Welded mounting brackets on the large downdraft tent stove. Three brackets are used to hold stove wires that provide insulated mounts for the stove. The other brackets can be used to mount V- wire pot support around the stove to support overhanging pots.
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Large downdraft tent stove for a pyramid tent

Summary.

This post is about making a DIY large downdraft tent stove. It is designed for heating, cooking and snow melting in a large ultralight winter backpacking pyramid tent that can be comfortably shared by multiple people.

[Add a photo of the assembled stove with pots on the cooktop in the tent]

Introduction to the large downdraft tent stove

Note: This post is not quite complete, but it should still stimulate the neurons of those rare people who enjoy comfort when backpacking in cold and snowy weather on skis, boots or snow shoes.

When backpacking ski touring (or terrestrial winter backpacking) a spacious tent makes living in the snow (or rain) much more comfortable. The comfort for multiple occupants can be greatly enhanced by having a small but powerful stick-burning downdraft tent stove in the tent. If the stove is centrally located (stove location optimisation) within an ultralight pyramid or bell tent with a DIY universal stove jack the heat of the stove can be equally shared by all tent occupants.

Additionally, if the tent can be pitched without a central tent pole a central snow pit can be formed so that the stove can be mounted on bush poles to provide good access for cooking and comfortable heated seating around the stove. My large DIY pyramid vestibule tent has all these features including easy stoop-in weather-protected entry during bad weather. Also, the vestibule allows the deployment of a large netted and internally operated ‘window’ that provides improved ventilation in bad weather and falling snow. The vestibule provides a draining/drying area for wet and frozen clothing that does not compromise sleeping positions. Lastly, the tent has a zippered sky vent along the vestibule ridge line that can be opened in good weather. “This vent provides ultimate ventilation and is great for observing the night sky from a warm bed. It even will allow me to stand up to surveil the surrounding snow and skiing conditions, for the next day, while wearing only tent slippers.”

If shared by a group of people such a tent/stove combination can make the weight carried per person xxxg/person. This is much less than the weight of most single-person tents.

A view looking down on the Kiss hot tent stove cooktop at night showing off its heating power. It is sitting up on insulated wooden bush pole legs, with a stick storage/drying rack below. “It might be small, but it is a serious source of heat for comfort and cooking in a small backpacking tent. A much larger stove will be more appropriate for a tent with three to four hungry skiers.”

A larger and more powerful stove than my small KISS downdraft tent stoves will make the stove more effective for a hungry group of skiers who share a large tent.

This post describes the making of a large downdraft tent stove with the same simplicity and clean hot burning of the KISS Stove. It will have a lot more power to provide more heating and simultaneous simmering of a dinner pot and boiling water pot for hot drinks and snow melting while making a slim and smooth pack-friendly load that needs no assembly at camp.

“I think the inclusion of a large but compact downdraft tent stove, that can be fueled by dead sticks, makes the winter group camping package complete.”

The large downdraft tent stove design

Stove size matters

The simplicity of my downdraft tent stoves should be self-evident. However, effective scaling up of a downdraft stove’s size for increased radiant heating and cooktop capacity necessitates a proportionate increase in the sectional area of both the stove pipe and the fuel stick feed tube. Together, these increase the power of the stove so that a suitably high temperature can be maintained on the cooktop and on the sides of the stove surface which also act as heat a radiators for our bodies.

In this flatish downdraft tent stove design, the turbulent air/wood gas inferno below the fuel tube rapidly settles into a laminar flow as the expanding gas flame spreads out through the fire dome’s primary burn chamber and rises to its ceiling. This means that the cooktop surface becomes somewhat hotter than the wall of the fire dome. Consequently, a squat fire dome height will provide a high cooktop temperature for cooking and general heating. It also will mean that the somewhat lower temperatures on the fire dome walls will provide constant radiant heating and it also will be a better shape for backpacking.

Radiant heat is most important in a winter tent

Convected versus radiated heat. Using convection heating, a small stove struggles to effectively heat all the air in a large tent with a large surface area that is exposed to freezing temperatures outside. In my experience, a small tent stove is most effective if it is used as a source of radiant heat that goes directly into our bodies, wet clothing, gloves, etc.

“The convected warm air is a welcome bonus but it rapidly gravitates to the tent apex where it becomes unpleasantly hot for people but great for drying clothing quickly. On the other hand, the radiant can be directly and instantly absorbed by our bodies and our blood circulation can take the heat all over our bodies, including our feet that might be down in a very cold snow pit. (as shown below)”

My slim oval tent stove is ~40 mm deep and has an oval cooktop that is 240*400 mm. It makes a versatile cooker with a backpack-friendly shape. At about 950 g it is ultralight if it is to be shared by a group of trekkers. The stove is mounted on four bush poles that have been pushed into the snow at the bottom of a snow pit that has been excavated in the centre of a 2,250mm square pyramid or bell tent. Snow steps (left) were formed to lead down to the snow pit “The pit provides comfortable seating and the radiant heat from the stove can warm even the toes of several intrepid skiers. “The spoil from the excavation was used to form a snow altar outside in case of sacrifice for good skiing snow for the next day was necessary.”
A large downdraft tent stove has been mounted on three bush poles.
A large downdraft tent stove has been mounted on three bush poles. The length of the poles can be used to optimise the height of the stove for sharing the radiant heat and setting the cooktop at an ideal height for cooking.

The higher the temperature of the stove surface the stronger the radiant heat. As an example, if the stove surface temperature is raised from 300 to 400C, this will nearly double the amount of radiant heat that will be distributed to the tent occupants and their equipment. My post on stove temperature and heat radiation from ultralight tent stoves describes this in more detail.

All this means that a large downdraft tent stove needs more power (cleanly burning more sticks/minute, still at a misery rate) to enable it to maintain a desirable and effective surface temperature for comfortable radiant heat for the multiple tent occupants’ bodies and their cooking, snow melting and hot shower needs.

A wider stove pipe and a wider fuel tube make an effective way of producing that extra power in a larger stove*. “Luckily, being able to use thicker fuel sticks makes for easier fuel collection and preparation. This easily offsets the higher fuel consumption rate as does having tent companions to help gather suitable fuel sticks. Most visitors to my tent come bearing an armful of nicely selected sticks!”

