Breaking Down the Burn

The key to any rocket stove/heater is obtaining a full burn to convert as much of the material in the wood into gases and those gasses into heat. The real experts here like Peter van den Berg can break this down into more specific elements than I'm close to understanding, but I'd like to bridge the gap of understanding from the perspective of a dragon apprentice.

Is a clean burn associated with an absolute temperature? IE, if there is a thermocouple at the base of the riser, will that tell you at what point you've obtained an optimal burn with an indexed reading?

More to my interest here, without dedicated instruments, what are reliable indicators of a clean burn?

In my experience, the best passive test of a clean burn is observing the external exhaust. Usually the exhaust should be clear, pure moisture with no white/grey/black particulates visible. But this becomes problematic the colder it gets, as the moisture condenses/freezes the more the temperatures drop. Exhaust ends up looking white even during the cleanest burn.

Are there other passively observable indicators that reliably show a clean burn?

Hi Coydon,
that is a very good question and I’m looking forward to the answers of the experts.

From my (still somewhat limited understanding) you should be able to tell a clean burning device by not finding creosote build up in any interior parts, especially the exhaust.

I would like to add to the question and ask where the thermocouple would make the most sense, bottom, mid or on top of the riser?

Benjamin Dinkel wrote:From my (still somewhat limited understanding) you should be able to tell a clean burning device by not finding creosote build up in any interior parts, especially the exhaust.

I agree and have had some signs of creosote build-up that prompted my concern. But pulling apart the exhaust stack is not exactly a passive observation, I'm not going to mess with that while I need to be running the system daily.

Coydon Wallham wrote:Is a clean burn associated with an absolute temperature? IE, if there is a thermocouple at the base of the riser, will that tell you at what point you've obtained an optimal burn with an indexed reading?

The short answer is no, not in a reliable way.
Long answer: years ago, I checked this with a thermocouple and the Testo analyser. It turned out clean burning (in a batchrocket) started much earlier than could be explained from the temperature readings. Most people know that in order to get a clean burn, the three T's (Temperature, Turbulence and Time) are all-important, and they are. But... with one of the factors increased, one or both of the others could be decreased. In my specific case, while getting turbulence up to new heights during development, the earlier the clean burn started. Even while the temperature opposite and halfway up the port, and one inch sticking inside the riser was about 300 ºC (572 ºF), the proverbial miles (km's) ahead of the 1000 ºC (1830 ºF) some sources mentioned.

Coydon Wallham wrote:More to my interest here, without dedicated instruments, what are reliable indicators of a clean burn?

Lots of turbulence in the riser and a lighter colour of the flames as compared to the firebox.
And the exhaust should produce nothing more than just water vapor which should vanish within 3 or more yards. Much like a high-performance natural gas burner would do. But you have observed that yourself already.
And third, smelling the exhaust should be like a laundry vent, lots of water vapor and a hint of ammonia. Most of the time there's also a faintly smell of wet charcoal (9-methyl ketone) or wet socks, absolutely nothing like the penetrating stench what most of us would recognize as wood burning.

Coydon Wallham wrote:Are there other passively observable indicators that reliably show a clean burn?

Not that I am aware of, sorry.

My opinion Croyden is that you have to determine what side of the scientific spectrum you wish to reside to determine the answer to your question. Having lived with and thoroughly enjoyed our batch box RMH for 9 months now I have dialed by my scientific curiosity and am fully comforted by the day to day operation of the system. I embedded 3 thermocouples into my build; 1 at the top of the riser, i embedded in the inner brick skin adjacent to the top of the riser and 1 in the exhaust port where the stove pipe meets the brick manifold near the floor. All 3 are connected to a data logger display for realtime monitoring.

I would consider my approach to be in the middle of the scientific spectrum for assessing burn performance. The two ends of the spectrum would be simple observation of the burn chamber and chimney exhaust with the other end being the method that Peter vdB uses with a Testo meter providing gas and thermal data. I believe my system burns as expected by Peter using Peter's specifications for my build. A batch will consistently burn down to coals in roughly an hour, there is never any visible smoke from the chimney and my exhaust port exit temperatures never exceed 250F while my riser temperatures run in 1,500F range during the active burn. As for cold outdoor temps and determining the cleanliness of the burn. We are consistently in the -10 to +10F range at sunrise this time of year. The exhaust from my chimney is white but it totally dissipates within 10 feet of the chimney exit indicating that this is steam. If it were smoke you would definitely see the smoke trailing off, especially on mornings where the winds are calm which is our normal condition.

Incidentally, the operation of my system has been so consistent I no longer leave the temperature display turned on. I'll flip it on a few times during a burn session just to check the temperature of the inside brick skin to determine if I want to burn a 2nd batch, or in the case of days like we had last week when it was -17F outside, a 3rd batch. My inner skin maintains a temperature of 350F when I first wake up and I target a temp of at least 500F this time of year for a 2nd or 3rd batch. All the while the system burns clean and never seen an over-fueled condition.

Bottom line for me... I could run with no temp probes if I wanted and I would be confident that the system is burning clean. Of course, this is dependent on the build design and following a proven method. Deviation in build design could affect performance and require more scientific methods for analysis. I hope this helps.

Benjamin Dinkel wrote:I would like to add to the question and ask where the thermocouple would make the most sense, bottom, mid or on top of the riser?

I'd be curious about that one also. It seems to me like BBRs vary so much it would depend on the individual design. J tubes however are more standardized, perhaps there is a universal rule for them?

Yes, a good rule of thumb: the hottest part is midway up the riser