TDI Downdraft Kiln Conversion Further Study

This Further Study post delves into some helpful tips and information gleaned from comments and questions within the Facebook TDI Group posts, conversations, and emails. The kiln build has evolved over the past 6 years with great input and experimentation of several serious potters. The evolution has resulted in more even firing temperatures and knowledge of the firing parameters and conditions. The overall inlet/flue design and dimensions have not changed. 

Build Features and Improvements

Flame Deflection Block: 

This is a big and important addition researched by Lyle Nicholson and Matias Azocar and has resulted in smaller top to bottom temperatures differences. The current deflector block I am using is cut from the end of a softbrick and the part that sticks out into the kiln from the wall and measures 2.5” high. It now has a bit cut off of each corner, as shown, as the middle area in the kiln was a solid cone higher and now I usually get around 1/2 cone difference, sometimes with spots at 1 cone different. It is cemented in place and I use a wire pin through into the kiln wall just in case. 

The original block and location. 

Now with a bit trimmed off.

Note: For some reason a few people think that having 1 cone difference top to bottom is unacceptable. From conversations with others and my limited large-kiln experience, it appears that most gas kilns have hot and cold spots and getting around a 1 cone difference is common. Potters pack the kilns remembering the various temperature spots with glazes that match. Reduction also varies around the kiln, again with glazes located accordingly. 

Peep Hole: 

The peep holes on most electric kilns are small and it is sometimes difficult to see the cones in the TDI kilns. A fix is to enlarge the hole and make a large peep plug out of softbrick. I only rounded out the middle peep hole and then made the plug using a hole saw, although it can be made by sanding it round. L&L Kilns uses a 1” diameter round peep plug that costs about $14 to $16 and is a good alternative. 

Burner Wall Height: 

The current burner wall height is around 5 inches, which provides enough separation of the down-flowing gases yet works with the deflector block. 

Rounded Flue Exit Holes in the Lid: 

To relieve stress in the lid, a rounded edge flue exit hole is recommended. The edges follow the same relative dimensions as detailed for the original cut-outs. It can be done with either a hole saw or shaped with sandpaper around a dowel rod. 

Propane Pressure Gauge: 

Initially the gauge was a 0-15 PSI (Pounds per square inch) and now we use a 0-3 PSI gauge that makes it so much easier to tweak small and repeatable adjustments. 

Note that the reason the gauge is at the tank and not the burner is that there is flow drag inside the 10 or 12 foot long supply pipe that creates a higher pressure reading at the tank verses at the burners. The higher reading makes for an easier and more accurate pressure reading given the quality (low cost) of the 0-3 PSI pressure gauges we are using. 


The orifice in the MR-750 MUST be changed to a #50. The MR-750 usually comes with a #38. A #50 is smaller than the #38, so if a #38 is used way too much propane will be dumped into the kiln causing it to stall out before reaching the higher temperatures. There is a balance between the amount of propane and air that results in a proper or complete burning of the propane gas. 

How Fast Can I Fire the TDI Kiln: 

Because of the relatively thin walls of the electric kilns as compared to larger store-bought gas kilns, there is a proportionally higher heat loss through the walls at higher temperatures and it thus requires a powerful burner. Combine this factor with the smaller size and smaller amount of ware in the kiln and one is able to have a very fast initial heating and ramp up.

After candling, the initial ramp up to 900C/1652F usually takes about 2 hours. Body reduction then takes about 45 minutes. The climb to around cone 7, 1235C/2255F, takes around 3 hours. Then there is the last hour of 60C/140F per hour climb to cone 10. 

So, the total firing time to cone 10 is about 7 hours including about a 15 to 30 minute candling/warming while setting up the temp meter, water bucket for the propane tank, chair, snacks, and a book. Lower cone reduction firings will obviously be somewhat less, but the same basic procedure. 

Keeping a Firing Log: 

I suppose I’ve already stated this several times, but without an accurate log of the firing numbers and conditions (PSI, dampening, & degree/hour change), it is difficult to establish baseline numbers that will make the firing outcomes consistent and repeatable. Firing is both science and art. The science is the knowledge and understanding of the baseline numbers. The art is the packing, visualization of the flame path, knowing the hot/cool/reduction areas, and the subtle tweaks during firing that produce the desired results. 


Because the kiln can be heated to 900C/1652F in about 2 hours, many have switched to using self-supporting cones rather than cone packs. I personally had several cone packs explode even after long drying times and low humidity conditions. I also use cone holders while using up the regular cones I already have. Self-supporting cones are easy to place around the kiln as witness cones. 

Temperature, a Pyrometer, and Cones:

The placement of the pyrometer probe is usually in the middle section of the kiln and closer to the flue wall than where the burner gases are rising. It should not be near the flame for obvious reasons. Note that when packing ware it is very important to not have items to close to or blocking the probe otherwise the reading may not be accurate. The pyrometer reading can be used for most of the firing and is accurate enough for the body reduction phase and for calculating degrees per hour. For the end of the firing, I think the cones should be relied upon. After a few firings, you may notice the temperature is within a few degrees of the same reading when the cone shows you’re finished. For example, the self-supporting cone 10 chart shows 1285C and I consistently get 1273C to 1278C on the pyrometer, so I know I’m close but still use the cone as the primary indication. 

