creosote buildup on exterior of vent pipe

Hi
I built a rocket mass heater in my wood shop last fall and for the most part it heated well but I have a buildup of creosote on the outside of the exhaust pipe.  I wand to make the mass bench larger and possibly change the outside exhaust pipe but would like to know if anyone has any idea why I would be getting creosote on the outside of the pipe.  Attached are pictures of my setup

If a RMH is built correctly, it will not leave any measurable amount of creosote in the exhaust gases. So if you are leaking creosote, you have a problem with the system's construction or operation as well as a problem with chimney joints. A very short mass flue path such as you show (I don't see much room for a long flue) will not extract all the heat from the gases, leaving the chimney exhaust hotter... which is exactly what chimneys traditionally do, being hot enough that creosote doesn't condense. If you cool the gases more with a bigger mass, you will have even more condensation until you fix the original functional issue.

Now it is time to look carefully at all aspects of your J-tube construction, the barrel, manifold, etc. What temperatures do you get on the barrel surface, at the top and along the sides? How well does the system draw when in operation, and how hard is it to start? How much ash do you get from a few hours of burning?

Hi Glen thanks for the quick reply
the temperature I get on the top and sides of the barrel when the stove has been running for a while average around 400 to 450 degrees.  I typically run the stove two to three hours a day and have about a handful or so of ash every three days.  Lighting the stove is sometimes easy but not always though I did have less trouble after using it for a month or so occasionally there is some smoke blow back especially when first starting up.  But there has always been some condensation on the outside of the exhaust pipe .

I'd say you need to rebuild the exhaust pipe in such a way that all the crimps are pointing down. This way there won't be black condensation fluid on the outside of the pipe. Instead, the water is running back into the heater where it will evaporate again when the mass is heating up.

As far as I can see in the pictures the buildup is minor, probably caused by condensation during starting up. Unless the rocket is still very hot, every time the heater is lit there's some condensation in the exhaust pipe. This is quite normal, even the burning of bone dry wood will produce water, much like natural gas.

It is a standard stovepipe installation practice that the crimps should be on the bottom end of stovepipe sections, for exactly the reason Peter said. I would take his highly experienced word that the amount of condensation is not excessive or indicative of an internal problem, and will be mitigated by proper stovepipe installation.

400 to 450 degrees (F) is at the bottom end of temperature ranges to expect from a standard RMH barrel with standard internal clearances. 800 or more is very common on the top. It looks like you have a 6" system with a 55 gallon barrel, so I would expect side temps to be lower as the heat is spread out, but top temps ought to be able to get hotter than that unless you have a large gap between top of heat riser and top of barrel.

Thanks for your input I will change the way the crimps are running

After looking at other posts about how their stoves are working I decided to take part of mine apart and post some pictures of the manifold construction.   the space between the heat riser and the barrel top is 2 and an eighth inches and I know the top of my stove never got hotter than 550 degrees.  I have attached photos of the riser and manifold.  I suspect the manifold is much too large Thoughts?  Thanks

The manifold can't really be too large. At most you would have a big chamber where hot gases would linger and make the area especially warm.

What material is your riser, and what are its dimensions including thickness? The inner top looks white indicating good heat (or a coat of ash), while the entire outer surface is sooty indicating that none of it got really hot. I would think the top of the outside would see enough heat to burn off soot.

A 2 1/8" top gap is maybe larger than required for a 6" system, but not by much.

The riser had a 6 inch pipe on the interior  with a 10 pipe on the outside.  The two inch gap between is filled with a mixture of vermiculite and fireclay  The 6 inch pipe has burned away.  From the brick base where the barrel sits it is 32 inches to the top of the heat riser.  From the bottom of the jtube burn chamber to the top of the heat riser it is 52 inches. Hope this answers your questions. thanks in advance

Paul, is this a big amount of soot we can see on the upper part of this photo?

https://permies.com/t/58754/a/43454/IMG_5361.jpg

No I think what your seeing is that the last inch or so doesn't have the outer metal collar all the way to the top.  the whitish color at the top is the vermiculite and fire clay.  I have done some reading up on this and am going to add more insulation around the riser.

Paul, if it's still time, insulate the burn tunnel too. I would say feed tube too. But some have problems with coaling, when doing this.

I get condensation too, but my stack is outside. Hot fires and dry wood help:)
I had quite a stalagmite outside in the first winter. When we increased the height of the stack we got a better rocket, and less water.
I just added mass this summer, so I don't know how the water production will be when the weather turns.
Also, my heater uses outside air, and has door that I can close.

Linda,
When the whole of the stack is outside and especially when it's not insulated there will be condensation inside. Unless the exhaust temperature is high enough it's unavoidable, 1 kg (2.2 lbs) of bone dry fuel will produce around 0.5 liters (1.06 US pints) of water. When the crimps are in the correct direction this liquid water will run back to the heater until the temperature of the exhaust is above dew point so it will be discarded as water vapour into the air.