Do-It-Yourself Kiln Power Vent

When I put my electric kiln in the basement, I discovered that it put out enough fumes to give me a headache when firing. So a kiln vent was in order. Commercial kiln vents are quite expensive for what they are made of and what they do. The function of a kiln vent is to create a slight negative pressure inside the kiln, so hot gases are not escaping anywhere except through the vent ducting to the outdoors. My kiln like most electric kilns is a top loader, so I didn't want any extra hardware on the lid. The logical, most out-the-way place to collect the exhaust was from the floor of the kiln. I found a 12' x 8" x 4" heating boot at Home Depot with a 4" duct adaptor. This was to be my hot air collection box. I drilled 4 evenly spaced 1/4" holes through the floor of my kiln to fit within the open top of the ventilating boot which was attached to the kiln bottom with short, sheet metal screws.

To dissipate heat coming out of the vent, so it wouldn't damage the fan, I put an adjustable shutter on the front of the box so a volume of room temperature would be sucked in and mixed with the kiln exhaust air

Because of the inadvertant small openings in the body of the kiln (at the peepholes and around the lid, etc.) I drilled just a single 3/8" hole in the center of the lid to allow for air inflow into the kiln. My thinking was that I was moving so little air through the kiln that I didn't need much of a fan to keep air moving slowly through the kiln and out the exhaust. I bought a little duct fan for $24 and hooked it up. The little 4" duct fan I purchased didn't develop enough suction to reverse the bottom to-top-heat flow in my kiln, even with the shutter on the box totally closed. A smoking match held at the hole in the lid was not being drawn into the kiln, therefore I did not have negative pressure in the kiln, and exhaust gases could be escaping into the room. The solution for this problem was to spend a bit more money on a more powerful fan. I chose a Vortex 4" inline fan rated at 172 cfm. It was $125 online (a couple of years ago).

When I installed the fan in my vent piping, I discovered that it was easily able to maintain negative pressure in my kiln.

The vent comes out of the box with aluminum flex tubing, continues in rigid galvanized tubing, and finishes out through the wall in another length of aluminum flex.

It actually drew more air out of the kiln than I wanted, even with the shutter on the vent box under the kiln left wide open. I had to open a secondary inlet in the vent piping between the kiln and the fan to reduce the suction enough so the air flow through the kiln was slow enough to just barely draw smoke from an extinguished match downward into the lid vent hole when the kiln is at maximum temperature. This ensures that my room air is uncontaminated and that I am not sucking too much cool air into the kiln and thereby pulling off too much heat.

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Comment by Samantha Tartaro on December 1, 2015 at 11:20am

Thank you so much! I will definitely be doing this in the near future. :-)

Comment by George Lewter on November 30, 2015 at 7:39pm

Samantha, There are two reasons to vent your kiln. 1 is to eliminate noxious gases from your indoor environment. 2 is to assist in the the development of colors that are better in a highly oxidizing atmosphere. 

For fume control, you only want to pull enough air out the vent to be sure that the bottom vent hole(s) is the only place where gas is escaping the kiln -- all other cracks or openings in the kiln envelope should be drawing air into the kiln. It doesn't take a lot of air flow to achieve this if you don't have a lot of gaps and holes in your kiln. If you can easily climb in temperature at 150 or more degrees an hour at the top of a cone 6 firing, then the vent will not cause your kiln to fail to reach cone  6. Hot air being pulled from the kiln is mixed with a large excess of normal room temperature air right in the boot, immediately bringing the temperature down to just slightly warmer than the room temperature. The volume of hot air being pulled out is in the range of what you could suck through a small soda straw, and you are mixing that with something like 100 cubic ft per minute of cool air in the boot. Your hardware guy is mistaken. Commercial versions of these vents are available from the major kiln manufacturers if you are uncomfortable doing it yourself.

To achieve a constant strong oxidizing atmosphere you want to pull a little more fresh air through your kiln, This is achievable by drilling one or more small holes in the lid of your kiln. This also gives you a way to monitor the draw. Is the flame from a match being drawn down into the hole and how strongly? If this is happening then it is very unlikely that gases are escaping anywhere except at the vent.

Comment by Samantha Tartaro on November 30, 2015 at 9:41am

Hi! I'm a newb and I have a question about this process. I went to a hardware store to get everything to make this work for my space, and the person who helped me seemed to think that either the kiln wouldn't keep heat, or that the tube/boot might not be able to stand the heat of the air from the kiln. Are these things I need to be worried about? I have everything and now I'm just nervous to move forward. any advice will be much appreciated :-)

Comment by KimberLee Torkko on November 4, 2012 at 5:12am

Thanks, George.  

Comment by George Lewter on November 3, 2012 at 8:55pm

KimberLee, the pictures you are referring were posted by member, Ted, who has left the network and deleted his pictures. We cannot recover them. I deleted my comment that moved them to this page.

Comment by KimberLee Torkko on November 3, 2012 at 12:49pm

I cannot see the pictures that were sent as a comment by George Lewter.

Comment by Robert Young on August 31, 2010 at 12:42am
Nice pics! The details you described make the entire process easy to understand.
Comment by JUDITH FREDERICK on August 30, 2010 at 10:44am
Great explanation. Thanks so much. I will have to work on one for myself.
Judy

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