Ultralight Tent Stove- Part 4 Getting the Best out of the stove

This post is the last in a four-part series on my efforts to design a truly ultralight tent stove. My aim was to efficiently and cleanly provide cooking and warmth from a small number of bush sticks to enhance the pleasure of skiing and camping in a winter wonderland. It is about getting the best out of it and taking care of the stove.

 

Part 1 is about my dream of an ultralight heat source for warmth when skiing and snow camping and my lessons from failures. It also tells how I was teased and spurred on by the amazing protracted and intense heat energy that could be released by the efficient combustion of a hand full of bush sticks. Part 2 is about my best stove so far, the ultralight tent stove or Micro Snow Stove Mk15 that largely met all my defined requirements. Ultralight tent stove- Part 3 is about the very important basics of safely and effectively using the stove under backpacking conditions that are invariably difficult when heat from a stove is most needed. Please read this article about getting the best out of the ultralight tent stove in context of the other three articles.

Tools, What I Take and What I Make In The Bush. 

DIY bow saw. This simple little saw is made from part of the leg of a discarded aluminium beach chair and a small bush saw blade. Weighs only 150 g it is a serious wood cutting instrument and easy to make.

Modified anvil-secateurs. My favourite secateurs are the anvil type (rather than bypass cutters) from Aldi and they weigh 240 g. I replace the plastic anvil with a stronger one made from a 42 mm long piece aluminium ‘L’ section. The replacement anvil is held in place by a steel rivet salvaged from the beach chair. Lastly, the anvil is shaped to match the shape of the original plastic one.

Splitting knife. I use those cheap TV steak knives “…..yes but wait there’s more type that are found in every op-shop…” and grind off the teeth to leave a smooth sharp blade and I use it as my regular camping knife. I make a blade guard out of a flattened piece of ½” polypipe. I make sure the metal of the knife is good quality, not too hard to be brittle, not too soft as to bend and capable of holding a good sharp edge (it should be able to cut out a bean can lid and still be sharp enough to easily pair wood. Lastly, give it a good workout splitting wood at home to test its quality before taking it on a trip. My splitting knife doubles as my general purpose knife for fishing etc, and cooking knife so it adds no extra weight. I find a quality knife with a riveted hand (as shown above) is much more long-lasting than those with moulded plastic handles that crack easily with the impact of splitting. 

Wood sawing stop. I use a spring head roofing nail (rather stout and not easily bent) and drive it into a suitable sloping branch on a tree (using the wooden mallet) to make a safe sawing stop for efficiently cutting firewood billets etc.

Bush mallet and wood splitting anvil. I cut a solid piece 400*50-60 mm diameter section of branch to use as a bush mallet (for wood splitting etc) and another bigger piece to use as a wood splitting anvil inside the tent. In bad weather, a warm tent is the best place to split and prepare the firewood sticks and I drive the splitting anvil deeply into the snow at a convenient location in the tent. The location should be such that it is comfortable to work at while sitting in the pit but not too close to things that could be damaged by a ‘miss-hit’ or the action of the bush mallet.

I staple some hay band loops (shown in the above photo) to the wooden tools and hang them in the snow gums to use during my next visit. As my father taught me, I take a little care with making these hand tools as this makes them safer, more functional and comfortable to use. Also when left hanging above the high snow level at a campsite they are usually respected as tools and are there for my next visit.

Bush stove mounting pole. I cut a point on the bottom end to make it better to drive into snow and soil. I finish the top end by paring it down a 30 mm diameter, allowing for removal of bark (it should be a tight fit in the stove mounting socket). Now I can hear some of you saying “… the stove heat is going to char the post top…”. The answer is no and I do a careful test of the temperature of the socket that goes over the post and I should always be able to touch the mounting socket with my forefinger and feel no pain. The socket has such a large surface area for radiating heat and such a thin connection of stainless steel (which is a very poor conductor) that it does not get hot.

Tool hangers. The very small tools that I use with the stove are easily misplaced or lost. Consequently, a selected twig or laterals branches on a wooden tent pole can be used to safely and systematically hang the tools ready for use.

Wood drying rack. I select two straight branches of ~ 600 *25-40mm diameter for the drying rack frame while collecting firewood billets. I split these branches to make two longitudinal and about seven cross beams that can be docked to suitable lengths with the secateurs.  The drying rack hangers have loops of ~25 mm diameter on the lower ends. These are designed to receive two cross sticks through the loops.

