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rocket mass heaters

Has anyone built this rocket water heater?

Yen Yus

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posted 2 years ago

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Just looking for tips on building my own. I need an efficient way to heat water for my underfloor heating with wood.

thomas rubino

rocket scientist

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posted 2 years ago

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Hi Yen;
I personally have not built Tim Barker's water heater myself.
But it is tried and true setup. It does work safely and well!
You will be happy with the results.

For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!

Yen Yus

Posts: 92

posted 2 years ago

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Thank you for your kind reply. I have no experience with rocket heaters so I was wondering if there was a more updated and detailed guide somewhere. It seems simple enough, which is what I like. It's just that I have to build up a little confidence before attempting to build it. Thanks again.

Yen Yus

Posts: 92

posted 2 years ago

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Also is there a design for a rocket stove that has detailed plans you can suggest?

thomas rubino

rocket scientist

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posted 2 years ago

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Hi Yen;
Building a simple J tube rocket like you would need to create Tim Barker's water heater is very easy.
You will need apx 20-30 bricks. In a perfect world, you would use fire bricks but regular clay bricks work just fine to start with.

You are creating a brick box. Apx 2' long and less than a foot wide and 16" tall . That box will have a vertical 16" deep square hole of 5.5" x 5.5" on one end.
A 5.5" horizontal square tunnel of no more than 12" long and another 16" tall 5.5" vertical square on the opposite side. Your riser (chimney) is built over one of the 5.5" holes.
Your riser should be 48" or so tall.
Cob (clay/sand) or sticky mud is piled around your brick box as an insulator, the thicker the better.

I suggest you acquire some clay bricks and follow my measurements, as the sizes are very important if your rocket is going to roar.
Play with the bricks outside, dry stack them to get an idea of what you are building. Add a riser and light off your first rocket!
Note that when bricks are dry stacked you will have air leaks and your rocket will not perform as well as it will when you have cob/mud sealing it all up.

That is a 6" rocket stove an 8" rocket would have 7.5" holes.

For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!

Yen Yus

Posts: 92

posted 2 years ago

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Thank you for all the info Thomas. Do you suggest insulating the j tube with rockwool or just mud? Do you think I'd need an 8 inch stove to heat up 500 liters of water?

Also how do you think Geoff suspended the barrell full of water on top of the j tube?

Peter van den Berg

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posted 2 years ago

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Geoff Lawton presented the rocket water heater, but forgot to name the inventor. His name is Tim Barker, he wrote a comprehensive article about it 10 years ago. See https://www.permaculturenews.org/2012/11/23/rocket-stove-hot-water/
Water that goes from liquid to steam (the phase change) will expand roughly 1500 times. Be aware that playing with boiling water in a closed vessel is very dangerous.

regards, Peter

Yen Yus

Posts: 92

posted 2 years ago

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Thank you Peter.

thomas rubino

rocket scientist

Posts: 6480

Location: latitude 47 N.W. montana zone 6A

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posted 2 years ago

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Hey Yen;
I would seal the rocket with a layer of mud to begin, then you can cover that with rock wool if you like.
The water tank can be supported by a metal stand or it could be suspended with straps or chains from overhead.
I always prefer an 8" J for ease of loading and a longer burn time.

As an apprentice rocket scientist, I suggest you start with just building a J tube alone out in the yard.
Build it dry stack and burn it. Then seal it with mud and see the improvement.
Once you have done that your confidence level will be on the rise!

Then move on to how you want to support your tank, leaving room to construct your J tube underneath.
Build your support structure including the water tank first and then build a J tube below.
After that is accomplished then I would start looking at how to use that hot water for your heated floor.

For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!

Yen Yus

Posts: 92

posted 2 years ago

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Sounds like a plan. Thank you for your help. I'll update here as I progress.

Yen Yus

Posts: 92

posted 2 years ago

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Ok so I've made some progress. I found the fire bricks, which are 20x10x5 cm. So in my placement I have 20 cm on the inside gap on the firebox and 30 cm on the outside. Which is about the size of an 8 inch j tube I believe. That's fine but when I try to do the bridge in the middle my 20 cm bricks cannot span the gap because my gap is 20 cm.

How can I solve this?

thomas rubino

rocket scientist

Posts: 6480

Location: latitude 47 N.W. montana zone 6A

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posted 2 years ago

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Hi Yen;
Make your tunnel 19or18cm wide so your bricks can span the gap and then make the tunnel that amount taller.
You keep the same volume it's just a slightly different shape.

For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!

