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velacreations

Rocket Mass Heaters

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We just built a Rocket Mass Heater, and it works great. It burns sticks, instead of logs, and 40 minutes of burning gave us heat for more than 6 hours afterwards.

8188121171_e2a2f9af7d.jpg

here are more photos:

Rocket Mass Heater - a set on Flickr

And an explanation of it:

Rocket Mass Heater - VelaCreations

This is not a cob model, it is a metal box filled with sand with a RMH in the middle. We wanted to have it so that we could change it if we needed to.

ande likes this

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Cool, thanks for posting. I've always wanted to bug someone who's build one as I have always been puzzled by rocket mass heaters. There's two questions I have.

1) What keeps cause the flow of air to go from inside to out and how do you prevent it reversing a filling your house with smoke? I've seen some plans with really long horizontal chimney's and I would be concerned smoke and CO getting into my house. I've not aware of any certified rocket mass heaters in Canada. Therefore, you could only have them in an uninsured cabin.

2) Where do these efficiency numbers come from? 1/8 the amount of wood??? Sorry, but that doesn't pass reasonability test my Dad always taught me to apply. Most modern wood stoves have lab certified efficiencies in the mid 80% range. So you can't really improve that by 8 fold. See this link for a list of efficiencies on wood stoves Wood stove heating efficiency comparison chart

From what I can tell, all they really have going for them is the thermal mass. Thermal mass is useful when your heating demands don't require the stove to run throughout the night.

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The thing that keeps it moving in the right direction is the heat riser in the center. The heat rise in ours is about 3ft tall, and it is insulated, creating a big draft that pushes the exhaust through the system. This draft is so strong it makes the fire burn sideways (see my photos) instead of upright. The heat riser gets really hot, like 1200 degrees F or more. This creates an enormous positive pressure.

The efficiency is from 2 things. One is that because you are making the exhaust snake through the system, you are capturing most of the heat. The final exhaust temps are about 150 F. In a regular wood stove, the majority of the heat rises right up the chimney and out of the house.

The other thing is the thermal mass. Not only are you capturing more heat, you are storing it. So the stove gives off heat for hours after the fire has gone out.

In our test run, we put in a handful of sticks, none bigger than 3/4" in diameter. All together, the bunch was maybe 5 inches by 4 inches. It burned for 40 minutes, and the sand in the RMH got so hot you couldn't touch it (probably about 150-200 degrees). It stayed that temp for over 6 hours. How many normal woods stoves are hot for 6 hours after a 40 minute fire?

So, we are getting more heat out of less wood. That's where the efficiency numbers come from. Heat your house with 1/8 the amount of wood as you would with a normal stove. We used to have a regular wood stove in that spot, and I can tell you, to keep it putting out heat for 6 hours, we would definitely burn more than a handful of sticks!

There are some discussions over at permits.com on the exact efficiency stuff.

Edited by velacreations (see edit history)

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Hi Abe,

Congratulations on your new rocket heater........that's a great achievement. The rocket mass thermal mass heater has been on my To-Do list for a long time and your effort simply reinforces my commitment to building one soon.

Gary

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It turned out to be a lot easier than I thought it would be. But doing it my way, and making a metal container to hold the thermal mass made it even easier. Once you build the core, you can just fill the container with sand. Quick and easy.

This thing has been working great. It got a little cold yesterday, so we ran the RMH for about 3 hours in the evening. The mass was still warm this morning!

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Hi

Can you tell me how many square meters of living space you can heat, keep in it steady in, let's say for the sake of argument 68 F (20 C) if is out temp around 32 F (0 C) ?

I have russian stove to heat 3 rooms and natural gas furnace in 2 rooms for back up ( which are on if it's below 14 F).

btw, like all yours creations. ;-)

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Well, we rarely have temps of 0C, most of the time it is around 10C min, even in winter. But, occasionally, it does get lower. We are heating about 70 m2 with this stove. When we have lower temps this winter, I will let you know how it performs.

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I did take extensive photos, so if you follow that link above, and then on the page, click on a photo to go through a slideshow of the whole thing.