Notes for newbies to downdraft tent stoves*: My unusual down draft stove design can be thought of as a glorified smoke generator that cleanly burns the generated smoke in a slight excess of oxygen. The shape of the internal flame guide makes the stove behave like a giant spark arrestor with no need for air controls or dampers or spark filters.

These simple two-hole stoves just run flat out with no need for further controls.

Consequently, these downdraft stoves are unlike all other tent stoves that burn batches of wood. In batch-fed stoves, the early burning of the batch can be hot but smokey combustion. The excess wood gas, can’t be fully combusted. Then, at the end phase of the burning of the batch, the residual fuel is mainly charcoal. It burns very hot and cleanly but does not produce wood gas or a moving flame to distribute that heat around the stove surface or to maintain a strong stove draft. It also can create destructively high local temperatures that can distort and destroy the metal of the stove. This is described in my more detailed post on the great differences between burning wood and charcoal.

In great contrast, all my recent downdraft tent stoves burn fuel cleanly and efficiently on a continuous self-feeding basis at a steady rate as the sticks fall down the fuel tube on demand. “They are in effect self-feeding wood gas generators that maintain a steady hot gas flame.

They can experience temporary flame-outs that can be easily re-ignited with a small flame. This little video shows such re-ignition.” This design means that, give or take a few glitches, at any given time there will be a constant gas flame and strong stove draft. A steady balanced mixture of combustion of both wood gas and charcoal results.

“Both fuel types magically contribute to the balanced destruction of the other in the gently pulsating inferno within the stove.” The wood gas production maintains the moving flame, the distribution of heat to the fire dome and the strong stove pipe draft. The inward rush of air through the fuel tube and around the toasting fuel sticks prepares them for pyrolysis into charcoal and wood gas. The focused jet of air preferentially burns the accumulated charcoal and prevents charcoal choking and reverse burning that can be experienced with other similar stove designs. The radiant heat from the charcoal combustion dries the incoming fuel sticks and pyrolyses more wood gas from them to sustain the stove draft and wood gas fueling cycle.

Splitting of thick frozen or very wet sticks may be required to accelerate the thawing, drying and pyrolysis process to produce an adequate supply of wood gas to sustain satisfactory stove temperature under harsh winter conditions. “Luckily, this preparation can be done in the comfort of a warm tent.” This downdraft burner/fuel tube arrangement ensures a good balance between the two fuel types is maintained for this clean powerful and efficient combustion. It also means that at any moment in the fuelling process there is a steady heat feedback into the thawing and drying of the self-feeding stick fuel in preparation for its gasification and combustion This contrasts greatly with the fuelling combustion in any batch-fed stove.

Optimized location of a large downdraft tent stove

If the large downdraft tent stove is primarily a radiator for a shared winter tent, it is best placed near the middle of a pyramid tent so that the radiant heat is shared evenly with the tent occupants. If the tent is large and has no fixed floor, a snow pit can easily be dug out, with entry steps, to increase the usable volume under the tent. This can provide comfortable standing space and heated seating (on insulated sleeping mats) on benches around the stove. The same benches and mats can convert over into sleeping places around the stove.

The use of such a snow pit will necessitate mounting the stove up on legs or similar (discussed later) to effectively optimize the distribution of the heat to the tent occupants. It will also require an extension to the tent pole (as in Darren’s FTR tent stove) or the use of an alternative support for the tent apex.

No tent pole?

My various DIY pyramid tent designs all have what I call a Load-bearing hem at the apex. This allows great flexibility for the tent to be pitched with a formal centre pole, a rough bush pole of any diameter or no pole at all, as in the photo above where the apex is suspended from an overhanging branch of a Snow Gum. This offers great flexibility with regard to snow pit excavation. It also means that when using a bush pole, it does not need to be cut to a precise length and apex tension can be adjusted from inside the tent. These issues are discussed in more detail in my post; Tent stove mounting for all seasons.

The load-bearing hem has the load-bearing drawcord fitted to hold up the tent on a formal pole, bush pole or an overhanging branch via an apex cord that can be tied to the drawcord. “It is the most versatile and strong apex fitting and can be easy to sew by a crap sewer like me.”

Terrestrial stove mounting

The stove is designed for both terrestrial and snow camping as both may be needed on one trip. The undersides of my downdraft stoves do not get particularly hot (just hot enough to charr organic matter (or scorch skin if contacted) and a nice temperature for gently drying fuel sticks). Consequently, for terrestrial camping, they can sit on the ground that is cleared of combustible organic matter. They can be sat on a bed of sticks that are shielded by aluminium foil or on a bed of sand, gravel or rocks. Even better they can be sat on a big flat rock that will become a hot table and a heat bank that will last for hours for terrestrial camping.

An example of mounting a tent stove on a rock slab that is placed on three other rocks to form a level hot bench.

Stove mounting while camping on deep snow

More effective insulation is required when running the stove on deep snow, otherwise, the stove or the rock support will just keep melting down into the snow. Following my previous success with using bush poles as stove mounts, this new stove includes an option to be mounted up on three insulated bush poles that can be driven into the snow (or soil), as shown in the photo below or on snow in the oval stove above. The poles can also be used to form a storage and drying rack for fuel sticks.

Ultralight tent stove mounting on three bush poles. There is great stability to support fuel sticks on the drying rack and a big dinner pot. “The tent has no pole as it is hanging from a ‘sky-hook’ (my washing line in this case).” The mounting pockets or brackets on the side of the fire dome also act as the mounting points for the ‘V-wire’ device that supports the overhang of the dinner pot. They also serve a similar function to support a second cooking pot when the fire dome is inverted to become a blower stove for outside cooking.

Primitive prototype large downdraft tent stove

A large DIY tent stove that is made from part of an oil drum. It is mounted up on a second drum to form the fire dome base and to make a stove stand. The open hatch at the front of the fire dome is an experimental port for adding big chunks of supplementary wood fuel and for incinerating turds in combustible bags.

I had already made a large experimental downdraft tent stove from mild steel 22L cooking oil drums. It was designed for base camping situations where the modest weight and volume were not an issue. “It was playfully called the Alpine TurdBuster Stove because it had a side hatch that would allow the addition of big chunks of extra wood fuel and a turd in a bag to be incinerated while making a cup of coffee!”

