TDI Downdraft Kiln Conversion

TDI Downdraft Kiln Conversion 2023 Revision

The November 2023 book revision presents new and revised text and delves into refinements, tips, and information largely gleaned from comments and questions within the Facebook TDI Downdraft Kiln Conversion group posts 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 from the original 2018 blogpost. 

The Firing the Kiln section was largely revised and was included and updated to the Firing the TDI Conversion blogpost, dated April 2022. The October Further Study 2023 blogpost includes the Deflection Block and other items and was also updated for the revised book text. 

The Amazon link to the book is the same: https://www.amazon.com/dp/B084DH88GH

The original blogpost: 2018/07/gas-kiln-conversion-downdraft

Firing the TDI Conversion blogpost: 2022/04/firing-tdi-downdraft-kiln-conversion

Further Study 2023 blogpost: 2023/10/tdi-downdraft-kiln-conversion-further

The purpose of the book is to explain the conversion of a typical 23” and 28”- wide electric kiln to a TDI propane fueled downdraft kiln. The TDI kiln came from me wanting to continue high-fire pottery after the local school had equipment issues and is the result of research and various prototypes made back in 2016. The original TDI kiln firings were good and I was asked to share the design dimensions and build. The idea for writing this book is to provide much more detail of the conversion and firing processes and help promote at-home or small studio reduction firings that can produce very reliable and repeatable results.

My advice:

  • Plan a stock build by following the book. The TDI kiln works very well and the design has been proven by many happy potters.
  • This will require some effort. The more you put into understanding gas reduction firing and the TDI kiln conversion process, the easier the conversion and the more successful your firings will be.
  • Before beginning the conversion, read all of the book and try to visualize and understand the building process. Before firing, read the firing sections to completely understand the firing process. Every detail has a reason behind it.
  • Plan the initial firings (and every firing) with a list of gas pressures, flue dampening sizes, and temperature target points. Try and visualize the gas flow path through the ware, packing accordingly. Use a firing log to document glaze, kiln, and firing conditions. Be patient. After the firing, analyze the pressure/dampening/temp-change-per-hour numbers and choose numbers for the next firing. Eventually, you’ll have confidence and a set of pressure/dampening numbers that makes firing outcomes relatively repeatable.

What is not in this book and has to be learned elsewhere is a basic knowledge of the reduction high-fire process. Britt says in his The Complete Guide to High-Fire Glazes, “Without a proper understanding of kilns and firing principles, successful firings are merely random occurrences”. I recommend the “Kilns, Firing and Safety” chapter in this book. Search the internet for “reduction firing”. A good firing procedure resource was from Val Cushing. Search for: Val Cushing High Fire Process pdf. Another one that explains terminology, energy parameters, and the building process is the Kiln Book by Frederick Olsen, which can be a reference for converting different sized kilns or for making design changes.

TDI Kiln Overview

The following provides a quick summation of important items in the build and firing process and sometimes why certain things are done versus other possibilities.

Kiln Size for Conversion

The manual details two kiln interior sizes: a 23” width by 27” tall, and a 28” width by 27” tall. These are approximate dimensions since different kiln manufacturers have slightly different interior sizes. The two recommended burners will handle larger and smaller sized kilns and there is a discussion of possible larger sizes depending on using one or two burners. The inlet and flue cross-section sizes are set constants based on either using one or two of the burners discussed.


Both the MR-750 and the LSMIITTH cast iron propane forge burner have been used and work. These are not expensive relative to the overall cost of having one’s own gas kiln. For alternative burners, provided are specific references to the amount of heat (BTU’s) verses propane pressures during the typical firing so as to help in assessing other burner alternatives and the potential amount of heat required.


The hole in the burner that the propane gas flows through is termed the orifice size. The TDI build is based on a #50 orifice, also called a #50 drill size or 0.07 inches. The MR-750 comes with a #38 orifice (larger hole size) and is removable and MUST be changed to a #50. The LSMIITTH burner has a fixed smaller #70 orifice hole that can’t be changed. It must be drilled to a #50 sized hole – a #50 drill bit can be purchased online. Yes, smaller drill number – larger hole size.

Pressure Gauge/Regulator

The TDI firing process references pressure in Pounds per Square Inch, or PSI. A 0 to 3 PSI pressure gauge is used and is mounted to the pressure regulator attached to propane tank. There is a hose of about 8 feet attaching the pressure regulator to the burner. This gauge location and the long hose makes it easier to adjust the gas pressure during firing and provides a finer adjustability at low pressures. The 0 to 30 PSI regulator shown in this manual provides very acceptable fine adjustments and is not too expensive.

3.5” Burner Inlet Hole in Kiln Base

For an efficient and complete combustion of the propane, there is a specific amount of air required. The air going through the burner itself is called primary air, and the air going around the top end of the burner and through the inlet hole is called secondary air. This must be in balance and is based on the orifice size and the pressures used for the TDI firings.