After placing the cross beams in place I  like to put some cooking foil on the rack to reflect radiated heat back and it also provides a storage/drying space for all sorts of small wood pieces that come about from preparing fuel sticks and these are always handy for stove management. It is a bit of a fiddle fitting the foil around the stove stabilizing strut, but in my experience, well worth it, particularly when base camping for multiple nights at the one site.

Tent lighting. Winter gives us short days and if you are like me, you ski all day and come back to the tent only when you must. This means that most of your time in the tent with the ultralight tent stove burning will be night time. Consequently, good lighting is critical for running and managing the stove. The stove can provide some nice light through the burner glass, and enough light to see that your mates are laughing at your jokes. However supplementary lighting will be needed.

I use a very small overhead LED lamp hung from the tent apex and carry backup lights including solar powered ones. I find that tea-light candles do not work well or at all when snow camping, the wax will not melt enough and the wick is starved of molten wax and the wick material burns away. My modified alpine camping candle is an interesting alternative, but that is a story for another day.    

Use of waxies. Waxies are small strips of newspaper and or egg carton that have been soaked or stewed in molten candle wax. I always carry plenty of waxies as they weigh very little, pack flat and easily in any space They have multiple uses and don’t; smell, spill or leak. Their real magic is that they will never get wet. It is just a shame that they are not edible otherwise they would be perfect. I also make similarly shaped fire accelerants out of strips of ‘2L plastic milk bottles’. Once these are set alight on one end they can be used to drip flaming plastic onto the coals or sticks to spread fire and help stubborn fuel to start burning.

I use thin paper waxies to put a starter flame up the primary air port to ignite a fire starter, start the flame in hot coals or reignite the burner after a flame-out situation. This saves me from destroying long nose lighters as described in Part 3 (Flameout Recovery). The waxies can be folded into a ‘V shape and be put up the primary air port and the tail end can be safely lit by any means and the flame will run up into the burner.

Preparation of waxies. I cut multiple sheets of newspaper into strips that are about 100 mm wide and roll them up together in a coil and tie a string around the circumference to hold the coil together and form an ‘extraction handle’. I get an old saucepan from the op shop and melt down old candles or jam wax so that the paper roll will be flooded by the wax. I lower the coil of paper in and let it become saturated with wax. Using the string I remove the paper slowly from the wax allowing it to drain back into the pot. Then I quickly put the roll on some more newspaper sheets and quickly peel the layers apart while the wax is still warm and spread the individual strips out separately to cool on the newspaper. When the strips are cooled I bundle up about ten strips and cut off at intervals of about 20 mm to make the finished waxies.

Great fire starters but they stink. Having extolled the virtues of waxies it grieves me to admit that apart from some thick Eucalyptus bark soaked in wax I have found no better fire starter for the ultralight tent stove than the commercial white fire starter blocks that many people use to start household wood-fired heaters. A small piece/s (1 cc volume) will easily start the micro stove if there is adequate fine tinder to kindle the burn. I think both the success of the fire starter and the fragility of the flame of the fire starter is due to the slow release of the hydrocarbon fuel from the white matrix (that is not really that flammable). It provides protracted gentle burning and for this reason, it is superior to waxies that burn much brighter but for a much shorter time. The fragile burn of the fire starter means that you must light it in place because if it is first lit and then dropped into place the shock will extinguish the flame. So this is where a long nose lighter or preferably a waxie is needed to gently start the burning of the fire starter that is normally out of reach of normal matchsticks or normal lighter flames.

Now I have the bad news. The hydrocarbon in fire starters permeates most available small plastic containers. The escaping vapour can easily contaminate food in your pack and quickly diminish the burning value of the fire starter itself. For this reason, I use an aluminium M&B style screw cap container (which are hard to find these days) or preferably I use a blood glucose test strip tube or an aluminium Berocca tube and cover the plastic press-in lids with aluminium cooking foil membrane to stop the hydrocarbons escaping through the lid. This may sound a bit iffy, but it works well and the glucose test strip tube is thin enough and short enough to fit inside the rolled up 2,000 mm flue pipe which is described in detail later. I also find it best to cut up the fire starter at home with a sharp knife. I find that many blocks of about 10*10*5 mm fit efficiently in these small tubes.

