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.
- 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.
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.
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
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.
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
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
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.
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.
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.
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.