Gas Kiln Conversion - Downdraft

November 2023 - A second revision of the TDI Downdraft Kiln Conversion book has been uploaded and published and is available at the same Amazon link previously provided. The book presents clarifications of previous text and enhancements and ideas from many people, including from the Facebook group. A major revision was to the Firing the TDI kiln sections. Oval kiln conversion is discussed as well as more detail relative to the optional or alternative parts. New text not included in other prior blogposts is included in this short blogpost: 

October 2023 - A new blogpost with details of the current design features with some helpful information has been added: http://www.sebastianmarkblog.com/2023/10/tdi-downdraft-kiln-conversion-further.html

April 2022 - This updated blogpost presents Firing the TDI Conversion section from the new 2023 book revision. It details the methods for the initial test firings, judging reduction, and also the Evening-Out-the-Temperatures process at the end of the firing:

January 2021- The Facebook Group TDI Downdraft Kiln Conversion has been created to provide a place to share information including building the conversion, firing schedules, and pottery results. www.facebook.com/groups/4124895130900261/

February 2020 – I appreciate the many people have given me feedback relative to the post or questions with their conversion. From this communication, I’ve realized that I assumed a level of understanding and construction ability above many people and have not adequately explained to all how to get it done using tools typically owned. To remedy this, I have put together a conversion book that explains the process in more detail, shows why specific items are important and how to measure them, and current sources for tools and components.

The TDI Downdraft Kiln Conversion book is available on amazon at: https://www.amazon.com/dp/B084DH88GH

Thanks, and if you have any questions, I can still be contacted at borissquare@gmail.com

The following details gas downdraft conversions of two kiln sizes – a 23” by 27” and a large 28” by 32”. The goal was to fire cone 10 reduction. The design is relatively inexpensive and easy to construct from commercially available parts. Simplicity of both design and use was a major objective. Both kilns fired very well and excellent reduction and repeatability were obtained. Photos, firing schedules, and pressure/flue size settings are provided following. Specific components/methodology were employed and using different burners or dimensions may not produce the same results requiring recalculation of the inlet and flue sizes. I would also suggest reading and understanding the entire process before doing a conversion.

The photo below of the 28”x32” conversion essentially provides the design. The 28”x32” was an Olympic gas updraft and thus had smooth walls. The 23”x27” was a Skutt electric and had the element grooves.

There is a tremendous amount of information available concerning kiln designs. There are also numerous examples of kiln electric to gas conversions in both text and video. Through research, I noted that there were some very insightful comments in website documents written by Marc Ward with Ward Burner Systems. Marc also provides equipment including drilled orifices in case you don’t want to buy a #50 bit and drill yourself.

Ward wrote “Draft is the life's breath of a gas fired kiln”. Most of the kiln construction and conversion problems, even from documents written in the 60’s and 70’s, related to issues with draft. Kilns are constructed and then flue sizes and external flue piping are rebuilt and tweaked until the kiln maybe works. Numerous books and articles detail flue/chimney heights and size calculations, and many seem to be providing a flue of around three times the draw length of the kiln, including the exit length. The draft is created by the ducted rising hot air, so the higher the chimney the more suction/pull. From a simplicity standpoint, the attached exterior chimney adds both expense and difficulty to the construction process.

Many electric to gas kiln conversions have been built with an interior flue made with firebrick. Three issues with this design are the amount of space the bricks and flue take up, the added difficulty in building and cementing the bricks, and the oft-requirement of additional flues added to the top of the kiln to increase the draft/suction.

Discovering a German design that uses cordierite shelf material for the flue provided the success for our small kiln conversions. The beauty of the German design is that the thin shelves become super-heated, providing energy to the exiting gases creating a better draft. The shelves also speed up the conversion process. Once all of the parts were obtained and readied, the 28”x32” kiln conversion took about two hours.

Another important consideration is the ratio of primary air to secondary air for the burner. The burner hole drilled in the bottom shelf is 3.5”, which provides for an adequate ratio of primary to secondary air relative to this specific design and the draft it produces. With the flue undampened, the kiln will fire in complete oxidation.