Materials were basically some 16 gauge metal, some square tubing, some 22 gauge, lots of screws, sand, and bricks.

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Hi vela

I read explanation for your RMH, and I have a question ( maybe because I just came for birthday party and it's past midnight :tongue:) .

The second part is thus the heat riser. The smoke and heat from the fire moves laterally and then up into the riser. Our riser is a thick piece of pipe, 32" tall. It is surrounded by a tube that holds 3 inches of perlite. The perlite insulates the riser, increasing the draw and thus sucking on the fire. There is then a barrel around tsecondhe perlite tube, which allows the smoke to move from the riser, down and out to the stove pipe. This barrel has a lid welded to it and can be used as a hot plate for cooking, and believe me, it gets very, very hot.

Isn't be better to heat (smoke) first goes from larger pipe (second barrel around the perlite tube) to smaller (center-perlite) tub.

This way heat can better transfer to outside (room).

Cheers

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the purpose of the perlite tube is to create a huge draft to push smoke through the system. It is insulated with perlite to heat up quickly and then the smoke spreads out to heat the outer barrel.

If the smoke went through the outer barrel first, there would be no draft, and you would experience a backdraft of smoke in the room.

The insulated heat rise (perlite tube) is the heart of a rocket stove. It is what makes the stove work.

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One more question Vela

Is this solution with bulkhead between inner pipe and barrel will transfer heat better and increase heat mass and don't mess with RMH performance?

post-5185-13795790859706_thumb.jpg

Cheers

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The efficiency is from 2 things. One is that because you are making the exhaust snake through the system, you are capturing most of the heat. The final exhaust temps are about 150 F. In a regular wood stove, the majority of the heat rises right up the chimney and out of the house.

The other thing is the thermal mass. Not only are you capturing more heat, you are storing it. So the stove gives off heat for hours after the fire has gone out.

Actually there is one other very important factor. Wood stoves only burn at their rated efficiency when they are wide open, burning as hot as possible, which you seldom do unless you are 80 years old and like your house to be 90 degrees F :)

I have read that a fire banked down for the night only burns at about 20% efficiency no matter the stove. With a RMH not only are you storing the heat from the exhaust but you are burning wood full out which burns the wood gasses as well, giving as much as 90% efficient burn AND storing a large percentage of the exhaust heat.

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Actually there is one other very important factor. Wood stoves only burn at their rated efficiency when they are wide open, burning as hot as possible, which you seldom do unless you are 80 years old and like your house to be 90 degrees F :)

I have read that a fire banked down for the night only burns at about 20% efficiency no matter the stove. With a RMH not only are you storing the heat from the exhaust but you are burning wood full out which burns the wood gasses as well, giving as much as 90% efficient burn AND storing a large percentage of the exhaust heat.

Yes, you don't need to run the RMH at night, it puts out heat all night long. We ran it for about 4 hours the other day, in the evening. It will still warm in the morning.

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RMHs are basically a simplification of Russian/Masonry stoves (Masonry Stoves, Masonry Heaters and Masonry Fireplaces (also known as the Russian Fireplace), plus Masonry Ovens and Earth Ovens: How to design and build your own very efficient, low-cost masonry heaters, ovens, and earth ovens.), where you have a monolithic central fireplace with a switch-backed flue that burns a load of wood full out storing the heat in the stones/brick to be radiated over the day. Apparently they have been using these in Europe for quite a long time.

The RMH has the advantage of being simple to DIY and is better suited if you want a long run to spread heat, whereas masonry stoves tend to have the stove central, possibly built so it fills a corner of every room. The masonry stoves also are often quite visually striking. Search google images for masonry stove and drool over some of the beautiful stone fireplaces they show

https://www.google.com/search?num=10&hl=en&site=imghp&tbm=isch&source=hp&biw=1813&bih=995&q=masonry+fireplace&oq=masonry+f&gs_l=img.3.0.0l10.1503.4085.0.5405.9.9.0.0.0.0.98.783.9.9.0.pchatac..0.0...1.1.nrhQQf0Im2g

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there are some big differences, though, especially with how a RMH works. They are not a simplified masonry stove. The whole "rocket' part is the real trick, having the insulated heat riser that makes the difference. This gives substantial velocity and complete combustion for an efficient burn ad the ability to snake through the masonry, while maintaining draft. That is why they are able to do long (13m or more) horizontal exhaust sections.