However, the side entry of the turd was not a practical or very elegant process as shown in this short video. ” If they were frozen stiff on the snow, they would be much easier to roll up in paper and load, so someday I may attempt to make a more functional supplementary loading hatch. “But I will Keep It Simple Stupid and stick with my proven design that has only two holes so that this current stove, like me, stays simple.”

A large experimental downdraft tent stove (Turdbuster) is shown located near the centre of my large pyramid tent with a vestibule. It provides welcome warmth in the tent. However, it failed to satisfy its primary function of providing direct radiant heat to the tent occupants and the cooking when in a freezing environment while snow camping.

The stove worked well enough, but the high thermal conductivity of the mild steel was inferior to stainless steel for producing optimum radiant heat. Mild steel would also have a much shorter life than stainless steel. The fire dome depth/thickness that would accommodate wood chunks (and turds) was too much as it made the fire dome volume too large to produce optimum radiant heat. The protruding rim on the cooktop was not good for allowing multiple cooking pots to effectively share the large surface and it certainly would not make a suitable backpacking load.

However, it did show that such downdraft stoves could be scaled up and could burn big wood chunks and turds efficiently. “The remaining questions were: Could I greatly improve the surface temperature, make it a slimmer pack-friendly load and good for cooking multiple pots for multiple people?”

Making the large downdraft tent stove

A large downdraft tent stove body

Breaking with my hard rubbish-sourcing tradition I found a suitable dish in a bargain homeware shop. The dish that forms the fire dome is made of spun stainless steel and appeared to have Goldilocks’ dimensions for backpacking and cooking (shown below).

The large downdraft tent stove’s fire dome has a diameter of 280mm and a depth of 53mm and a weight of 1,100g. It has a smooth and robust shape for backpacking. It also has that all-important small beaded rim around the opening. This rim bead provides a benign edge, and crush strength and critically allows my traditional attachment of a disk of stainless steel to form a base plate for the fire dome (discussed later under; A base for the large downdraft stove fire dome ).

A stainless steel dish that will form the fire dome of a large downdraft tent stove. It has a beaded rim that can be used to attach a stove base.
A stainless steel dish forms the fire dome of a large downdraft tent stove. It has a rim bead (on the bottom) that provides a benign edge, and crush strength. Critically, it allows my traditional attachment of a disk of stainless steel with crimped aluminium foil to provide an air seal over the base plate for the fire dome.

Fuel and stove pipe ports

I started by cutting two 63mm diameter holes in the dish and left a space of about 24mm between them (as shown in the photo above). This space means that they can eventually be partitioned from one another by a divider that I call a ‘flame guide’ that controls the spread of the hot wood gas and flame (more details later under the heading: ‘Flame guide’).

“Putting an asshole beside a mouth might not look like intelligent design, but it works.” This odd arrangement maximises the area of the cooktop that can be used for cooking pots. It also allows the stove pipe to become a support an ultralight fuel stick guide that can allow the use of thick and very long lazy fuel sticks that can be more than 1,000mm long (more details later under the heading: ‘Fuel tube’).

Insulated stove mounts

Another post on stove mounting describes many ways of mounting a tent stove. I have found that the bent mounting wire (HT farm fencing wire or titanium wire) shown below makes the most versatile ultralight mounts when attached to bush poles that have been driven into the snow (or soil). ‘Four legs provide the best wood rack potential, but I have found that having only three legs makes it easier to set the cooktop level and it still can make a good wood rack.”

The three mounting brackets for this stove are folded out of stainless steel from a discarded rubbish bin (the procedure is described in my post DIY Hand tools for tinkerers, under the heading: ‘weld on hinge forming’). These brackets have a very low profile that makes them pack-friendly when welded to the fire dome and they have a small vertical circular opening that can receive the pole mounting wires described below. The same or similar brackets can also be used to mount V-wire pot supports (discussed later).

Pole clasps and rubber whipping. The mounting wires and pole clasps have evolved by simplification over several years and many versions will be seen in my posts. The ones that I find easiest to use, with cold winter fingers, are shown as examples in the photos below.

The clasps are made from welded stainless steel foil (0.1mm thick) that wraps around a wooden bush pole of any diameter. Glued knobs of RTV silicone rubber on the inner face of the clasp provide enhanced grip on a smooth wooden pole and they also can grip on an aluminium central tent pole if required. Please see this link for more ideas DIY backpacking silicone rubber uses.

Thin long strips of rubber, that are cut from truck or tractor inner tubes, are used to tightly whip the clasp to the wooden pole. “These rubber strips may look tatty, but they don’t perish like natural rubber and they cost nothing (unlike silicone rubber).” The rubber strip is permanently attached to the clasp on one end using a welded cleat. On the other end, a simple knot is tied to form an easy grip and a cleat stop. When the tight whipping is done, the end can be held tightly in a cleat formed between the pole and a small protrusion that is welded to the clasp.

A stove mounting clasp that has been whipped to a mounting bush pole with a long strip of inner tube rubber. The clasp has a tiny tube with a blanked of bottom. It was formed forming by rolling and then welding. The clasp is held tightly in place with stretched rubber whipping strip. The long leg of the bent HT wire is inserted into the clasp tube and the shorter leg can be inserted into the stove mounting bracket on the fire dome.
A stove mounting clasp that has been whipped to a mounting bush pole with a long strip of inner tube rubber. The clasp has a tiny tube with a blanked-off end. It was formed by rolling and then welding a pocket on one side of the clasp. The clasp is held tightly in place with stretched rubber whipping strip. The long leg of the bent HT wire is inserted into the clasp tube and the shorter leg can be inserted up into the stove mounting bracket on the fire dome. “This is a primitive but very versatile ultralight stove mounting device where its size and weight belies its strength and flexibility and height adjustments with cold hands is a breeze.”

Swinging mounting wire

The mounting wires have two bends in them as shown in the photo above and below. This means that the poles can be set wider than the fire dome perimeter and the wood pole top can be kept away from the stove’s strong radiant heat. This provides more space for the wood rack below the stove and a wider protective barrier around the hot stove. Also, the accuracy of the pole placement is not a critical factor while hurriedly setting up camp after a long day of skiing. The design means that each leg of the bent wire can just swing to fit the matching mounting brackets on the fire dome.