In order to have even temperatures between the bottom and top of the kiln, there is a consistency of past designs/descriptions of having close to a 1 to 1 ratio between the width (or depth) and height of the kiln. The width/depth is related to the path of the flowing gas/heat from the burners up, around, and down to the flue entrance. For the 23”x27” electric to gas conversion, the flue entrance is 2” high and 2” kiln posts are used to support the ¾” bottom shelf, yielding an interior of approximately 23” by 24” tall. Also using 2” posts and flue opening for the 28”x32” downdraft conversion, the interior measures approximately 28” by 29” tall.

The flue wall is made from square kiln shelves and the conversion utilizes the inexpensive MR-750 Venturi Burners. The brass orifice that comes with the MR-750 is drilled to #38 (0.1015”) and is too large. Order new ones from Ward Burners and have them drill them with a #50 drill bit (0.07”). Or, if you have a #50 bit, order “Brass Spud Orifice #70 Drill Blank Starter Hole” from www.thebbqdepot.com - $3.14 each.

In order to ensure controlled and consistent firings, an adjustable pressure regulator was used along with a decent quality pressure gauge. The 0-30 psi pressure regulator was purchased from a local grill/propane company and the brand was Marshall Excelsior, made in the US. The pressure gauge was average quality 0-15 psi. A standard 12 foot (¼” Inside Diameter) propane hose was used and can be purchased with 3/8” flare fittings attached. The hose does not have to be high pressure so any ¼” ID rubber hose can be used and grill companies can make them with the flare fittings attached.

A propane tank fitting is attached to the pressure regulator input and the gauge is T’d in directly to the output, rather than placing the pressure gauge at the other end near the burner. Due to the slight flow restriction of the 12’ hose, the pressure readings will be higher and thus allow for a more sensitive adjustment during firing. It’s also easier to set pressures at the regulator.

Following are dimensions of the two conversions. Since shelves and kilns vary, the dimensions may change slightly.

23x27 Specs

A – Flue to inside burner wall
B – Flue width
C – Flue depth
D – Flue flat dimension
Flue Area
22.3 Sq. In.
E – Center hole for single burner

F – Burner hole diameter
Burner hole total area
9.6 Sq. In.
Flue shelf size
2@ 16” square x 3/4”
G – space between burner and burner wall/kiln wall
Approx. 1/4”

28x32 Specs

A – Flue to inside burner wall
B – Flue width
C – Flue depth
D – Flue flat dimension
Flue Area
35.6 Sq. In.
E – Burner/hole center distance
F – Burner hole diameter
Burner hole total area
19.2 Sq. In.
Flue shelf size
2@ 20” Square x 1/2"
G – space between burner and burner wall/kiln wall
Approx. 1/4"

In both kilns, the flue wall was placed on a small 2 inch post. Thus, the flue opening for the 28x32 was B-17.25” times 2” or 34.5 square inches, less 1 square inch for the small post, yielding 33.5 square inches.

Photos 15 and 16 show the 28x32 burner assembly, which must be made first. The MR-750’s are screwed into the iron pipe fittings. The MR-750 has ½” NPT threads and ½” NPT pipe and fittings were used. Required are: 4-elbows; 2- gas cock valves; 2-‘T’s; 2-6” length threaded pipe; 2-1” pipe; 3-2” pipe; ½” NPT to 3/8” male flare (for 12’ hose connection); needle valve and adapter fittings to get it into the steel pipe ‘T’; ¼” copper tubing; and, the copper tube T. Thread seal was also used on the pipe threads. Note that the burner photos show a cap over the hose flare fitting, used to keep dirt out.

After the burner assembly is made, an accurate measurement of the distance between the burners can be done, then the holes in the kiln bottom can be drilled with the 3.5” hole saw. A pilot hole was drilled first as the hole saw was not deep enough for a single cut on one side. For the 23x27 kiln, one burner is used and an ‘L’ shaped burn assembly can be made to make it stable. Later, to position and secure the burners, several daubs of Liquid Nails Construction Adhesive were used. Due to the height of the kiln stand, a ½” shelf was placed under the 28x32 kiln burner assembly to get a ½” space from the burner to the kiln bottom. The shelf was glued to the floor. Liquid Nails is very hardy (and easy to use) yet can be cut apart with a razor knife if it needed for repositioning, etc.