Sifted ash supply. This is a small but important item to carry when camping on deep snow. I use fine ash to form an air seal around the join between the stove body and the inverted access lid. At home, I use a fine metal sieve to obtain a small supply of fine ash dust. I put about four teaspoons full into a small zippy bag and this will last a long time as it takes very little to fill the crack. It is worth sieving as it will pack into the seal better without coarse particles. After spreading the ash around the joint it should be compacted a little to form a better seal. Fine soil will also do the job if you forget the ash. Several times I have thought; “why is this stove not working as well as it should? Then I discover that I have forgotten to add the ash seal. It makes a big improvement and luckily it is easy enough to do while the stove is running.”

Flue pipe setup. Rollup flue pipes of stainless steel or titanium foils are just like magic for ultralight backpackers. They make an ultralight tent stove possible by making the flue pipe so light and compact. When assembling my flue pipe at a campsite I tease any uninitiated bystanders by asking them if they believe that I have a two meter long flue pipe in my hand. No is the normal reply. I release my flue pipe from the confines of its storage tube. It is a source of wonder.  In an instant the tightly coiled roll of metal turns into a flue pipe faster than anyone can see. These flue pipes have been around for some time, but “…as you probably have grown to expect by now…”, I have made my own little improvements to this technology to make them more compact and their handling and packing more user friendly and safer. My flue pipes are smaller than others, to match my small stoves, and I have not seen reports from others who have used roll up flue pipes of 40 or even 30 mm diameter as I have. The small diameters bring with them special challenges when it come to forming the flue from the initial flat/ish sheet of foil (that is naturally rolled the opposite way) into a long round tube, but it can be done. Once formed for the first time the flue pipe needs a ‘good- burn-in’ to give the metal a ‘heat memory’ of this new shape. The initial forming is worth doing very well as botched job initially will result in an ugly flue pipe for ever. I agree with others that the breaking in process is best done at home where you have flat surfaces to work on, plenty of time, tools and hopefully a team of spare hands (in gloves) to help you gently wrestle the beast into shape. It is hard to believe that such thin foil (the thickness of photocopier paper) can have so much fight. After heat forming the flue pipe will be a lot easier to form the next time, but the unfortunate offset to this improvement is that the foil is now much more difficult to put back into its ‘rolled up’ backpacking form, particularly as the rolling process is starting with; sharp edges bristling everywhere, a dirty black coating and a spring curve that is not easily flattened. It is easy to put nasty crinkles in the metal that are impossible to remove “…an Australian Prime Minister, who lost his trousers, famously said…… life was not meant to be easy…”

Here is some more magic. To make the rollup safe, efficient and easy, I make what I call a ‘snail-tail- rolley’ out of titanium or stainless steel foil. It has a precise circumference to allow it to fit concentrically inside and outside many other round stove components. It has a tangential snail-tail to safely receive the nasty bottom end of the flue pipe foil. It holds the foil safely with sharp edges and corners away from hands and flattens it in readiness for the foil to be rolled up around its outside without significant kinking.

There is more magic with the snail-tail-rolly! When the roll-up is completed, I slip the flue pipe off the rolley and hold it in one hand and with the other hand I carefully grasp the wind up handle that is welded to the bottom of the flue pipe. By counter-rotating both hands the flue pipe can be wound up like a clock spring until it is small enough to fit inside the rolley for storage. I carefully allow the coiled metal to unwind until it is snug in the rolley making sure to do this with control so that the rollup handle is held firm and does not slash my hand. The tangential snail tail also makes a gentle spring keeper for the flue pipe rings and the flue mounting socket when they are stored on the outside of the rolley.

If all my words don’t make sense, this little youtube video of the unpacking and packing of the flue pipe might help. ” If a picture is worth a 1,000 words, a video is worth 10,000.”