A tent stove leg connector clasp with a swinging mounting wire shown ready for assembly. When the wire is inserted into the matching tube formed on the clasp, the wire can rotate to accommodate the fitting of the wire to the fire dome mounting brackets and both ends can then rotate to fit between the mounting legs and the stove.
A tent stove leg connector clasp. A swinging mounting wire is shown ready for assembly. When the wire is inserted into the matching tube formed on the clasp, the wire can rotate to accommodate the fitting of the wire to the fire dome mounting brackets and both ends can then rotate to fit between the mounting legs and the stove.
A tent stove leg connector clasp with a swinging mounting wire sho A tent stove leg connector clasp with a swinging mounting wire shown assembled withe wire inserted into a pocket on the clasp. The wire can rotate to accommodate the fitting of the wire to the fire dome mounting brackets and both ends can then rotate to fit between the mounting legs and the stove.
A tent stove leg connector clasp with a swinging mounting wire inserted into a pocket on the clasp. The wire can rotate to accommodate the fitting of the wire to the fire dome mounting brackets and both ends can then rotate to fit

The pole clasps (shown above), rather than the pole cap that I previously used, can easily be adjusted up and down the poles to set the cooktop level. This means that the pole’s of a particular diameter and with clean-cut pole ends are no longer critical for stove mounting as was the case when using the previous mounting caps. “Any pole can protrude well above the stove if required and the use of this structural feature may be the subject of another post.”

A tent stove leg wire mount whipped tightly to a wooden pole with a thin strip of pneumatic tire inner tube. One end of the rubber ribbon is held to the clasp with welded clip. The other end has a knot tied in it that allows the whipping to be stretched tightly and cleated between the protruding welded stainless steel foil peg and the wooden pole. The pole can be driven into the soil or snow to make a ultralight and insulated tent stove mounting leg. The bent wire can swing to engage the mounting bracket that is welded to the tent stove fire bowl.
A tent stove leg wire mount whipped tightly to a wooden pole with a thin strip of pneumatic tire inner tube. One end of the rubber ribbon is held to the clasp with a welded clip. The other end has a knot tied in it that allows the whipping to be stretched tightly and cleated between the protruding welded stainless steel foil peg and the wooden pole. “Easy to do with cold camping fingers.” The pole can be driven into the soil or snow to make an ultralight and insulated tent stove mounting leg. The bent wire can swing to engage the mounting bracket that is welded to the tent stove fire bowl.
A prior example of an ultralight tent stove mounting and wood drying rack using two bush poles and the tent pole. It is happily supporting a big dinner pot. The pole clasps, in this case, are whipped in place with a nylon cord. “The rubber whipping is much better to use with cold winter hands.”

Pot overhang supports

A stove that can simultaneously cook, simmer and snow melt in multiple pots or lay flat kettles has obvious benefits for a group of snow campers. “It is a delight to have plenty of hot drinks for rehydration while dinner is simmering while more water is being prepared from snow.” To enable the use of multiple pots on a stove it is useful and safer to have temporary and light extensions around the cooktop that can provide extra support for pots that must protrude beyond the edges of the cooktop.

These wires are especially important when the pot for fast boiling is using the majority of the stovetop and other pots are simmering or snow melting off to the side. This stove can have three or more removable ‘V-wires’ for this overhang support and they can be shuffled around the perimeter where needed. “These wires also help to prevent accidental contact of the cook’s clothing with the hot stove.”

The mounts for the V-wires are similar to those described above for the stove mounting brackets and in some places, a single bracket serves both purposes.

Welded mounting brackets on the large downdraft tent stove. Three brackets are used to hold stove wires that provide insulated mounts for the stove. The other brackets can be used to mount V- wire pot support around the stove to support overhanging pots.
Welded mounting brackets on the large downdraft tent stove. Three brackets are used to hold stove wires that provide insulated mounts for the stove. The other brackets can be used to mount V- wire pot support around the stove to support overhanging pots.
Three large pots sitting on the large downdraft tent stove with the support of V-wires mounted around the cooktop.
Three large nesting pots sit on the large downdraft tent stove with the support of V-wires mounted around the cooktop.

Flame guide

The flame guide is the major dividing baffle that causes the flame and hot gas to take a long path from the charcoal blast zone (below the fuel tube) across the fire dome. Then the flame and gas turn sharply and turbulently back through a narrow tunnel on the back side of the flame guide to flow to the stove pipe port that is adjacent to the fuel tube. The stove has a central baffle that helps the wood gas flame to mix with the air and spread the flame and heat over the fire dome. The flame guide and baffle have another function of acting as a very effective spark arrestor, so no other device is required for this purpose.

Tabs have been folded on the edge of the guide and baffle so that they can be used to hold the guide in place, either by direct welding or preferably by using welded holding cleats. The use of cleats means that the baffle and flame guide can be easily replaced if they burn out after lots of use.

A stainless steel flame guide that will be fitted under the bottom of the fire dome of a large downdraft tent stove. It has a folded tabs that allow it to be welded to the underside of the fire dome.
A stainless steel flame guide will be fitted inside the fire dome of a large downdraft tent stove. It has folded tabs that allow it to be welded to or held by cleats that are welded to the ceiling of the fire dome.
A stainless steel flame guide that has been loosly fitted into the fire dome of a large downdraft tent stove. It forms a small channel for the hot combustion gases to flow back through to reach the exhaust port.
A view of the underside of the fire dome of the large downdraft tent stove. The flame guide is sitting in the fire dome (held in place with a clothes peg), showing how, when clipped with cleats (or welded) in place, it will divide the fire dome to form a large primary wood gas burn chamber (left) connecting to the round fuel/burner port hole (left). On the other side of the flame guide, a much smaller secondary burn chamber or a tunnel (right) is formed. This smaller chamber helps to complete the gas combustion and causes the flame to flow back through it to reach the exhaust port (right) without taking ‘shortcuts’.

I use stainless steel sheet metal from discarded trash cans to make the flame guide and baffle. It has a longer life than thinner SSor titanium metal foils. Their location means that they have little opportunity to dissipate heat to the outside world. “They get as hot as hell on one side and as hot as hades on the other.” The constant high temperature combined with surplus oxygen that is required for clean hot combustion means that metal will eventually corrode.