The burner is placed below the kiln and has a vertical flame path up the wall. This may mean that the kiln stand needs to be set on small bricks for the 9” long MR-750 to fit underneath. A vertical flame path is very efficient and helps maximize the pottery ware area. Side entry with deflector blocks was considered but the loss of kiln setting area combined with the loss of heat radiated through the deflector blocks made this a less desirable option for such small kilns. Noted is that the LSMIITTH burner is only about 7 inches tall after assembled with a 90 degree elbow.

Flue Cross Section Sizes

The TDI flue dimensions are detailed and minimum flue cross section areas are explained. These dimensions were calculated and based on the BTU’s generated by the burners so as to obtain both reduction and also Cone 10 temperatures. Making them larger might just waste a bit of ware setting space and making them significantly smaller might cause the kiln to stall and not reach your desired temperature.

Propane Tank Sizes

I would recommend using two 30lb propane tanks for most firings. A single-burner TDI kiln will use about 20 pounds of propane and having a second tank is just a precaution. Use one for firing, and then the second one for the next firing knowing you have a 10 pound reserve should something go amiss. The two-burner TDI kiln may consume around 30 to 35 pounds of propane so both tanks will be used in the firing. I find that 30lb tanks aren’t too heavy to move around and put in the back seat of the car, securely seat-belted, to take for refills. An empty 30lb tank weighs about 25 pounds, and about 55 pounds full.

Pilot Lights

The basic burner set up uses a very simple pilot light and, after several years of firing the TDI kilns, I’m not sure it is really needed. The original 1/4” tube with a hole in it is a very inefficient pilot setup and a better alternative is presented in the manual. For the first 2 to 3 hours, the kiln should not be left unattended in case the wind blows the flame out or there is a regulator failure, although this is not very likely and I haven’t experienced it. Once the interior is red hot, or around 900C/1652F, the propane will ignite from the kiln interior and a pilot is not needed. This is then the time after that I might feel comfortable leaving the kiln for a short bit of respite. However, without a BASO type safety system, the kiln should not be fired unattended.

Firing Guidelines

There is a long section on firing the kiln and doing the initial test firings to learn the attributes of your TDI kiln. Having a plan and approaching the firing analytically will speed up the learning process for getting repeatable results. Even if you are an experienced potter and gas firer, one still has to learn the PSI/dampening setting numbers to achieve anticipated results.

The firing temperature numbers and schedule that I use and refer to are largely from the writings of Val Cushing and what was taught to me by Walford Campbell. The clay and glazes I use are for Cone 10 firings. Others may use or teach different schedules and, like any gas kiln, other firing temperatures can be used for different clays and glazes.

Even Top to Bottom Kiln Temperatures

At best, kiln temperatures within gas kilns can vary around the kiln 1 cone or more and the expectation of having completely even temperatures is not realistic. A temperature evening-out process is described in the firing section that may be recognized by potters experienced with firing updraft kilns. Kiln packing also has a significant effect on hot and cold spots and just has to be learned and is also described in a section following. Know that many people are getting relatively even temps in their TDI’s with hot and cold areas anticipated. Rather than blaming the kiln, record and analyze the ware placement and the evening-out process during firing. Take photos of the packing to help you remember. If you’re still getting poor results, maybe pose a question on the TDI Facebook group.

Shade for Seeing the Cones

Available on Amazon for looking into the peep hole to see the cones - Forney 57005 Lens Replacement Hardened Glass, 4-1/4-Inch-by-2-Inch, Shade-5 Green. I like Shade 5 as it isn’t too dark.

Alternative Sized/Shaped Kilns

The only advice I can give regarding different sized kilns is based on my experiences with the two stock TDI conversion designs and information from potters who have done oval TDI conversions. Two important parameters are burner power relative to the cubic foot size and heat loss through the walls.

A single MR-750 burner will provide enough power for a 7 cubic foot kiln with about 30% additional BTU power available without changing the inlet hole size or PSI gauge, equating to a maximum single-burner kiln size of around 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.

This is a good spot to introduce the term BTU, which is British Thermal Unit and how heat energy is measured per hour. One gallon of propane produces approximately 91,500 BTU’s per hour. A 30 pound tank holds 7 gallons of propane.

The MR-750 or LSMIITTH burner with a #50 orifice puts out the approximate BTU’s listed below. For the 23” kiln TDI conversion, the typical firing pressures range from approximately 1 to 2.5 PSI (pounds per square inch), indicating 55,700 to 88,100 BTU’s. The 28” kiln has similar pressures but usually not much over 2 PSI.

The guiding dimensional factors in the conversions are the burner-inlet size and the relative flue cross section area. The burner inlets are 3.5” in diameter. The flue cross section areas were made larger than my calculations showed as a safety factor and to account for parasitic drag in the flue. The approximate flue cross section area for the TDI single-burner is 22 square inches and for the two-burner conversion it is 35 square inches. These areas work for the stock TDI’s but may not work if the kiln size and/or BTU’s requirements are significantly increased. That’s part of the experiment.