Tent/flue gland. I like to see the flue temperature maintained at a safe level (below 100 C) where it passes through the tent canopy. Even so, I still provide the canopy with extra protection by the use of a tent/flue gland. So what flue gland do I use? The answer is more complex than the question. “…and yes you guessed it… it should be the subject of another article and you are probably guessing that I have taken a different path to others ….. and yes I have a shoebox full of not quite right tent/flue glands to show you….”. Conventional fibreglass reinforced silicone rubber sewn in ‘stove jacks or boots’ are used in North America. I resisted permanently sewing this heavy clunky piece of hardware into my ‘beautiful, slick, soft, light and stuffable silnylon tent canopy that only weighs about 800g’, adding significant bulk and weight to be backpacked around on every trip regardless of whether or not I was going to use it.

I tried several insulated tubular flue glands that fit with a concentric air gap around the flue pipe (see photo below). I designed these to protect both the sloping internal tent surface before the flue pipe passes through the roof and on the other side as the pipe exited from the roof. These were made of 0.1 mm stainless steel or titanium foil with a thin zirconia ceramic insulation layer that was covered by either silnylon fabric or a layer of 0.025 mm stainless steel foil with a silnylon rain flashing fixed to it. These glands were very effective and they could be put unobtrusively through my tent surface by opening up a small length of the French seams and back stitching the two end of the new opening. The stretch of the nylon fabric allowed the tent gland to be inserted and when it was removed the opening would naturally heal and not be a source of rainwater leakage. This arrangement also made it possible to have multiple experimental stove locations in the one tent or multiple tents in which the stove could be used (see if you can see these openings in the photo in Part 3).

However, these designs suffered from some deficiencies: the glands were delicate and bulky and could not easily fit into the stove box (with all the other essential parts) for backpacking, they were also complex to make and fiddly to fit and adjust to both the slope of the tent and the flue pipe and lastly it was not simple to pass the assemble flue pipe up through the gland without catching on the sharp edges of the many flue retaining rings.

My latest flue gland design (h) is much simpler and still does not add significant weight or bulk to the tent canopy. This design uses a removable rectangular aluminium sheet gland with an elliptical hole in it and it is inserted up into a waterproof pocket that is permanently attached to the outside of the tent. A micro cord-lock and cord is used to lock the gland in place. When no flue pipe is required I substitute the aluminium sheet with a rectangle of clear plastic sheet that is cut from a ‘shirt box’ and it becomes an oval window. This simple aluminium sheet gland works well as it has minimal contact with the flue pipe to transfer heat from the flue and it also has a large surface area to act as a heat sink. In strong winds, the oval hole in the gland can interact with cylindrical flue pipe and act as a ratchet to lift the flue pipe off the flue connector on the stove. This problem was solved by using the flue pipe tie down ribbon described earlier and its use would be advisable under all conditions with any type of flue gland. Also, the flue pipe retaining rings still catch on this flat sheet gland, but with experience and appropriate tilting of the pipe to exploit the elliptical shape, the job is quite easy.

I repeatedly monitor the flue temperature using some simple ‘finger-zzzt tests’ that is described here in Part 4. Where the flue penetrates the tent canopy, I expect to find the temperature well below 100 C, which is well below the melting point of nylon which is between 210-265 C depending upon the polymer type 6 or 66 (antron technical bulletin).

However, I always; use a protective flue gland, make the tent penetration as far as possible from the stove as well as the monitoring the flue temperature at the canopy penetration point. To illustrate this importance of this, I caught a very short ‘runaway burn’ on camera while my daughter was filming many rapid-fire night time shots of the stove on her U-beaut camera. The duration was so short that we were both unaware that it had happened until we were reviewing the many photos taken. Needless to say, this is why a flue gland should always be used. At this point of writing this article and looking at the photo, I think I found the explanation for the runaway burn. The primary air port regulator is completely removed and the stove flue got hot enough to ignite the soot and the extra air that is drawn into the stove is burning the light soot deposit in the bottom of the flue pipe. The bottom line is you always should expect the unexpected and have a big safety margin.

Alternative stove mounting with snow pit. Tent snow pits make snow camping just that much more comfortable and a tent with a partial floor or no fixed floor make it possible. There needs to be a safe margin between the finish of the snow pit and the point where the stove mounting pole is driven in. I think it is best to drive the pole in and pack the snow firmly around it and let it freeze up a little before cutting the snow pit. If the mounting pole is curved the curve can be used to put the pole further away from the pit while still maintaining a perpendicular pole top and providing more lateral support for the pole.