The baffle with three holes in it (shown below) spreads the flame and creates turbulence to better mix the oxygen into the wood gas flame. In this design, the holding cleats that are welded to the ceiling of the fire dome will easily allow for the removal, modification and replacement of the baffle.

The curved flame guide in this stove design is experimental and it departs from my usual practice. Previously, I divided the circular or oval fire dome into two uneven chambers with a straight flame guide. The larger of the two chambers is for primary gas combustion. The tongue of wood gas flame after reaching the outer end of the flame guide makes its way back to the exhaust port through the smaller chamber. The sharp turn around the flame guide causes turbulence and good mixing of wood gas to help it to complete most of the combustion within the chambers.

Turbulence baffles

In this new and larger stove, the secondary chamber is curved around the side of the fire dome so that the primary burn chamber is a much larger proportion of the fire dome that is very wide. The secondary burn chamber is limited to becoming a curved tunnel that will return the gas to the pipe. This means that a greater proportion of the wood gas will be burnt in the primary chamber (as compared to my previous designs) and combustion and heat exchange should benefit by having a mixing baffle in the large chamber. It will stop the flame from taking a straight course across the primary chamber.

The flame guide that has been fitted into the fire dome of a large downdraft tent stove. It is held in place by cleats that have been welded to the inside of the fire dome. This means that it can be replaced when it may eventually burn out. A central flame-splitting baffle is fitted to another cleat so that the flame will be mixed with oxygen for cleaner combustion and better distributed of heat over the fire dome for more even cooking temperatures.
The flame guide that has been fitted into the fire dome of a large downdraft tent stove. It is held in place by cleats that have been welded to the inside of the fire dome. This means that it can be replaced when it may eventually burn out. A central flame-splitting baffle is fitted to another cleat so that the flame will be mixed with oxygen for cleaner combustion and better distributed of heat over the fire dome for more even cooking temperatures.

A base for the large downdraft stove fire dome

Base plate with an aluminium foil cover? Some of my very small downdraft stoves used only crimped aluminium (pie dish foil) for the stove base. However, I did use a small sacrificial but protective steel disk (bean can lid) and a bed of ash (or soil) to form a heat protection shield.

This shielding protected the aluminium from the inferno in the charcoal bed that forms below the fuel tube where the temperatures are the highest. Eventually, I moved to add a full disk of stainless steel to provide protection to the aluminium across the whole base. “A little weight compromise for a great convenience benefit and better backpacking strength.”

This full disk of stainless steel makes the base much more robust while in backpacking mode and much easier to confidently set up, knowing that the protective shield will not slide out of place. “Who wants to have read hot coals falling down in a tent?”

For the new stove, the full stainless steel disk is held in place and also sealed from inwards air leakage by using a cover of aluminium ‘disposable-baking-dish-foil’. The edge of the oversized aluminium disk is crimped to the beading of the bowl.

Sacrificial charcoal pan/heat shield

A downside of the downdraft tent stove design is that the clean, steady (flat-out), efficient and hot combustion of the wood fuel depends upon the extremely hot combustion of the wood residual charcoal. The charcoal combustion occurs in the air blast zone that is immediately below the fuel tube where the tips of the fuel sticks collapse into chunks of charcoal as the tips are depleted of wood gas. This charcoal combustion zone reaches temperatures in excess of 1,200C which is much hotter than the wood gas flame (est. 800-900C). Consequently, the stove’s longevity can be enhanced by the use of a sacrificial/replaceable heat shield.

The shield can be made of roofing iron, tuna tin-can sheet metal, stainless steel foil or even titanium foil and can be fitted to shield the hotspots on the nearby fire dome, flame guide and stove base. I use thin sheet metal for the liner on backpacking stoves and thick metal (such as roofing steel) for base camp ones (as in the Alpine TurdBuster Stove). Thin metal makes fabrication easy and saves backpacking weight. However, it will reduce its survival time.

A cut-down tuna tin as shown below is a simple and inexpensive way of providing effective shielding to the stove base, the nearby portion of the flame guide and the fire dome wall. (The tuna tin shield mounting location is shown in the photo above.)

A modified tuna tin that has been cut down to form a protective sacrificial shield for the fire dome wall and flame guide.
A modified tuna tin that has been cut down to form a protective sacrificial shield for the fire dome wall and flame guide. A shallow lip of the tuna tin wall is left on the open side (front) to act as a charcoal and ash keeper below the blast zone. The ash is a good thermal insulator and the charcoal burns best if retained largely in this blast zone.

Protection from chemical erosion

The success of this downdraft stove design depends upon destructively high charcoal combustion temperatures that are always present in the small charcoal combustion blast zone located below the fuel tube. Unfortunately, a chemical excess of oxygen must be present in this zone. This excess oxygen is essential for the complete, hot and clean combustion of the wood gas in the remainder of the burn chambers, so it can’t be avoided. Also, other corrosive chemicals such as hydrochloric acid are produced during the combustion process.

When these corrosive chemicals are combined with high temperatures they cause the destruction of nearby metal surfaces. It is somewhat ironic that my striving for clean hot combustion stove designs increases metal decay while in contrast, dirty, smoky, inefficient batch-loaded stove designs can largely protect similar metals from decay. Nevertheless, even a batch-loaded tent stove will go through a destructive ‘charcoal+excess oxygen’ burn phase if fresh wood fuel is not constantly added to make enough protective wood gas.

“This does not surprise me. Carbon is a wonderful element, from diamond jewels, abrasives, carbon fibres, purifiers, gun powder, activated charcoal, capacitors, insulators, conductors, electrodes, motor brushes, graphite lubricants, the building blocks of terrestrial life and coal, a vexatious element involved in climate change (if you have not got your head firmly stuck in the political sand or some other orifice). What an element, so full of wonder.”

Me

My ideas about metal decay in my downdraft tent stoves may seem a little odd, but I was pleased to find an informed Youtube discussion of similar issues with metal components in rocket mass heaters that similarly burn clean and hot. They conclude that metal was unsuitable in the hottest components.

“We can’t avoid metal in our ultralight stove designs but we can protect the worst problem areas with protective shields. It may give the stove a long life that may exceed our available time to enjoy using it in the snow. Having a removable base makes the replacement easy and possible.”