Drill burner holes in kiln bottom and position on stand.

Photo 1 shows the placement of the first shelf and the groove cut in the soft bricks on the 28x32. Start by marking and sawing on the bottom kiln section first. To establish and mark the groove position, a ½” wide 20” wood piece was placed on the top of the bottom kiln section (or use a ruler) and squared up by getting a constant C dimension. The actual flue shelf/wall can also be placed on the kiln wall to mark the groove. Note that the shelf may be recessed slightly into the groove.

Two lines are needed, inside the flue and outside. Measure from inside and outside of the wood piece, or the actual shelf, to the center kiln brick corners – ends of dimension D (inside kiln). Mark the lines on the bottom kiln section bricks and using a stiff saw, eyeball a square cut. The shelf width was 20 inches and the width of the grove was lightly wider.

Carefully stack and align/secure another kiln section. Measure, mark and cut, and repeat until all sections are cut. Afterwards, use a piece of coarse sandpaper attached to the edge of a board to smooth out the cut (Photo 20).

Photos 2, 3, and 4 shows the placement of the flue wall shelf pieces. Using twisted or straight pieces of Nichrome kiln wire about 1.5” long and a pair of needle-nose pliers, push the wire against the flue wall and into the brick, with approximately 3/8” sticking out and holding the flue in place.

I then jammed some ceramic fiber into the space on the side of the wall, first wetting it in a soupy refractory cement. The ceramic fiber helps holds the wall in place and seals it. After the bottom flue wall is in place, the top wall can be held in place and marked for cutting. Photo 5 shows the top wall piece, which was about 10” tall. The 10” cut off piece was then used as the burner wall (Photos 11 & 15). A standard cheap tile saw was used to easily cut the shelf.

Install the top flue wall the same as the bottom. A very small amount of cement was placed between the wall pieces and a small amount of fiber was used as the cut was not perfect (Photos 6 & 7).

Photos 9 and 10 show the placement of the bottom 2” shelf posts and also soft brick pieces cut to 2” tall that are used as baffles. The baffles make the exiting gases return from the burner side of the kiln and helps produce more even temperatures over the bottom shelf. The un-baffles areas were approximately 8” to 9” wide so as not to constrict the exit gas flow, providing around 16 square inches per side.

Photo 11 shows the 26” diameter by 3/4” shelf cut to fit the kiln. The cuts were based on the shelf being slight off center towards the flue as shown so as to have the gases exit through the unbaffled areas under the shelf. As shown, the space along the edge of the shelf provided an area of approximately 16 square inches per side for exiting gases. Cuts to the shelf were made with the tile saw.

Photo 11 also shows a soft brick baffle between the burners measuring 1” wide and around 5 inches tall. This keeps the 10” burner wall shelf piece from inadvertently leaning into the burners.

Photos 12 and 13 show the exit hole though the top. Note that it is to the side – this is the strongest place to put it but care must still be taken when opening the kiln lid to not overstress the bricks around the opening. The cut is carefully made with a flat saw and marked by measuring the flue on the top kiln section. Since the 28x32 was originally an updraft kiln, the round center exit hole in the lid was filled with cemented and pinned soft brick.

Photos 26 to 32 show the 23x27 electric to gas conversion. Missing from Photo 31 were soft brick baffles placed similar to the 28x32 to control exiting gases.

Firing Schedule
The attempted/ideal firing schedules were based on information from Val Cushing, John Britt, and Walford Campbell, and were tweaked for the two kilns. Initial issues encountered were too fast a warm-up and very fast firings. The too fast warm-up once caused a top cone pack to explode. There did not appear to be any adverse effects from the fast firings, however, Δ10 reduction firing in 4 hours just seemed too short a time. For both kilns, a firing time of approximately 7 hours appeared reasonable and included a 30 to 45 minute initial warm-up.