The other consideration is heat loss through the walls for larger kilns, including oval shaped kilns. L&L Kilns did a BTU analysis for their Easy-Fire and Jupiter kilns and it is available on their website. For their 23”x27” kiln with 2.5” thick brick at 1285C/2350F, the heat loss was listed at 18,593 BTU’s per hour, or about 907 BTU’s per square foot of kiln surface area. For the 23”x27” kiln with 3” thick brick at the same cone 10 temperature, the heat loss was 15,439 BTU’s, and for the 28”x27” it was 20,204 BTU’s, or about 763 BTU’s per square foot.

So, this means that the 23”x27” TDI kiln with 2.5” brick, at near cone 10, and with a propane pressure of around 2.5 PSI, 18,593 BTU’s of the total 88,100 BTU’s being produced by the burner are going through the wall, and only about 69,500 is heating the pottery. Larger oval kilns have more surface area and thus even greater heat loss through the bricks.

Alternative Pilot Lights and Ignitors

The pilot shown in the original build is pretty simple and basic. There are many after-market or replacement pilot light assemblies that can be used. One that is easy to install is the MCAMPAS Pilot Burner and 750 Millivolt Thermopile Assembly available on amazon. Using a hacksaw it can be detached from the mounting plate and connected directly to a 1/4” copper tube. This one is less susceptible to blowing out in the wind. Another inexpensive item is a piezo-electric grill ignitor and MCAMPAS makes one with two leads called 2 Set Piezo Spark Ignition Kit. The ground lead is attached under a burner clamp. The photos following show how I attached them. I attached a screw to the piezo tab and then used a hose clamp to secure the screw to the pilot light. 

Alternative Burner

The MR-750 burners are usually easily available in the US but this isn’t always true of other countries. There are many burners available with Forge Burners being the most prolific and lowest cost option. Forge burners are typically designed to have a tight blue flame to be used in small foundry kilns/containers. For the small ceramic gas kiln, what is best is a larger more turbulent flame. 

A forge burner that appears available in most countries through Amazon or AliExpress is the cast iron LSMIITTH propane forge burner. The nice thing about this burner is that the burner pipe is threaded with 3/4” pipe threads allowing a 90 degree elbow to be used, which means it can easily be placed under the kiln. The venturi burner is rated at 100,000 BTU’s and has an adjustment for primary air. The orifice hole that comes with the burner is smaller, around #70, so it can drilled to a #50 (0.07”). It will require a pipe thread 3/4” 90 degree and around a 2 1/2” threaded 3/4” pipe nipple. Below is a product picture. 

After attaching the 90 elbow and the nipple, and drilling the orifice to #50, below is the burner I tested. The burner is 7” tall and so should fit under most kiln stands. I placed it about 1/2” under the kiln bottom. 

For the burner tests, I just used a large clamp to hold it in place. In the photo below, shown is a mount made of a U-bolt and an inexpensive strip of galvanized metal called a Strap Tie, drilled for the U-bolt. 

A note about drilling the LSMIITTH orifice. I suggest to unscrew the handle and remove the needle valve stem before drilling and then after make sure there are no metal bits inside. The needle valve has two lubricated O-rings and metal bits may stick requiring removal with a pin or the like. 

The MR-750 with a #50 orifice will provide around the same BTU’s at the listed pressures. Assuming a proper and complete combustion, it is the pressure and the orifice size that determines the BTU’s, so the LSMIITTH burner with a #50 orifice will do the about same. However, I think there may be some flow restriction in the needle valve part of the LSMIITTH burner and the pressures were a couple of tenths higher at higher temperatures. It may be prudent to get a 0-5 PSI gauge, just in case. Since the LSMIITTH burner is throwing the same amount of propane in the kiln, the inlet hole size and flue area needn’t change. 

For the 23” kiln TDI conversion, the typical firing pressures range from approximately 1 to 2.5 PSI, indicating 55,700 to 88,100 BTU’s. The 28” kiln has similar pressures but usually not much over 2 PSI. So an alternative burner selected should have at least the capabilities listed. Burners of significantly more power should be double checked to ensure that they burn properly at lower than intended BTU’s and/or gas pressures.

Noted is that the set up and adjustment to the burner plate (primary air) should be done in place under the kiln and with the lid closed. A burner tested to open air will have unlimited secondary air and the quality of the burn may not be indicative of the combustion confinement within the kiln. The plate should be adjusted so as to provide a more bluer flame with limited yellow. A mostly yellow flame usually indicates incomplete combustion needing more air. Incomplete combustion may not provide for sufficient heat and the kiln may stall at higher temperature, and the atmosphere may be in continuous strong reduction. I adjusted the LSMIITTH burner plate to about 1/2” and it seemed to work fine. 

Note: If you are going to use the LSMIITTH cast iron forge burner, do not use the 2.5” deflection block presented in a prior blogpost. The LSMIITTH forge burner flame is very tight, just over 1”, and not a nice blustery mess like the MR750. During a test firing and using a mirror, I could see that the whole flame was hitting the deflector block and shooting towards the first shelf, well below the middle of the kiln. I did not continue to work with the LSMIITTH burner past doing two test firings, but I think that a wide deflector block up near the top just below the lid would be better with maybe a slight bump block down low to give some turbulence to the flame. I would cement and pin both blocks in the same way previously discussed.

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