Refractory coatings

While I have tried DIY protective refractory coatings with some success, a sacrificial and replaceable internal metal shield or liner is a cheap, simple and more effective solution to this problem.

Ash or soil as an additional shield

I also form a small lip on the open side of the above tuna tin shield. This helps to retain the charcoal chunks below the fuel tube where it should be burnt for the best effect. The lip also helps to retain a deep layer of wood ash in the liner. The accumulated ash layer (from the combusted charcoal) is an excellent insulator and also protects the liner base and the stove base from excessive heat. Consequently, ash or soil can be added before light-up to accelerate this natural insulation process.

I often add charcoal to the base of the stove to help the stove quickly establish that balanced combustion of wood gas and charcoal at startup. Found charcoal is usually, unavoidably mixed in with ash and soil so all can be added at the same time.”

A modified tuna tin that has been cut down to form a protective sacrificial shield for the fire dome wall and flame guide.
A modified tuna tin that has been cut down to form a protective sacrificial shield for the fire dome wall and flame guide. On the open side of the shield, a sallow lip is left to act as a keeper for the charcoal and the protective ash layer in the high-temperature blast zone.
The sacrificial shield made from a tuna tin has been place in the fire dome below the fuel entry port where the most intense and hot combustion of charcoal occurs under the blast of oxygen rich incoming air.
The sacrificial shield made from a tuna tin has been placed in the inverted fire dome below the fuel entry port where the most intense and hot combustion of charcoal occurs under the blast of oxygen-rich incoming air.

Fitting the stove base

The circular base for this stove was cut from 0.1mm thick stainless steel foil after scribing around the beaded lip of the fire bowl. Alternatively, sheet metal from a discarded stainless steel rubbish bin can be used if lightweight is a lesser consideration.

Stainless steel and aluminium foil disks for the base of the fire dome of a large downdraft tent stove. The stainless steel disk has the same diameter of the bead on the original dish. The foil disk is somewhat larger and it can be wrapped around and crimped to the dish bead. This holds the stainless steel disk in place and also seals the base from excessive inwards air leakage. The stainless steel disk also acts as a heat shield for the aluminium. An additional heat shield is also located immediately below the fuel tube.
Stainless steel and aluminium foil disks for the base of the fire dome of a large downdraft tent stove. The stainless steel disk has the same diameter as the bead on the original dish. The foil disk is somewhat larger and it can be wrapped around and crimped to the dish bead. This holds the stainless steel disk in place and also seals the base from excessive inwards air leakage. The stainless steel disk also acts as a heat shield for the aluminium. An additional sacrificial heat shield (made from a tuna tin) will also be located immediately below the fuel tube to provide extra heat shielding in this particularly hot charcoal blast zone.

Next, a slightly larger disk of aluminium foil can be cut from a disposable baking tray or tart dish. The stainless steel disk and then the beaded rim of the fire dome can be placed on top before the excess aluminium foil can be gently rolled and crimped up and over the fire bowl beading to hold the base in place.

Supplementary stove base clips

During hours of backyard testing, I found that for the scaled-up large downdraft tent stove, my traditional crimping of the aluminium base cover was not strong enough. “I found this out the hard way while testing the fueling of the stove lazily with big long and thick fuel sticks while doing other household chores. With my back turned, a large stick must have ‘hung up’ and then fell down and caused a crack to open up between the stove base and the fire dome flange. Luckily I was not snow camping!”

Consequently, I needed a lightweight and low-profile holding device that could provide extra holding strength. The welded stove base, such as the oval stove shown above, is very simple, secure and fast to set up at winter camp. However, it is not removable for internal stove servicing. Luckily, in some previous smaller stove experiments I had successfully made an effective ‘string of clips’ (as shown below).

The 'band' of clips used to hold the stove cover disk on tightly.
The experimental ‘string’ of clips was used to hold the stove fire bowl cover disk on tightly.

I made a similar, but much larger, ‘string of 16 clips to hold the stove base SS disk and foil cover securely in place.

The fire dome base with supplementary clipscover of stainless steel foil disk and then crimped aluminium foil cover clips.. The the string of 16 clips has been added to provide supplementary holding forces for the base covers.
The fire dome base with supplementary clips on the cover that is made of a stainless steel foil disk and then a crimped aluminium foil cover. The string of 16 clips has been added to provide supplementary holding forces for the base covers.

Other fittings for the large downdraft tent stove

Roll up stove pipe

The ~60mm diameter of the stove pipe for the large downdraft tent stove makes it much easier to form into a pipe from the parent flat strip when compared with my smaller stove pipes that were ~40-50mmdiameter. While the stove fittings are apparently very simple, they can be made with subtle differences with effective diameters that allow them to fit compactly inside one another while backpacking. This makes the bundle very pack friendly, efficient and strong.

I start with the rolled up stove pipe that is rolled up within its own retaining rings (without their removal), as described in my post, Part 9- Large roll up stovepipe forming. This little video of a 65-second roll up of a 3,000*60mm stove pipe may help explain this magical process*. This particular roll-up is done with the assistance of a second person, but it can be done solo. However, assisted forming and rolling up is very appropriate for a tent and stove that will be shared with others.

Note* The magical roll-up and roll-out method initially was all about preventing crinkles from forming in the pipe foil (as is the case for other methods). However, with practice, it has also become a quicker method of deploying and packing up the pipe while camping under difficult winter conditions as the holding rings are not removed during the process.’

The outside diameter of the rings determines the size of the other two components that if made a little larger can fit over this to form a compact backpacking bundle.

A roll-up stove pipe that has been formed new within its holding rings and it has stayed in the rings during rolling out and rolling up.
A roll-up stove pipe that has been formed new within its holding rings and has stayed in the rings during rolling out and rolling up.
A roll-up stove pipe that has been deployed within its holding rings. It can be rolled up within the rings also.
A roll-up stove pipe that has been deployed within its holding rings. It can be rolled up within the rings also.

