12 rocket heaters - burning wood 5 to 10 times more efficiently

How do you keep the inside surfaces of the exhaust from getting covered with soot?

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I'm building a masonry heater, which is sorta the rocket stove done vertically, and out of expensive materials. $2,000 vs. $20 These are a few of my thoughts on the stoves.

Airtightness and cob construction: Meloc, I don't think they are being defensive. The Masonry Heater Association did a study on makeup air and the requirements for an outside source. If the rocket stove is anywhere near as efficient as the masonry heaters, the airflow is not that great. Even in very airtight homes there isn't a problem getting enough air. So, if you don't need much air, and cob homes aren't very airtight, then it makes sense that its not worth the while. They are already achieving very efficient combustion, and pulling most of the available heat out. As far as an airtight cob house goes, I believe it is not what you want to aim for. You have all those tons of damp clay that has to dry out. You don't want to leave that in the living space.

Low exhaust: It sure isn't up to code. If I'm not mistaken, most building codes say the chimney has to be 2' higher than anything within 10' of it. But, then again, if you're building a cob house, are you building to code anyway? Efficient burning and maximum heat extraction are probably what allow the exhaust to be so low.

Soot at the exhaust: Masonry heaters do not have to be cleaned, for the most part. Open the cleanout doors at the bottom of the vertical channels and sweep out a bit of fly ash every year or so. No creosote if properly dried wood is burned. (under 20% moisture) If the rocket stove burns as efficiently, there will be no soot at the exhaust. Soot is unburned fuel. Burn hot, add secondary air, and you should have no problems.

Korean floor heating: Reminds me of the roman heated floors. Warm feet make a warm body.

Draw/draft: In masonry heaters, after the first firing, as long as you don't wait days for the next one, the thermal mass is still warm. A warm chimney will automatically draw, even without a fire. One of the reasons to put a damper on a masonry heater is to stop this loss of heat after the fire is out. It'll keep pumping heat out of the chimney until its cold. The rocket stove should be the same with all of that thermal mass.

Michael

At the workshop, there was a guy who did nothing but build gobs of masonry heaters. He had a lot of excellent stories.

Soot: there isn't any. There is a lot of thought that goes into getting a complete burn. So I guess that is why there is no soot - the soot is all burned before it can get soot on something.

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So.....no creosote build-up, since it's not an airtight stove. It reaches 1500 degrees? The thermal mass soaks up the heat and radiates it into the home.

Basically, it's like a soapstone stove.

I take it that you would have to build a fire once daily to keep that thermal mass warm.

Did these people advocate a constant fire, or what?

What was the resulting temperature in the building?

With today's tighter homes, dryer exhaust, large range exhaust, power vented water heaters, etc., make-up air may indeed be necessary when using a wood burning appliance....

These people do not advocate a constant fire. More like .... build a fire when you want it warmer. The idea is that you build a small, short fire in the morning. You then stay warm all day and all evening. In the following morning, it is still warm, but you might build a fire to be warmer still.

It isn't as if the room gets suddenly cool. It's more like the heat in the mass has a half life. As the hours pass, it gets cooler.

I never saw a thermometer when I was there.

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Hey Paul,

Is there somewhere that those of us on dial up can read and see still pics of this? Thanks!

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Well, I did post some of the pictures to this thread:

rocket stove and butt warmer

It was with this thread where I first started the journey in learning about this stuff. Somebody in the thread replied that they knew of a workshop about it and I should go. So, next thing ya know ....

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So I got some feedback here about the draft.

The highlights are:

1) Which would you rather burn, fresh air from outside or the stale air from inside?

2) Most of the warm air in the room is near the ceiling and the draw is near the floor.

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it is really quite simple: if you have other appliances venting more air than the natural leakage of the house can accomodate without restriction, then the make up air will come down the flue, which makes for a smoky house when the stove is operating. In cold climates, it can become a serious problem. One needs to perform a leakage test to determine if outside air is required to prevent the stove from backdrafting. Sure, you can go ahead and open a window if the house fills up with smoke. I would rather not have to resort to this when it is -25F with a 30 mph wind outside......

paul,
how long does one of these stoves burn on an average charge...am I going to have to stoke it every 10 minutes or could it be made to handle a couple of decent sized pieces of wood...my intent is for use in a boiler application

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You put some sticks in it and how long they burn depends on the lengths of the sticks. The fire chamber is tiny, but you can gravity feed the fuel.

But once the bench is warm, it stays warm for a really long time. Days even.

As for a boiler - I think there are other designs you might want to look into. This design is for heating a home. But this design is modified from a design that is about cooking. Specifically about boiling water.

For other uses, have a look at this.

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I don't know how you fire the rocket stove, but its probably close to how you fire one of the masonry heaters. One or two firings per day, and the heater radiates all day and night.

The masonry heaters burn most efficiently when you let them burn quickly at one time, then wait to burn another charge. The inner surface of the heater has to have time to conduct heat deeper into the thermal mass. If you burn a second charge immediately after the first, you get less heat transfer, and wasted energy.

An example of hydronic heating with a masonry heater at this web site. Not exactly what you're looking for, but a good start.

To answer your question, it depends on how much heat you want to draw from the system. The above linked heater is fired twice a day. Once in the morning, and once at night. You load it with wood, light it, and wait until it goes out to shut the damper.

Michael

I am still traveling...so the use of the stove is to heat water for my homemade hotub/stocktank....is there any reason that I can't run the flue through the stock tank,and maybe place the combustion chamber inside the tank? if this is doable...it sounds like I would be able to heat the tank to a little over a 100 in short order

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That looks like a big lot of work. And the design looks like a fair bit of engineering.

I guess that's what I like about the design of the rocket mass heater. REALLY simple.

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That sounds possible to me. It would all have to be water tight and metal.

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There is a major difference in approach to the the two heaters. The masonry heater is built to code, can be insured, exempted from EPA standards, will last longer than I'll be alive, can have the firebox easily rebuilt without dis-assembly.

The rocket stove is cheap, uses scavenged materials, and will probably work. How long until the metal pipe rots out, and will the clay work as a firebox when it does? How much heat will it actually put out? Can they give you numbers, or just a rough idea?

It comes down to personal preference, and intention. Sorta like the difference between the adobe construction in the videos vs. stick built conventional house construction. One lets you build with local materials for not much money, and hope it lasts for a long time with no problems. The other uses more money, but has a proven track record and will meet code requirements if required.

Michael

Good points Michael,

Although I do know of one rocket mass heater that is getting checked out in portland to see if it meets code.

Also, I think it is possible to have a design that uses parts that are more .... professional. Although I do get the impression that if built right, one should last 50 years or more just with scavanged stuff. I do like the idea of welding something up to be .... better.

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Just in... the 2009 MHA annual meeting/experiments. One of this years demo builds/tests was a rocket/double bell heater.
http://www.mha-net.org/docs/v8n2/wildac09f.htm

If this was built indoors, with the insulated concrete base, proper distances to combustibales, etc., I think it would work well. Last a long time as well.

I scavenged 11 pallets of bricks, so this would be buildable using the clay mortar made from local clay. The firebricks for the burner, however, is still a bit of cost.

Michael

Michael, cob and adobe houses still exist (in the thousands) that were built as much as five hundred years ago, even in Britain's damp climate, and are still not only occupied, but in demand. So I would say that cob as a building material is the proven one, not stud-frame!

Just saying....


Kathleen