Changing the Design - Experimenting

This depends on one’s priority. If your priority is to be able to fire your gas-reduction pottery and that is most important, follow the TDI design as detailed and learn how to fire the kiln. It is not complicated and often there are people you may know who can help with pipe fittings, measuring, cutting shelves, and setting it all up. The stock TDI conversion per the manual works very well and produces great repeatable results once the nuances of the kiln are determined and recorded. 

If you are a person who enjoys building and experimenting, and I am one of those people, then experiment away. If you come up with an evolved better firing kiln then build and fire it and you can decide if you want to share the details and explain how to build and fire your design. There have been a few FB posts with nice looking side entry burners designs but it doesn’t help people to suggest something may be better than the stock TDI design and not provide specific details, dimensions, and firing procedures. 

By placing the kiln stand legs on bricks, one can use the MR-750 burner from below with a nice vertical flame so I am still unclear of the advantage of side entry except in countries where the MR-750 is unavailable. 

Over the last couple of years I have been amused by FB posts that simply and authoritatively state that the TDI kiln won’t work, sometimes with a reason why, without actually building one. Yet I dislike and find it misleading for people to post text that says the TDI kiln doesn’t work when significant changes have been made to the design. These have included smaller flue areas that have led to stalling, use of weed burners, side entry burners, not using a #50 orifice, and too small burner inlet opening. This doesn’t help the purpose of the FB group, which is to support those building a stock TDI conversion and those who have experimented, discovered, and shared improvements with design details. For example, a weed burner produces a wonderful turbulent flame and numerous people have suggested them as a cheap alternative but what about sharing the size of the inlet hole verses a specific make/model, primary verses secondary air, changes to the flue area, firing details and temps, etc. This might be helpful.

Climbing off the soapbox now. 

Oval Kiln Design:

Oval kilns provide a unique conversion and the potential for more even top to bottom temperatures with larger ware areas. Shown following are photos from Michael Buckley for a 16 cubic foot oval kiln conversion using two MR-750 burners with the flue area of about 30 square inches. Oval kilns typically range from about 10 to 16 cubic feet in interior size. This compares to around 7 cubic feet for a 23” x 27” kiln and 10 cubic feet for a 28” x 27” kiln. 

A single MR-750 burner will provide enough power for a 7 cubic foot kiln with about 25% additional BTU power available without changing the inlet hole size or PSI gauge, equating to a maximum single-burner kiln size of around 8.5 to 9 cubic feet. The two MR-750 burners used in the 10 cubic foot 28”x27” kiln conversion have a considerable excess of power and can likely work for kilns up to around 16 to 18 cubic feet. 

The guiding dimensional factors are the burner-inlet size and the relative flue area cross section size. As mentioned in the conversion manual, the the flue sizes were made larger than the calculations showed as a safety factor and to account for parasitic drag in the flue. The flue area for the original two-burner conversion was 35 square inches and I think 30 square inches would be the smallest I’d make. The difference in 30 verses 35 square inches accounts for less than 1/2” of flue wall displacement and loss of ware area.  

So in conclusion, the burners and inlet construction should follow the dimensions as stated for the 28” kiln conversion and placed on one end of the kiln, the flue is placed at the opposite end. The flue wall size will have to be calculated so as to have around 35 square inches of area. Search the internet for “area of a triangle” for how to calculate the area off of your measurements – a photo following shows how they divided the triangle areas to make the calculation.  

As of the December 2023 revision, I have not personally converted an oval kiln. Some friends had an oval planned for 2021 but had to put it on hold. The kiln we were going to convert was an old Blue Diamond with an interior width of 25” by about 41” long and 29” tall. With about 14.5 cubic feet, I decided that two MR-750 burners would be sufficient given that they have more than enough power for a 10 cubic foot 28” conversion. We went with using 18” by 18” square kiln shelves for the flue wall, recessed into the kiln wall. The flue cross-section area was about 37 square inches. The layout of the burners and flue was to be similar to the Buckley oval discussed, with the burners on one side and the flue at the opposite side.

The larger ovals have sizes that range from 10 to about 19 cubic feet. If I were tasked to convert one of these larger kilns, I would still plan on using two burners as I think they most likely have enough power to reach Cone 10. It might take a bit longer during the initial climb phase and sequence from body reduction to Cone 7, but I think it’d work. Firing propane pressures for the two-burner 28” conversion generally range from 1.5 to at most 2.5 PSI. The MR-750 burner will still work well at 2.5 to 4 PSI and should provide sufficient power.

12/29/2023 – Perry Brown provided the FB group some good information from his 16 cubic foot oval conversion. He is using natural gas at 7” WC pressure and MR100 burners. The burners put out 90,000 BTU’s at 7” WC, so 180,000 BTU’s total. He was able to get to 1200C before it stopped climbing, only 85C left to get to cone 10. He added a third burner and was easily able to get to cone 10. This showed that the issue was not stalling due to draft, but not enough heat energy. Perry’s conversion had a flue cross section area of about 37 square inches.

The takeaway is that for 16 cubic foot oval conversion, more than 190k BTU’s will be required. For a propane conversion using MR-750’s, it will probably require at least 4 PSI, putting out around 120k BTU’s total, or maybe slightly more for safe measure. This is in the good burning range of the MR-750, which is rated up to about 10 PSI before things go awry. It may also be necessary to use a 0-5 PSI gauge rather than the 0-3 PSI.

Cheers and good luck. 

No comments:

Post a Comment