The roll up stove pipe connector/guard tube

The sacrificial guard tube/stove pipe connector that has been inserted into the exhaust port of the large downdraft tent stove. It has been made from the sheet metal from a coffee tin. It makes a tapered connection between the stove's exhaust port and the stove pipe. It also greatly reduces the heat damage done to the formal stove pipe by dissipating heat in this turbulent section of gas flow while the flow settles to laminar flow that heats the pipe much less. The protruding seam acts as a stop to prevent the tube falling into the stove and as a stop to prevent the stove pipe falling down over the connector.
The sacrificial guard tube/stove pipe connector has been inserted into the exhaust port of the large downdraft tent stove. It has been made from sheet metal from a coffee tin. It makes a tapered connection between the stove’s exhaust port and the stove pipe. It also greatly reduces the heat damage done to the formal stove pipe by dissipating heat in this turbulent section of gas flow while the flow settles to a laminar flow that heats the remainder of the stove pipe much less. The protruding seam acts as a stop to prevent the tube from falling into the stove and as a stop to prevent the stove pipe from falling down over the connector. Two stainless steel rings hold the rolled metal tube in shape while it is being fitted into the stove and into the stove pipe.

This tube has a dual function. It is used as a tapered adaptor to connect the exhaust port and the roll-up stove pipe that can have different diameters. Its second function is a sacrificial guard tube to protect the stove pipe from thermal damage.

The first 200mm of a normal stove pipe on this type of stove gets very hot because of the combination of high gas temperature, periodical delayed wood gas combustion in the pipe and a high heat exchange rate that is caused by turbulence in the same zone. The turbulent flow occurs when the exhaust gas changes direction sharply from the fire dome and into the stove pipe.

These combined conditions cause rapid metal decay in the turbulent zone and will eventually destroy the bottom of a normal stove pipe (stainless steel or titanium). As an alternative to periodically trimming off the damaged parts of the pipe, I have found that the sacrificial guard tube makes a much better solution. “It can be made of something as cheap as a discarded coffee tin and can save a lot of grief.”

A fuel tube (left) made of welded stainless steel is inserted into the fuel port of a large downdraft tent stove. A tab in the tube seam limits it protrusion into the fire dome to about 10mm. The tube holds fuel sticks upright as they wait to feed down into the combustion zone as the residual charcoal is burnt away.
A fuel tube (left) made of welded stainless steel is inserted into the fuel port of a large downdraft tent stove. A tab in the tube seam limits it protrusion into the fire dome to about 10mm. The tube holds fuel sticks upright as they wait to feed down into the combustion zone as the residual charcoal is burnt away.

Such a connector/guard tube can be rolled out of sheet metal (much like the fuel tube described below), but I do not seam weld this tube. Instead of welding, I put strategic cuts in the overlapping metal at each end of the tube. By interacting with the opposing edge of the rolled sheet, these set the diameter of each end of the tube and prevent it from collapsing. The ends of the seam can form tabs or stops that can prevent the tube from dropping into the fire bowl and also stop the stove pipe from dropping down over the connector guard/tube*. For more details please see my post: A sacrificial guard tube for a tent stove pipe and A guard tube as a flue pipe adaptor and a storage tube.

Note* This connection method is unsuitable for other stoves that may burn less cleanly, as sooner or later tar combined with condensation water from damp bush wood will eventually leak out of the top joint, then run down to burn on the outside of the adaptor or the cooktop and make annoying smoke in the tent. “It is only suitable for a clean burning stove, such as a downdraft stove. The overlapping of the top joint would need to be reversed for stoves that burn less cleanly.”

Lastly, because this tube is rolled but not welded it can be rolled up to fit over the rolled up stove pipe and inside the fuel tube to make a compact backpacking bundle.

Stove pipe anchor and more stove pipe magic

Buffeting alpine winds can cause the stove pipe to lift off the adaptor or the stovetop. Consequently, it is prudent to anchor it down to a stove leg or similar fixed object. For this, I connect a ribbon of stainless steel to the bottom stove pipe retaining ring. Then I tie this ribbon down to the stove leg situated below the exhaust port. To make this method work, the diameter of the adaptor needs to be made wide enough to make the stove pipe flare or swell a little so that the ring can not be pulled down and slip off.

[Add a photo of the stove pipe anchor ribbon, and tie-off cord]

Fuel tube

I find that a fuel tube that is made as a light and rigid welded tube gives the best control of the fuel stick loading. The rigidity allows it to be jiggled and easily tilted a little from vertical for easier fuel stick flow. However, a roll up alternative can still be used. I roll the tube using the methods described in my posts Halfpipe metal curving anvil and Rolling curves in titanium and stainless steel foils, depending on the metal thickness.

Having rolled the tube ~100-180mm long, I weld the seam so that the tube has an external diameter of ~63mm to fit tightly inside the fuel port. I do the welding so that there is a slight taper that allows it to protrude into the fuel port by about 5-10 mm only on one end. A folded tab on the side seam of the tube can be used to adjust the amount of protrusion if required.

On the top end of the tube, I finish it with a 5mm wide rolled edge. This gives the tube more rigidity and softens the otherwise razor-sharp edge of the foil from which it is made.

[Add a photo of the formed fuel tube showing the seam, insertion limiting tab and rolled top]

At the other end of the tube, it is welded with a slightly larger circumference. This slight taper allows it to be easily fitted over the rolled-up stove pipe and stove pipe adaptor while backpacking. “It in effect becomes the strong backpacking container for all the other minor round stove fittings.”

[Add a photo of all the minor round stove fittings packed up in the fuel tube.]

[Add a photo of the stove pipe adaptor and burner tube wrapped around the rolled up stove pipe alongside the fire dome]

Here is a little video of the intense fuel combustion and the simple fuelling with bush sticks. It is tempting to make the fuel tube longer to better support longer fuel sticks. This strategy may work well when using straight-sawn timber for fuel at home, as seen in most demonstration videos. However, it does not work well when using irregularly shaped found bush sticks that the stove is designed to use. Importantly, it also increases the risk of triggering reverse burning, as it can behave like another stove pipe.

If the convenience of using long fuel sticks is required, extra support for them can be provided by a separate ring or U-shape device that can be suspended from the adjacent stove pipe.

urning long thick fuel sticks a s lazy fuelling of a large downdraft tent stove. The long stick is stabilized by a holding ring that is attached to the adjacent stove pipe.
Burning long thick fuel sticks as a lazy way of fuelling of a large downdraft tent stove. The long sticks are stabilized by a holding ring that is attached to the adjacent stove pipe.