Noted about the firings is that the adjustable burner plate on the MR-750 burners was kept at around 5/8 to 3/4 of an inch throughout the firing. Reduction was made solely by restricting the exit flue, as shown in Photo 25. Soft bricks were used for dampening and in the following schedules the flue dampener opening is the distance between the bricks. It was found with both kilns that the flue area was larger than it needed to be. Both kilns will fire cleanly in oxidation dampened to around 9 inches. For the 28x32, this means that the flue has an excess area of approximately 9 square inches, equivalent to moving the flue wall around ½ inch, adding ½ inch to the kiln area. However, changing the flue dimensions may affect the firing – it would require experimentation.

As mentioned, the regulator in our conversions connects to a 12’ hose with a ¼ inside diameter. Varying the length or diameter will change the pressure verses flow characteristics and so the pressures will have to be found by experimenting. The valves on the burners are turn full on during the firing and gas flow is controlled by pressure only. When pressures are adjusted, tapping a fingernail lightly on the gauge is required to get an accurate reading.

The pilot light system is simple and is made from ¼” copper tubing, crimped at the top and drilled with a 1/16” hole. It works to light the burners. The 28x32 is warmed up with one burner and the pilot light was blown out and had to be relit to start the second burner. The pilot gas is shut off once the burners are running. We do not leave the kilns unattended during firing and no thus Baso safety system was employed.

Reduction commences at 900C and the kiln is stalled for 45 to 60 minutes with a constant temperature target of around 925C and not to exceed 950C. The gas pressures and flue dampener openings may have to be tweaked for your kiln and very small dampener changes can stall or increase kiln temps. A balance between gas and dampening will be found. No black smoke or soot is made during reduction. The appearance of soot shows over-reduction settings. Soot is carbon – reduction happens with unburnt fuel and carbon monoxide gas.

After the heavy reduction, the flue dampener is opened up and the temperature is allowed to increase quickly up to about 100C lower than the Δ10 temperature. The dampener settings listed following provide for a continued light to moderate reduction during this climb. Nearing the end of the firing, cones and color are used to affirm that the temperatures are relatively even from the top to bottom of the kiln. The top has always been slightly hotter than the bottom and ware is placed accordingly.

Photos 35 and 36 show some kiln packing. In Photo 35, the shelves were staggered vertically and placed toward the sidewalls causing the flame to zig-zag down though the shelves making for even temperatures. Photo 36 had taller bottles and pieces that made for a more abstract placing, but also had good firing results. One must try to visualize the flame going vertically to the top and flowing over the ware and through the shelves. For example, a shelf placed against the side wall and over a shelf also placed against the side wall will create a ‘dead’ spot near the wall that could experience lower temperatures. For the 28x32 kiln, 24” half round shelves were used with approximately 1” cut off the sides for fit – as shown in Photo 38.

Photos 22 and 33 show the propane bottles used. Bottles are placed in a tub and a small trickle of water is sufficient to keep the gas from freezing up causing pressure/flow loss.

Photos 23, 24 and 34 show the stove pipes used to exit gases from the building. The pyramid-shaped hood directly above the kiln was made from 24” wide sheet metal roofing panels and pop riveted together. 6-inch steel stove pipe was used to connect to outside the building.

Photo 37 was included as it shows a small shelf placed to block wind gusts during a windy firing day.

The following schedules are based on firings and can set up guidelines as a starting point. The primary objectives are an initial slow to moderate warm up, then fast climb to 900C, 45 to 60 minutes in heavy reduction, moderate reduction to 1182C, 60C per hour climb to 1282C with possible requirement to even out temps by dampening. The hotter the kiln gets above 950C, the slower the increase per hour if nothing is adjusted. Small changes in gas pressure and dampener opening size can have large effects in temperature rise or stall. Keep a log to learn the kiln.

At the end of the firing, the flue is completely closed off and a thin soft brick piece is placed over the burner providing somewhat of a block for the burner inlet. 

For questions or your comments/improvements, I can be contacted at borissquare@gmail.com


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