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Hi Vela

Let me ask you something.

Is it important how big is gap (in your picture is 2 inch) between heat riser (perlite 2 inch tube) and lid (cooking surface) ?

I will go higher (more gap) to capture more hot gases. Like a bell on top of the heat riser pipe.

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Yes, the gap is important, as it determines the velocity of the hot gases coming from the tube below. You don't want it too big, because things will slow down. I do think you could make it work with a big bell, though.

the most important dimensions are the length of the burn tunnel vs the height of the heat riser. Generally, the higher the heat rise, the better draft you will create, but you want to keep than burn tunnel as short as possible.

all cross sectional areas throughout the system should be equal. So, if you have a 6" stove pipe (like mine), evey cross section should try and match that area. The burn tunnel cross section should be the smallest of all, and it is best to just match this with your exhaust pipe.

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Hi Vela

all cross sectional areas throughout the system should be equal. So, if you have a 6" stove pipe (like mine), evey cross section should try and match that area

I do some math calculation according dimension in picture. Volume of heat riser (perlite pipe) and volume of space between barrel and perlite pipe is in ratio 1:2 . This mean that hot gases slow down after they leave heat riser. I'm I right?

I explain again because of my bad english.:redface:

If I got it right, air will go like this through heater :

Enter through firebox, then speed up through burn tunnel and heat riser, then slow down (emits heat), then again speed up again through exit pipe.

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Hi Caca

Have you watched the videos web4ded have produced?

Latest is titeled: Rocket mass heater on steroids

Quote : This is the final design of the rocket mass heater for the aquaponic geodesic dome greenhouse. It goes a bit above-and-beyond what the typical heater does! Quote end

cheers

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Hi Vela

I do some math calculation according dimension in picture. Volume of heat riser (perlite pipe) and volume of space between barrel and perlite pipe is in ratio 1:2 . This mean that hot gases slow down after they leave heat riser. I'm I right?

I explain again because of my bad english.:redface:

If I got it right, air will go like this through heater :

Enter through firebox, then speed up through burn tunnel and heat riser, then slow down (emits heat), then again speed up again through exit pipe.

If you have a 6" (15cm) heat riser, then the cross section area is about 28 square inches. Circumference is about 19", so you want the gap between the heat riser and the barrel top to be about 1.5" (19 x 1.5 = 28)

in that photo, the space is 2" for an 8" heat riser. Cross section of the heat riser is about 50 square inches, and circumference is 25", so for 2" rise, that would give an area of 50", which matches the riser area.

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I didn't explain well.I talking about vertical gap.

If is in picture 55 gal barrel, then diameter of barrel is 23 inch, there for radius is r = 11.5 inch. 11.5^2=132.5 square inches. 132.5 x 3.14 = 416 sq.inch cross section area of barrel.

Vertical gap between barrel and fireproof insulate (perlit) is 1.5 inch. 1.5 x 2 = 3 inch gap for both sides.

Barrel is 23" minus 3" (1.5" gap from both sides) is 20 inch. r = 10 inch, 10^2=100 square inches. 100 x 3.14 = 314 sq.inch cross section perlite with heat riser.

416 sq.inch cross section area of barrel minus 314 sq.inch cross section perlite with heat riser = 102 vertical gap. Air goes through 8" (50.2 ") and through vertical gap (102). So, 50.2 : 102 is 1:2 ratio (heat riser : vertical gap between barrel and perlite) which means that air will slow down in that part.

I'm sorry being so boring about math, but I don't wanna be misunderstand again.

You do it by calculating cross section so I follow. I have same ratio by calculating volume of a cylinder.

If I boring you with this ,no need to answer.

Cheers

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