfortable to the touch, and cooled down about as fast as we did, once the fire was down to embers. So, we moved them closer - about 6 inches away - and doubled the number of bricks. It was an improvement, but not by much.

Last year,  we realized that (because heat rises), and most of the blocks were sitting only a little above the bottom edge of the stove, we would need quite a few more bricks, just to get them high enough to even absorb the heat. (I'd call that a 'DOH!' moment)

Since I actually do use the top for cooking, I've been reluctant to stack bricks there, as well as a little concerned about excess heat retention damaging the already slightly buckling stovetop), but I think I'll start experimenting with a single layer, on the back, which will then be (I hope) similar to having a 'keep warm' space, for things I would like to have for multiple serving times, without burning - while at the same time, adding to the mass, for residual heat. So, this winter, I plan to add 4 bricks on the back side of the top of the stove, and more height to the layers around it. I've been dismantling a small outdoor brick oven my husband made and abandoned for inefficiency, so there are plenty more bricks to add.

A standard wood stove, even new models, depend on the exhaust being hot enough for creosote to not condense in the chimney, as well as provide sufficient draft. A very nice one my wife bought last year to install in the (ranch style) family home required me to add about 6 feet to the height of the chimney for adequate draft. That was still a foot or so less than the minimum total height in the installation manual, but any higher and it would not have been possible to sweep the chimney. It did need sweeping several times last winter even with a nice insulated stainless chimney (though it would not have been unsafe if swept only once or twice - my wife is a fanatic about keeping chimneys swept since she has heated with wood for decades.)

You can have the efficiency of a rocket mass heater without the extreme mass of a standard design. There is one called the "Cottage Rocket" designed by Chris McClellan (Uncle Mud) who is active on Permies; this is the size of a 55 gallon drum. There is also Matt Walker's "Tiny House Cookstove" which is larger but still reasonably sized for a modest house.