Experiments to cool the hot tent stove
This post describes possible ways of making the KISS tent stove run a little less hot so as to slow the rate of decay of some critical flue pipe components.
This post is a prelude to other posts; Part 2. that describes a sacrificial guard tube to accommodate high stove temperatures. Also Part 3. Other benefits of a flue guard tube that make the KISS tent stove even more versatile and backpack friendly. Lastly, Part 4. KISS Stove flame guide options
Note: The above guard tube approach to the high-temperature problem is ideal for those (Like me.) without enough self-control to resist the lovely warmth that comes from running the stove flat-out within in a tent in a freezing world. Nevertheless, even if the guard tube is for you, the experiments in this post make good background reading to understand the metal decay problem and other strategies for mitigating damage.
Introduction
I have had an enquiry about the KISS tent stoves ‘flat-out’ burning action being a little bit too hot for its own good. The question came from Nick who has asked so many good questions about my stoves. So thanks again to Nick.
The area of concern was the lower end of the rollup flue pipe where it connects to the stove. “Over the years I have dealt with this issue by simply snipping off the ~50mm of damage and accepting that the flue pipe will be just a bit shorter. I have one pipe that has so many hours of powerful burning that it has been ‘trimmed’ twice. “It is still-going-strong.”
Having an extra-long pipe to allow for trimming over the years is a simple solution. “It could still be sold to ‘the little people’ years later when it got too short for you.” However, this may not be an acceptable solution for others.
I have commented sporadically on this and related issues in many posts. However, it is time to consolidate this in dedicated posts, particularly as my latest KISS Stove (and related stoves with J-burners) burn so consistently hot and cleanly with a surplus air intake. “The Stella temperature performance when seen will be believed.”
I add other such problematic parts to the discussion. So for clarity during the following discussion, I will list them, define the problem and consider individual mitigation strategies that slowly merge into just one or maybe two strategies.
Some of these modes may or may not be practical enough to be implemented in real stoves in the field. However, they should inform users about the metal decay factors that they, as stove operator, can influence. It should allow them to better manage the balance between high stove temperature for great alpine comfort and a lesser, but a very satisfactory performance that should improve the longevity of stove parts or focus the damage on cheap, replaceable and sacrificial parts.
Other possible ways of reducing metal damage
Restricted fuelling. The most obvious solution is to not drive the stove so hard by only providing a ‘light-loading’ of fuel sticks. “This is not rocket-stove science.” This will produce less heat and also ‘dilute’ the temperature of the hot gases with extra air that can get past the fuel sticks. The higher proportion of air in the combustion mix could contribute to stronger oxidation of the stove metal, but I doubt it.
“However, from personal experience, this will be ‘easier said than done’ in a cold winter tent. It will always be tempting to add just pop ‘one-more-stick’ into the fuel tube to make the stove ‘babble’ and make the radiant heat just that bit stronger and be toasty warm. If you don’t do it your enthusiastic friends probably will. They quickly learn about making heat, but the lessons about stove damage are much slower! Clearly, the best design is one that invites a generous fuel loading and no-more.”
![]()
Experiment 1. Restriction of burn rate with a thinner flue pipe
The power of the KISS Stove is driven by the flue pipe draft. Consequently, another quite obvious solution would be to simply de-power the stove with a thinner pipe. Normally the stove has a 40mm dia flue pipe that by good luck or by good design seems to just ‘nail it’ to make the stove red-hot. I had a 30mm flue pipe on hand to give it a try.
The area ratio of the smaller flue is 56% of the large one and this is a big reduction. I was surprised at how well the stove worked. It reached about 350C with heat well distributed over the cooktop with a little poorer distribution of heat down the fire dome wall. “The side-wall heat is what you want for optimum radiant heat for campers bodies.”
The flue pipe down low was less hot as a result of this change and should last a lot longer, but I would not be entirely happy with the heat output when I know what the fully powered KISS Stove feels like.
The still impressive stove performance did not encourage me to have such a small flue pipe on the KISS Stove. However, it did inspire me to investigate a compromise of a 35mm dia pipe. This compromise would mean it would still be powerful but could fit easily inside the KISS Stove fire dome while backpacking.
This is reported along with other related issues in other posts in the series; Part 2. A sacrificial guard tube for a KISS Stove, Part 3. Other benefits of a flue guard tube and Part 4. Stove flame guide options.
Experiment 2. The emasculation of the magic J-burner tube.
If nothing else, this little experiment emphatically demonstrates that the J-burner tube drives the impressively powerful combustion performance of the KISS stove.
I have never done this rapid change over between ‘with’ and ‘without’ the J- burner tube in place while essentially burning the same bundle of sticks.
Normal J-burner mode. I established strong combustion that is typical of the KISS Stove with the J-burner tube in place and estimated the temperature and its distribution. There were very high temperatures on the cooktop (350-500C) and fire dome wall (250-300C).
As expected, only a short portion of the fuel sticks was pyrolysing and supporting the strong babbling gas flame. At the same time, the residual charcoal from the stick pyrolysis was burning completely and supporting the strong pyrolysis.
This meant that there was balanced combustion of wood and charcoal so that it did not accumulate more charcoal. So there was no possibility of causing charcoal choking.