Off-snow stove mounting. Throughout my articles, I put emphasis on the use of the stove in the snow. However, it also has application for cold wet weather backpacking camping. Without snow or where it can be cleared away for the tent site the stove can be mounted on a bush pole that is driven into the soil. A simple alternative is to use a big flat rock as a stove mount (This applies if the ground is too rocky for a pole mounting). The flat rock can be elevated on three other rocks to level and lift it and form a useful drying space below. The rock gets quite hot and can effectively dry fuel sticks (on top and underneath) and if it is a large rock it is nice to use it as a hot table to keep meals and drinks warm and dry gloves etc. If big enough it will still be warm for some hours after the stove has run out of fuel.  The benefits and pleasure of this feature I will leave to your own imaginations. I recall a walk to Mt Feathertop with my daughter and we were ‘flogged’ by rain and cold weather and enjoyed retreating to the tent with a hot rock in the middle and cooking pancakes on the stove while we warmed up, dried out and recovered. Because the bottom of the rock gets uncomfortably hot this mounting method is completely ineffectual on a deep snow surface (sorry no photos yet).

Flue pipe length. When a mounting pole is used to mount the stove, the roll-up flue pipe can be considerably shorter, than when there is no pole, to enable it to pass through the tent canopy. Consequently, a longer flue will be required if you wish to use the one flue pipe for both purposes. A short clearance beyond the tent canopy of ~30cm seems to be adequate with a stove that does emit sparks. This height also makes it easy to cover and uncover the flue pipe exit with the snuffer can to keep rain out when you stop/start using the stove. This can be done from the comfort of the tent doorway, a least with my bell or tipi tents. “This is particularly nice on a cold night when you are ready for sleep or a cold morning when you want a short period of warmth to wake up to, have coffee, breakfast and boot up in before braving the elements for a day of skiing.”

General Maintenance

Refractory coating for stoves. This is such a complex subject that it deserves a separate page DIY Refractories. I have made similar stoves to the ultralight tent stove out of thin (approximately 0.1 mm) titanium or stainless steel. Briefly, in my experience when the stove is clean burning (usually oxidizing conditions) and designed to put out intense radiant heat, the life of the stove will be very short if the metal in the hottest parts is not protected by a suitable coating. I have tried high-temperature paint (rated to 1093 C) and vitreous enamelling and both failed to withstand the heat or oxidation. Despite initial signs of success of the enamel coating started to show signs of decay after only 10 h of burning.

My most successful (and luckily the cheapest) coating has been my homemade concoction of sodium silicate that is doped with a mixture of other refractory minerals DIY Refractories. This is amazing stuff and it works well as a stand-alone refractory render coating for stainless steel or titanium and also as a glue, filler and ‘black body’ colouring. The render requires meticulous surface abrasion and cleaning before application and a careful ramping of temperature to cure the render properly. The following is an example of the benefit of the refractory coating of ultralight tent stoves made from 0.1 mm stainless steel; the stove operational life increase from about 30 h to about 100 hours. With substantially thicker stainless steel (0.3 mm) used in my current designs, I am expecting the operation life to extend to 200 to 300 h, but only time will tell. Luckily, this refractory render also has the wonderful attribute of being easy and cheap to repair, if the render gets damaged.

Cleaning and drying. It is important to keep the ultralight tent stove as dry as possible. At home, I clean the stove body by brushing with a stiff brush and inspect for any damage to the render. I sand off any flaky patches with medium aluminium oxide sandpaper and recoat damaged area with a thin coating of refractory render and dry in an oven at ~100 C and then bake in oven at ~200+ C to fully cure if possible. Repairs can also be done in the field as it only requires a small amount of render a little sandpaper and a micro brush. Controlled temperature ramping will not be possible in the bush and the surface may blister and ‘puff’ before the final curing takes place, but more substantial repairs can be done when home. Details on render in DIY Refractories.

The roll up flue pipe will give hundreds of hours of trouble-free service if prepared correctly and used carefully and most importantly stored properly. Storing the rolled up flue pipe with moisture in it can result in rapid corrosion. Keeping it dry when packing up in heavy rain or a snowstorm is nearly impossible and will not be your highest priority. Always unroll the wet flue pipe and dry it as soon as possible before long-term storage.