Lastly, a downdraft tent stove can be used without a fuel tube or with a very short fuel tube. This arrangement is particularly suitable when chunky fuel such as wood chips, DIY fire starters strips*, dung, nuts, combustible waste or wood pellets are being used instead of bush sticks. If the tall tube is loaded with these chunky fuels it may block the air supply too much and cause the stove to smoke excessively.

Note about DIY fire starters*. My post on DIY starter strips describes how a range of very high-energy light fire starters can be made for starting fires when there is no dry fuel available. “They are worth their weight in super down on a cold night, cost nothing, weigh bugger-all, are waterproof, don’t stink, and each has a very different burn characteristic to match every pyromaniac’s requirements.”

A quiver of micro DIY ultralight fire starter strips from left to right; thin waxies (thin strips of wax-soaked newspaper for a quick source of flame), thick waxies (wax-soaked egg carton lid strips for a persistent source of heat and flame) and milk bottle flame drizzler strips (for drizzling flame down onto other fuel).

Large downdraft tent stove specification

This large downdraft tent stove weighs xxxg including a 2,200mm long roll up stainless steel stove pipe and other minor fittings. The stove has a 285 mm diameter cooktop and the fire dome wall is about 60mm deep with a circumference of 896mm. The combined weight of the stove and vestibule tent is only yyyg/per person when shared between four people.

Large downdraft tent stove performance

Fuel stick combustion rate. The wood combustion rate is a misely~12.5g/minute or 750g/h. “So each of the four tent occupants only to collect about 1.5kg of fuel sticks to provide about 8h of heated comfort on a cold snowy evening.”I estimate that the heating power of the stove is about 1,600 watts. This estimate was based on assumption that; the combustion was complete (no smoke), the sticks contained 20% moisture and the heat exchange is 70% effective.

A view looking down into the clean, efficient, compact, hot, persistent, balanced and self regulating inferno of charcoal and wood gas in an a large downdraft tent stove.
A view looking down into the clean, efficient, compact, hot, persistent, balanced and self-regulating inferno of charcoal and wood gas in a large downdraft tent stove. “This pulsating inferno is the heart of the stove’s success but the cause of a metal decay problem that needs to be managed.”

The cooktop with a 285 mm diameter temperature typically reaches 250-400C. The whole cooktop can provide useful water boiling, simmering and snow melting.

The temperature of the large downdraft tent stove being measured with a thermocouple. It was slow to reach a final stable reading. An infrared thermometer was much quicker, but it grossly under estimated the surface temperature.
The temperature of the large downdraft tent stove is measured with a thermocouple. It was slow to reach a final stable reading. An infrared thermometer was much quicker, but it grossly underestimated the surface temperature. The temperature is being measured on the side that is opposite the downdraft burner and behind the turbulence/mixing baffle. It shows that the flame a heat spreads well away from the burner to make a big and effective cooktop.

The hottest part of the cooktop (adjacent to the fuel tube) can boil 500ml of water in about 6 minutes. “This is not jet boil speed, but fast enough when it provides unlimited heating in the comfort of a winter tent when the sunset comes so early in the evening.” With the addition of the ‘V-wire’ pot supports a mix of boiling, simmering and snow melting can be simultaneously achieved.

A view looking down a large downdraft tent stove that is capable of symultanously boiling water, simmering dinner and snow melting to make water for a group of 3-4 skier that share a large ultralight winter tent.
A view looking down a large downdraft tent stove that is capable of simultaneously boiling water, simmering dinner and snow melting to make water for a group of 3-4 skiers that share a large ultralight winter pyramid tent.

The cooktop temperature range is somewhat lower than that of my much smaller KISS tent stove which often reaches 500C (extensive red glow at night). However, the relatively large fire dome surface area of the large downdraft tent stove will provide abundant radiant heat and cooking power for the occupants of the large backpacking tent. “As a practical indication, the radiant heat from the stove is so strong that I can only sustainably hold my hand at a distance of 300-400mm from the fire dome without pain and this test was done in the open air with a significant breeze. From experience with smaller tent stoves, I expect the warming effect of the large downdraft tent stove to be much greater when inside a tent.”

On the bright side, the lower temperature should also result in much less decay of the stove metal as per the smaller and hotter stove.

Conclusion about the large downdraft tent stove

This large downdraft stove is a good addition to my series of downdraft tent stoves and will suit larger groups of backpacking skiers who share a large pyramid tent. It appears to have all the good downdraft performance characteristics of my smaller downdraft stoves. It provides generous radiant heat for the tent occupants. Its large cooktop provides for simultaneous snow melting and boiling water for copious hot drinks while simmering a big dinner pot. The slim pack-friendly stove can be carried in its assembled form so that it will be quick to deploy when the skiing for the day is done. It would be equally suitable for skiing trips from a base camp or terrestrial winter backpacking camping.

I used the same calculation of energy transfer as for my earliest downdraft stoves. I assumed that the wood has 20% moisture content and combustion is complete because the exhaust gas is clear. If I assume 70% of the wood’s energy is transferred as radiant heat+air heating for the tent this would be equivalent to a 1.6kw tent heater. “A welcome concession to comfort in a winter wonderland tent.”

A photo of the 500g of fuel sticks that were burnt over a 400 minute test period. The combustion rate was only 750g of damp wood per hour.
A photo of the 500g of fuel sticks that were burnt over a 40-minute test period. The combustion rate was only 750g of damp wood per hour.

Earlier, I downplayed the role of a large downdraft tent stove as an air heater for a tent in favour of it being a heat radiator. However, the air heating aspect of the stove can play an important role in reducing condensation within a winter tent. Tent fabrics, such as that used for my breathing polyester vestibule tent, are very permeable to water vapour and greatly reduce problematic condensation in the tent when it is made with fabric like silnylon. This moisture permeation rate can be greatly increased by warming the air within the tent relative to the very cold outside temperature.

While preparing this post I realised that in bad weather, which is good for skiing, this big stove/tent combination would, for the first time, allow me to share the hospitality of my warm tent with other skiers without sacrificing my sleeping place as I have previously done!

[Add a photo of the assemble large downdraft tent stove mounted on three bush poles surrounded by three sleeping positions.]

Tim

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