Note: This may not seem so amazing to you, but to me, after dozens of different stove designs (a bathtub full) it is. I even have a post where charcoal choking caused me to declaire in the last paragraph: ‘The end of the road for my ultralight J-burner tent stoves’ (Reverse burning in ultralight tent stoves).
“Maybe persistence+luck has paid off and I have designed the perfect stick burner for a tent stove. “That is a positive but rather bold statement.” Unfortunately, ‘Intelligent-Design’ did not take care of the stoves appetite for eating stainless steel flue pipe parts for dessert, but don’t despair, there is a solution to this hunger!”
Emasuculated burner mode. I quickly removed the fuel stick tube by slipping it off and upwards from the burning sticks as they were left in place.
After the change over the sticks were tilted over and were inserted deeper into the fire dome to rest in a sloping bundle with the bottom ends beyond the centre of the fire dome as shown in the photo below. It took about a minute for the burn zone on the sticks to slowly creep up to the entry hole.
With this different mode of burning the stove temperatures were about 100C lower and there was less heat distributed down the fire dome wall.
Once in this state, it stayed that way while there was wood to be pyrolysed. There was a characteristic soft gas-rich flame around the fuel opening. “It appeared to rise up from in the fire dome to the edge of the fuel hole and then it rolled down to disappear down beside and in amongst the fuel sticks. Also, the ‘babble’ of the J-burner mode was absent.
[Add video of the soft rolling gas flame at the entrance to the fuel port, include sound recording]
The portion of the sticks that were pyrolysing and supporting the gas flame was 2-3 times longer than when in J-burner mode.
The stove burned the wood robustly and easily and it behaved much like a dome stove operating with a side fuelling port Miniature Dome stove with hybrid fuel ports.
“Fortunately, this means that it will be much quicker to recover a strong burn after your mates have neglected their refuelling duties. This is because of the large charcoal bed, that will form. It naturally burns quite slowly and is just waiting for a few splinters to get burning with flames once more.”
Long charcoal stems form on the sloping sticks. Eventually, they snap off and fall to the fire dome floor. There they accumulate in a way that does not let them get preferential access to oxygen to burn, as they would have under a J-burner tube.
If using this mode of burning a return to J-burner mode would be required periodically to prevent charcoal choking.
The size of the flame and the delayed combustion of the wood gas was such that it was still burning gas in the flue pipe. It may contribute to component decay, but the presence of carbon in the unburnt smoke may also provide some protection for the metal surface when coupled with generally lower temperatures.
The use of the stove in the above two modes requires no modification. “All you need to do is change your mind.” The change over between the two modes can be easily done “on the run’ with burning fuel sticks left in the stove. However, it comes with some downsides that need to be managed.
[Add video of J-burner change over with burning long sticks]
Downsides of the emasuculated burner mode. I have previously indicated the issue of charcoal choking when the charcoal falls off the ends of the fuel sticks. This process unbalances the sticks and if not pushed in they will fall out into the tent!
Also, if you look at the above photo, you can see the glow of combustion high up on the sticks within the fuel port at the 7 O Clock position. This combustion will be present, but out of sight, from 7 O Clock around to 11 O Clock. This is incipient reverse burning and if left unattended, with dry fuel sticks, could develop into full reverse burning where combustion gas will exhaust into the tent.
Attentively, pressing the sticks into the fuel port will prevent reverse burning, but unlike the combustion with the fuel/burner tube in place, the operation needs constant vigilance to maintain safety.
Not all bad news. Fueling the stove without the inverted J-burner in place can be useful, practical and safe when using small chunky fuels. Providing the fuel pieces are dropped in below the edge of the fuel port, they will burn safely without the above-mentioned problems. Such chunky fuels could be wood chips, friable bark, short sticks, nuts, peat, dung and charcoal mixed with other woody or hydrocarbon fuel. “If chunky fuels are available, then this will be another way of keeping warm on a cold night.”
Note: Pure charcoal will provide plentiful heat, but it will promote high temperatures and poor stove draft that may be destructive to stove components. Mixing charcoal with a little wood or other hydrocarbon fuel tones down the temperature of the charcoal by shrouding it in wood gas and promotes strong flue draft.
Add photo or video of stove fuelled with other chunky fuels with charcoal
Experiment 3. Basal flue pipe venting
With the stove running normally (flat-out), I lifted the 40mm flue pipe up out of the connector cone and inserted a small piece of bent stainless steel scrap sheet metal in and lowered the pipe into place once more. It had ~3mm gap between the cone and the flue. This allowed a considerable amount of air to be sucked in and it cooled the flue pipe by about 100C. It also reduced the stove temperature by about 100C.
This strategy may help to reduce the metal damage and may be practically implemented by putting a small adjustable vent in the stovetop near the flue pipe connector. Venting air in this way would also act to cool the vulnerable entry lip of the connector cone.
Conclusion
The above methods of reducing stove heat all worked well while still sustaining a robust, convenient, and clean burn within the stove. It will not eliminate the metal damage. Consequently, a more comprehensive solution would be a more satisfactory. Such solutions are described in the related posts listed below.
https://timtinker.com/part-5-optimized-retaining-rings-for-the-flue-pipe-on-a-kiss-stove/
Tim