Yen Yus

Posts: 92

posted 2 years ago

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thomas rubino wrote:Hi Yen;
Make your tunnel 19or18cm wide so your bricks can span the gap and then make the tunnel that amount taller.
You keep the same volume it's just a slightly different shape.

Ok so I'll just squish the brick at the side together and make it 19 cm. I also found some refractory cement so I'll stick it to get her once I'm confident enough it's working.

And then I have to place two bricks on top of each other and that would make the tunnel 20 cm tall. İs that ok? The firebox gap would be 19x19 and the depth would be 20 cm

IMG_20220911_143247.jpg

thomas rubino

rocket scientist

Posts: 6480

Location: latitude 47 N.W. montana zone 6A

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posted 2 years ago

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Hi Yen;
If your tunnel was 20x20 before and we shrunk the sides into 19 then we would want 21 for a height.
You may find that when you add mortar you end up with 21 cm.
A tunnel can be made smaller but it should not be made any larger.

Now let us talk about mortar.
You mentioned you have found refractory cement.
I have used refractory in the past and I will not waste my money on it again.
Fireclay and sand are the mortar to use.
Clay mortar is cheap compared to refractory but better yet it is easily knocked off when you are rebuilding/repairing and you just crumble it up add water and reuse it!
Refractory is solidly attached, you need to break or cut it off and you certainly can not reuse it.

For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!

Yen Yus

Posts: 92

posted 2 years ago

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So it would like this

IMG_20220911_150009.jpg

Yen Yus

Posts: 92

posted 2 years ago

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So I could use clay from the bottom of my pond for example, mix it up with a bit of sand and voila? İt's difficult to find things here you see.

thomas rubino wrote:Hi Yen;
If your tunnel was 20x20 before and we shrunk the sides into 19 then we would want 21 for a height.
You may find that when you add mortar you end up with 21 cm.
A tunnel can be made smaller but it should not be made any larger.

Now let us talk about mortar.
You mentioned you have found refractory cement.
I have used refractory in the past and I will not waste my money on it again.
Fireclay and sand are the mortar to use.
Clay mortar is cheap compared to refractory but better yet it is easily knocked off when you are rebuilding/repairing and you just crumble it up add water and reuse it!
Refractory is solidly attached, you need to break or cut it off and you certainly can not reuse it.

thomas rubino

rocket scientist

Posts: 6480

Location: latitude 47 N.W. montana zone 6A

3323

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posted 2 years ago

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Yes from what I see this will work.
But it appears you only have 12 cm between the vertical openings?
That roof between the verticals can be up to 30 cm.
It will work at 12cm, if it were me I would lengthen it.

For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!

Yen Yus

Posts: 92

posted 2 years ago

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I believe the roof is 20 cm. I will put 4 bricks 5 cm X 4 =20 cm

Should I add more bricks to lengthen it another 5 cm. Then I could interlock the brick work then too I think.

I will make the riser out of bricks so I will use around 57 bricks in this design

thomas rubino wrote:Yes from what I see this will work.
But it appears you only have 12 cm between the vertical openings?
That roof between the verticals can be up to 30 cm.
It will work at 12cm, if it were me I would lengthen it.

IMG_20220911_152020.jpg

thomas rubino

rocket scientist

Posts: 6480

Location: latitude 47 N.W. montana zone 6A

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posted 2 years ago

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Yes, you could use pond clay but you would need to clean all debris from it, no rocks, wood, or leafy stuff at all.
Ideally, a sack of clean dry clay may be from a pottery shop mixed with builder's sand.

The reality of a rocket stove is no matter what you use as a mortar it will crack out after time.
When you use a clay mortar you simply use your finger to add more clay mortar to cover the crack.
You can do that literally for years.

Refractory is expensive.
It is hard on your hands and it will dry out in its bucket waiting for you to use a fingerful at a time.

Clay is relatively cheap, and sand is as well.
If your clay dry's out waiting to be used... add water!
Your hands will look and feel great after using clay.
Not so good if you use refractory.

For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!

Yen Yus

Posts: 92

posted 2 years ago

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Yen Yus wrote:I believe the roof is 20 cm. I will put 4 bricks 5 cm X 4 =20 cm

I will make the riser out of bricks so I will use around 57 bricks in this design

thomas rubino wrote:Yes from what I see this will work.
But it appears you only have 12 cm between the vertical openings?
That roof between the verticals can be up to 30 cm.
It will work at 12cm, if it were me I would lengthen it.

