Brick rocket stove with heat coil

Hello
I am Jeroen from Belgium and this is my first attempt at a rocket stove.
Inside the riser is a 9 mm coil where water is pumped through to heat my hottub.

The stove works great for let's say the first 45 min or so.
But then it starts to smoke and even burn through the wood and air intake.

I think it may have to do with the fact that the brik gets hotter in the intake than in the chimney.

Oh and it is stacked so I can adjust the schape quik but I think I get lot of draft in wrong places from these gaps.

Anny tips or ideas ?

Greets from belgium

Hi Jeroen;  Big Welcome to Permies!

So, Wow that's a large rocket stove you have there!
First thing I suggest you do is use clay and sand to mortar your bricks.
With a clay mortar if you want to make changes you simply pop them apart. Save your clay to rehydrate for reuse!
Without mortar your sucking air in at all the wrong places.  You want it rushing down the feed tube only.


The next thing I will say is be very very careful.  Water turns to steam in a microsecond... it expands and bursts thru pipes and connections... very bad thing.

Obviously this has been working for you so far but be aware it can and has happened.

Mortar up your bricks and see how she performs then.

I wil try to use some clay mortar to seal everything.

About the safety.
My stove now heats about 200 metric liters per hour from 20 ish to about 60 degreas.

And the coils is special made for swimming pool heaters from stainless steel.
The black tubes are not made from heat resistant material so the instant my pomp stops circulation by power failure or anything the hose almost instantly melts and bursts .
After that the coil just emptys out .

I don't see any dangerous situation but please tell me if my logic is faulty.
Greets jeroen

Hi Joroen;  

Yes , mortar up your bricks and I think you will get much better performance.
What size did you build your rocket?   Did you keep the same internal dimensions from the feed tube thru the burn chamber  and up the riser?

So about overheating. It sounds like it has already happened.  
Luckily your rubber hose is acting like a safety fuse and popping before your stainless coil does.

Be aware that if your coil gets hot enough, even while pumping it can flash to steam and blast hot water and bricks across your yard.
I would hope that your hose would melt before that BUT...  Always be ready ... you are after all playing with fire!

The feed tube and the riser have the same diameter but the burn tube is slightly less high.

Also I think I could make the burn tube longer.

Are there perfect ratios for a rocket stove ?
Greets jeroen

Your sizes sound OK as long as the burn tunnel is only a little bit shorter.
As far as your burn tunnel length.  It appears to be 5 bricks wide.
That would be about perfect if this was a mass heater.
As it is a rocket stove with an open riser.
I think you could modify things (experiment) with out to much worry.
Your riser is already taller than most. You might experiment by tying it shorter and see the effect.
I believe that once you clay mortar your bricks in you will see it perform much better.

About clay mortar and bricks.  
To start, all bricks should soak in a pail of water for a few minutes before applying the  mortar.
Good clean clay ( NO rocks) should be mixed with sand. 3 parts sand to one part clay.  Clay joint should be very thin , just enough to seal between bricks. Apx 1/16" or so thick. ( 1.5 mm)
In comparison a cement mortar joint is at least 1/4" or more thick. (6.3 mm)

Now for safety's sake and for peace of mind.
As we improve the performance of your rocket. The temperatures inside the riser are going to climb.
I strongly suggest monitoring the outgoing water temps . It is possible that the temps at your stainless pipes will rise into the unsafe zone.
You may notice a spike in the water temp and be able to stop feeding your rocket in time.

Please Please be careful!  

I have been looking around what I can buy online and local .
What I found is bags of 25 kg "fire clay"
Wich is powder used to make fireproof clay stones.

On the other hand I can also buy heatproof glue cement for making outside barbecues and so on .
I think tis could also work but is more permanent.

And option nr 3 is heatproof cement but I don really like the looks of the think joints.

What do you think would be the best option ?
Is the clay weatherproof in a wet climate?

Hi Jeroen;  The fire clay is by far the product of choice.
By using fire clay & sand you will be able to disassemble to make changes easily. Clay can be re hydrated and reused!
Is it weather proof?  Short answer is not really, but sort of . If the clay is exposed it will wash off in hard rain. However a brick stack that is mortared will not lose much .
The beauty of clay mortar,  add  a dab of water and some on your finger and it goes right back.
A down pour of rain, with high winds driving it sideways , could maybe wear thru a mortar joint maybe...but I doubt it.
Of course a cover of sorts would help stop any water damage in the rainy  season.

You can always use the bbq mortar later, after you are satisfied with your rocket design and you wish to make it permanent.

Remember if you use the clay. One scoop of clay to 3 scoops of sand. No rocks allowed.

Hi
Tonight I was testing the rocket stove again and I think I found a major error in my design.

I kind of broke the stove down from a j desing to a L shaped rocket stove to minimise the smoke out of the inlet.

Now that I could see into the fire chamber I see that the coil of the heater in the first  35cm of the riser was sitting   on the bottom of the fire chamber...
Practically blocking 75 percent of the flow from the fire chamber to the riser.
Because there is no space between the coils for air to pass through them.


I think I will just lift the coil 20 cm so it hangs in the riser a bit higher above the fire chamber

OR...

Should I put the coil in the horizontal part of the burn chamber and not on the riser so it lays in the fire and the riser can get hotter and create more draft ?

Greets
Jeroen

Once again I'm late to the party.
For the issue of dangerous pressure perhaps a stainless pressure relief valve.  Probably between the stainless output and the black hose so that if the pressure exceeds a certain amount it will open to prevent catastrophic failure.  In case of unattended operation you could have the valve output into or toward the area of the burn to shut down the heat at the same time.

Hi Trevor;
Any hot water system needs a pressure relief valve.
Those valves are generally plumbed outdoors or away from habitable areas.
If you have never seen a PR valve blow open, it is rather sudden and violent!
Having it blow back towards the fire would be a questionable choice... I could see it blowing chunks of wood out into the room.
Our friend Jeroen is playing with steam... not always a good idea.  I hope he has not found that out the hard way .

I thought it was outdoors.
I was just thinking of a way to prevent a waterline blowout and stop the heat at the same time.  It wouldnt have to be directly aimed at the fire, just enough to douse it.  The overall pressure is variable based on which valve one gets and may be even as low as the relief valve on a basic water heater.