Rocket stove creosote in the bench pipes

Hi,
I build a rocket stove at my parents place 4 years ago and we are super happy with it.
But it seems that we have some trouble with creosote in the pipe after the rocket stove in the bench.

Let me give some details on how I build it:
I used the book the rocket mass heater, builder's guide from Erica Wisner & Ernie Wisner. The rocket stove is perfectly burning.

I used refractory bricks for the burn tunnel and heat riser and did not insulate the heat riser at the time as I thought the big bricks would do the job.
Pipes are in stainless steel and connected and tapped with aluminum tape. I did not put an angle for the condensation to evacuate. But I won't be able to change that.
Barrel is an old oil barrel. Dimension of the wood feeder,  burning chamber and heat riser are the same.

We already had some creosote in the pipes at the time. So I started by changing the gap between the heat riser and the barrel to reduce it. I reduced it from 8cms to 5cms(3.14in to 2in)
Seemed to not have improve the problem that much in the pipes.
I tried to insulate with a metal + vermiculite around the heat riser.
Did not improve the problem with creosote in the pipes neither

temperature read on the barrel after few hours of burning measured with a laser instrument
top where the heat rise is: 190ºC to 200ºC (374ºF to 392ºF)
top around the barrel: 135ºC (275ºF)
around barrel bottom next to the wood feed: 55ºC (131ºF)

I am not sure what I could check or change now.
Are my pipes too long in the bench?
Is the hash box too big (behind the barrel?
Should I add a small metal pipe to inject oxygen in the heat riser?

thank you so much for the help you could give me. I am a bit out of ideas now.

Hi Jody; Welcome to Permies!

I know you followed the builders guide, but looking at your drawing , the dimensions seem off.
I realize it is just a drawing but It appears,  your feed tube is short and your burn tunnel is long.
Is this an 8" or a 6"?   feed tube depth on average is 16" , burn tunnel roof is 12" or less,  Your riser should be as tall as possible, mine is 54" from the burn tunnel floor to the top..
Your top of barrel temps are low. For a 6" I would expect 500-600 F on an 8", 800 F+
You mention creosote?  Is it really creosote or is it fly ash ?  
One of my early builds had too long a burn tunnel...   It burned fine but by mid winter there was enough ash buildup in the horizontal pipes that I had to do a major cleaning to use it the rest of the season and then do a core rebuild that spring.  Could your "creosote" be fly ash ?
You mentioned lowering the riser to barrel top gap from 3"+ to 2" . I run mine at 2.5" and have 800F barrel top temps. Something is not quite right.
Your transition area behind the barrel can be built to small but can not be built to large.So no issue there.
Pipe length without bends is 50' on 8" and 30' on 6" with each 90 degree bend is a 5' deduction in length. Vertical rise after the mass doe's not count as a deduction.
Your riser should be insulated. You want that heat to stay in the riser to burn up all the particulates (creosote)
If your fire is busy trying to heat the riser, it can't reach incineration temps for particulates.

Hi Thomas,

yes it seems to be creosote. Big black, as hard as rock in the pipes. Really hard to get rid off which is annoying.

Yes my drawing is off. I will try to get the measurement of the inside of the burn tunel.
there are 6 bricks (as I can remember) for the top of the burn tunnel until the heat riser. So it's not that big.

I will come back with dimensions on the drawing.

Maybe I should check that I put more gap again on top of the riser and the barrel

What sort of wood are you burning? How does the exhaust look when it's running? Any smell that you can detect?

A properly functioning RMH should have close to zero creosote formation, since all the volatiles are being consumed in the superhot zone of the burn tunnel and heat riser. My suspicions would revolve around wet wood, insufficient airflow, or incorrect dimensions.

Yes Jody;Since it is creosote, then something is bad off.
Could be dimensions or as Phil suggested it could be your wood is not dry.
You should be getting hotter on top of the barrel.  Because your not, then the temperature in the riser is not getting hot enough to consume the particulates.
That is the only way creosote can get past the riser.
If you can, get us some measurements. All of them. Depth of feed tube , length of burn tunnel roof, height of riser.  Size of core (ie) 7.5 x 7.5 or 5.5 x 5.5)  
What is your riser made off? Properly insulated ?  What is your transition to horizontal area like ?
The 2" riser top gap, should have increased the barrel top temp, So changing it back rite now won't fix your problem.  Later changing it to 3" or more is a good idea.  

Hi Jody;
I was reading your first post again.
Your rmh has been in service for four years now. Everything is dry.
Then I realized that you already gave us several of the measurements I asked for...
Looks like your fine on total length.
What is your vertical chimney like ?  Brick, metal ?  Indoors or outside ?  Is it open top ?
Is it possible, rain water is getting into the vertical pipe ?  
How steady is this stove burned ?  Starting in Sept thru May - June ?  How many hours a day steady ?

