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 ?