IMG_20220911_152020.jpg

thomas rubino

rocket scientist

Posts: 6480

Location: latitude 47 N.W. montana zone 6A

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posted 2 years ago

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I see, I was not reading your drawing correctly.
I think adding the 5th brick would be a good idea.

For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!

Yen Yus

Posts: 92

posted 2 years ago

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Ok then I'll use clay either from a pottery shop or from the pond. The clay from the pond is clean but I'm not sure if it has to have certain properties to be able to use it with fire. The clay from the pond is a grey colour.

Yen Yus

Posts: 92

posted 2 years ago

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Ok then the outside measurement would be 80 cm and the inside would be 70 cm. So if we do a 30 cm bridge then we would have two 20 cm openings. I will try it this way and report back soon . Thanks again for all your help.

thomas rubino wrote:I see, I was not reading your drawing correctly.
I think adding the 5th brick would be a good idea.

Yen Yus

Posts: 92

posted 2 years ago

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First of all I apologize for my terrible drawing skills.

So I got the bricks I ordered and dry stacked them. I did I terrible job of staking them. And I didn't stack the riser high enough. İn the first 15 mins it was smoking and wasn't heating enough. Then I loaded it up with wood and let it burn. After a few minutes it was drawing the fire up the riser furiously.

I will dismantle it and build it again, this time with a higher riser and report back. But even in its current state I managed to cook some sausages and chips. So I'm pretty hopefull.

Thanks again guys.

IMG_20220915_185614-2.jpg

Yen Yus

Posts: 92

posted 2 years ago

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This is the drawing.

IMG_20220915_171301-2.jpg

Yen Yus

Posts: 92

posted 2 years ago

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Hi everyone

So I took it down and this time stuck it together with a bit of clay. Made the riser longer about 49 inches in total. The first 3 minutes the flames were coming out of the input side.

Then all of a sudden while I was chopping wood the stove came alive and started to roar.There was no more smoke and the flames started to be drawn towards the riser.

I couldn't put my hand on the out put at that point but could on the input side. I'm guessing that my rocket stove is working.

The only thing is the top of the riser opening is 15 X 15 cm and the bottom of the riser is 20 X 20 cm. So I kinda made the bricks sag inwards after the halfway point. Not on purpose. İt was my shoddy brickwork.

So I wanted to ask you guys (before I cob it up) should I take it down and redo the riser? I believe it's working but I've never seen a fully functional rocket stove before so your input would be much appreciated. How do I know it's working properly?

IMG_20220918_151845-2.jpg

IMG_20220918_152100-2.jpg

thomas rubino

rocket scientist

Posts: 6480

Location: latitude 47 N.W. montana zone 6A

3323

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posted 2 years ago

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Hi Yen;
Congratulations on your first dragon flight!

And Yes rebuild the riser now while it is easy.

For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!

Yen Yus

Posts: 92

posted 2 years ago

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Thank you Thomas.. next I have to figure out how to use that energy to heat water.

I will come up with a few ideas and share.

thomas rubino wrote:Hi Yen;
Congratulations on your first dragon flight!

And Yes rebuild the riser now while it is easy.

Yen Yus

Posts: 92

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Hi Everyone

Hope you've all been well and busy.

I've been re doing my underfloor heating plumbing getting ready to connect my rocket stove to it. And in the meantime I tore down the shoddy brickwork and re did it. Then my guinea fowl and geese tore down my riser. So I decided to cob it up asap.

Unfortunately I have never done any cob work before so I forgot to add sand to my clay. I mixed alot of hay dipped in slip and some clay. Then I rendered it with lime because I noticed it was cracking alot (due to me not putting any sand). I found out where I went wrong after I rendered it with lime. That I should of added sand to the cob.

I've attached a picture of what it looks like now. I will do a couple more coats of lime render. But for now the cracking has stopped for the most part. I got a couple of small cracks but it's holding together nicely. Not sure what will happen when I heat it up though. Either way I have to get cracking as the cold weather is upon us.

I have a good idea of how I want to do it. I like Tim's and Geoff's design. The only difference I want to make is that I would make the water tank on top vertical instead of horizontal. Reason being that water heats from top down and I would like to start using the hot water before the tank starts equalising.

My question is where do I put the flu for the exit gasses. In my design the barrel would be at the top of the riser supported by four legs on the ground. Then I would cut another barrel and make it smaller and insert it into the barrel sitting on top of the riser. So naturally the flu would be at the very top. But I'm not sure this is the most efficient way. Also, how long do you think the barrel would last those kinds of temps?

IMG_20221008_085400-2.jpg

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