Digital temperature monitoring. Without any sophisticated measuring device, the temperature of the flue pipe can and should be monitored. Starting with the highest (and least hot part of the flue pipe) very briefly touch the flue with a finger (or digit) with a dab of spit on it. If it does not give an audible spit back (zzzt) it is not above 100 C. Next, I try a longer and longer finger contacts and should feel no sensation of heat pain. This should mean that the flue at the penetration point is well below 100 C. Next, I do the zzzt test in steps of about 150 mm down the flue pipe to find where the 100 C point is. Hopefully, it should be at about a third to two-thirds of the way down the flue pipe and a long way from the tent canopy. This means that there is a considerable safety margin if the unexpected happens (see the photo of runaway burn above) and there should be a good draft in the flue pipe to sustain the fire and the correct and safe air flow. It would be nice to be able to estimate the cooktop temperature from the flue temperature. I have tried to find the correlation between the zzzt height on the flue pipe and the stove cooktop temperature, but have not found any useful relationship. There are probably just too many variables and the biggest one I suspect is the proportion of the wood/charcoal that is burning at the time. I discuss these issues further in the combustion of wood and charcoal.

Low temperature monitoring. While I put considerable emphasis on monitoring hazardous high temperatures of the flue pipe, consideration of minimum flue temperatures is another important functional issue. Adequately high flue temperatures are required to: maintain stove burner draft or suction and prevent the condensation of soot, tar and water). In my experience when the flue temperature is inadequate there will be water tar mixture that will condense in the flue pipe and then dribble down to the stove where it will leak out of the flue pipe connector and burn on the top of the stove and make smoke in the tent. Under these circumstances, the comfortable grip point on the flue pipe will be very low down. To avoid this problem, and the consequent mess when it comes to packing up the flue pipe, always maintain an adequately strong burn, particularly with damp wood. Luckily this condensation condition is self-evident by the tar burning smell.

Very low flue temperatures also occur with the very dangerous reverse burning of dwindling charcoal embers (described in Part 2- How Does The Micro Snow Stove Work?). In this case, there will be no warning given except for the stove going silent (no gentle chuffing sound) and the flue pipe temperature will be exceptionally low. Under these conditions without flame combustion gases from the charcoal will rise straight up through the fuel tube and into the tent. The stove must go through this phase as it is shut down. To eliminate reverse charcoal burning I open the primary air port, take off the fuel tube and burner glass and it’s mounting and I cover the burner opening with a thin foil cover (micro ash shovel). This importantly leaves the flue pipe as the only path for the safe exit of the exhaust gases from the slow burn of the last charcoal which can have a high and dangerous carbon monoxide content . I also increase the ventilation of the tent until the stove is extinguished.

Cautionary note. While the temperatures on the outside of the upper reaches of the flue pipe can be safely monitored with the above finger zzzt test, the temperature of most of the gas moving up inside the flue pipe is considerably hotter than these test would indicate. Under extreme conditions, I have measured 100 C on the outside of the top of the flue pipe, while at the same time I have measured 300 C in the core of gas exiting the flue pipe under the same conditions. What you feel in the outside belies what is going on in the inside!

Insulated stove mount temperature test. The insulated stove mount is designed not to get hot enough to burn the top of the wooden mounting pole and this can be tested by carefully touching the part of the metal stove mount where it contacts the top of the wooden pole. It should never be more than comfortably hot to touch. It is most important to do this test when the stove reaches its highest operating temperature, but it should be done with care.

Risk management- the final word. In my family, I am infamous for my statement which goes like this when discouraged from doing something that others think is risky “Oh it is alright if you know what you are doing”. I said this one morning as I set off for work driving our little yellow three-cylinder Daihatsu Charade down through the paddock to ‘save’ the fragile steep farm track for my homecoming. The car slipped and luckily missed huge 100+ year old trees and farm equipment and I arrived safely back on the track. Just because you get away with a dodgy practice most of the time it does not mean you always will. Running a wood burning stove in a light nylon tent may appear very risky, but when managed well it is probably a lot less risky than running a gas or alcohol stove in your tent or vestibule. In either case is important to keep checking risk areas and maintain safety margins and don’t ignore warnings just because you have ‘got away with it’.

Tim

Tim

Addendum

This post dates back some time now and could be considered redundant. However, it contains some fresh ideas and lessons that may be helpful to others. In my opinion, it has been largely supplanted by my very light, compact, and hot KISS tent stove.