Maybe your hard black stuff is fly ash that sat in water and hardened ?


   

Hi,
Sorry was super busy last couple of weeks...

- I use only dry oak (has dried out 3 years + 2 years in the backyard) so I guess it's not the timber the issue
- the chimney is a maybe 4m high, I put a tube with double wall inside an existing brick chimney. The top is open, water can come in, but I have a T at the bottom before it comes in the bench. No water can come in the bench from the chimney.
- the stove burns quite often. I would say big fire in the morning, then depending on the cold. At least 4h per day I would say.
- it's definitively no fly ash. It's black, sticky and super hard to take out

I updated the image with the dimensions of the burning chamber

thomas rubino wrote:Yes Jody;Since it is creosote, then something is bad off.
Could be dimensions or as Phil suggested it could be your wood is not dry.
You should be getting hotter on top of the barrel.  Because your not, then the temperature in the riser is not getting hot enough to consume the particulates.
That is the only way creosote can get past the riser.
If you can, get us some measurements. All of them. Depth of feed tube , length of burn tunnel roof, height of riser.  Size of core (ie) 7.5 x 7.5 or 5.5 x 5.5)  
What is your riser made off? Properly insulated ?  What is your transition to horizontal area like ?
The 2" riser top gap, should have increased the barrel top temp, So changing it back rite now won't fix your problem.  Later changing it to 3" or more is a good idea.  

- the riser is made of refractory bricks (high temp) and is insulated with vermiculite + metal sheet around it to keep in place (maybe .2in)
I don't understand the transition to horizontal?
From the barrel to the bench, I have this chamber that get's the fumes down. It's diretly connected to the ground (tiles at the bottom). Maybe it's too cold and creates condensation?

Hi Jody;   Welcome back.
I have one important first question. What size are your horizontal pipes?  6" or 8"?  
I'm having trouble with reading your measurements.
I only see one measurement for the feed tube size, appears to say 7" is that 7 inches square?  A 6" rmh feed tube would be 5.5" x 5.5" square and an 8" would be 7.5" x 7.5" square.
Then I see it appears to be only 11.4" deep ? As a comparison, both of  mine are 16" deep.

Your burn tunnel roof is 11.8" add in 15" more  for the feed tube and riser  (if is built to be 8" )  and total length would be just short of 26"  That is what my 8 incher's are.

Now your riser. It shows 34" from the top of the burn tunnel roof. Add 4.5" for brick thickness and I'm assuming 7.5" for the depth of the burn tunnel So a  46" tall riser. My riser is 51" tall.
What are the inside dimensions, 7.5" x 7.5" ? Or 5.5" x 5.5"?  


The transition area (hash box)  is the spot after the barrel where your gas flow enter's the horizontal pipes.
Your tiles on the floor would not be causing any problems. It should be plenty hot there.
From your drawing it seems this area is large enough.

hey Thomas,

my pipes in the bench are 6 inches stainless steal pipes

The measurement for the feed tube size are 6.3'' x 7.1'' so I guess I am 6.7 '' square
For the burn chamber I read in the book that it should not be tool long

Rocket Mass Heater Overview and Terms page 7

Length: The minimum burn tunnel
length is whatever is necessary to tunnel

under the bell. With a smaller barrel or cus-
tom bell, the burn tunnel could be shorter

without causing problems, but the bell must
never overhang above the feed.

I guess I should adjust the rocket stove to the 6 inch pipe I have in the bench, I will have a look for the correct dimensions
thanks

Hi Jody,
I think we found your problem.  
Your core design is correct.
Your burn tunnel is correct length.
Riser height is a little low but OK .
I would increase feed tube depth to 16" or 40 cm
Proper size core for 6" pipes  is 5.5" x 5.5"  or 14 cm x 14 cm would work fine.   Your core is producing a little to much for your pipes to discharge.
I'm rather surprised it has worked as well as it has for 4 years.

When you decide to  go in for a repair you might consider a change in your riser as well.  
There is a new style of super insulated riser. It is so easy to build it is known as the five minute riser. Basically it is 1" thick ceramic fiber blanket fit inside a standard wood stove pipe.
Here is a link all about them.    https://permies.com/t/95849/Working-Morgan-Superwool-ceramic-blanket.

cool thanks so much for your help Thomas! really valuable info. I was trying to find why this wasn't working properly.

I think I will give a go to the 5min riser. It will help to have more space in the barrel for the fumes to circulate as I have a brick riser + vermiculite around it with a metal sheet.

Super Jody;
Take plenty of photo's during your rebuild.  
Share them with us in a new thread!
Your really going to like the 5 minute riser.