Build a Superior Kiln Vent For Less Than $50

What: Below are detailed plans, complete with photographs, that will enable you to build your own down-draft style vent system for your electric kiln. A number of commercial kiln vents are available for purchase, but they are outrageously expensive considering how simple the kiln-vent concept is.

Why: There are two great advantages to venting your electric kiln: First, noxious, corrosive and toxic gases produced during firing are vented outside. Secondly, down-draft style kiln vents counteract the tendency of the heat in the kiln to rise, thus evening out kiln temperatures.

Important: With some commercial systems, the blower motor is in contact with the underside of the kiln. This type of design has three major problems, and should be avoided. It places an electric motor and fan in close proximity to the potentially damaging high heat produced by the kiln. It makes the motor virtually inaccessible for replacement or servicing without completely disassembling the kiln. And, it can transmit motor vibrations to the kiln. These vibrations can cause ware to move during firing. I lost countless pieces due to vibration-induced ware movement when I used a commercial kiln vent that put the blower in contact with the kiln.

Overview: All commercial down-draft kiln vents work in the same manner. Room air is drawn into the kiln through small holes drilled near the top of the kiln. Kiln air is drawn out through small holes drilled in the center bottom of the kiln. The amount of heated air which exits the kiln is small, and will not significantly increase firing times.
This exiting air is mixed with room air in a "mixing," or "bypass" or "diffuser" box. This prevents superheated air from passing through the blower. Most of the air flowing through the 4" duct is room air.
The commercial systems place the blower inside. My blowers are outside in weatherproof housings. Each placement has its advantages. My plans can be easily modified so that the blower is inside.

If you can make a hole in a wall, you can make your own kiln vent.

Making a hole in your wall: Even if you buy a commercial kiln vent, you will still need to make a hole in your wall for the 4" flexible aluminum duct. This is probably the hardest part of making your kiln vent. If you can make a hole in your wall, you can make a kiln vent system. Be careful to avoid studs and wiring. If you can't locate your studs, find a friendly person who can. Drill a pilot hole, and then use a handheld jig saw. If you are going through a cinderblock wall, use a 1/4" masonry bit in a hammer drill to make a series of small holes in a circular pattern, and then knock the hole out with a hammer. If your kiln is in a basement, you may need to run your vent out a window. Remove a glass pane, and replace it with a board or Plexiglas that has a 4" diameter hole cut into it.

Do-It-Yourself Kiln Vent #1

This is my best Do-It-Yourself Kiln Vent design.

Materials: All materials should be available locally at a HVAC specialty supply house such as Johnstone Supply Co., http://corporate.johnstonesupply.com/ , or Granger. Most materials can also be found at Home Depot or Lowe's. The blower fan for Kiln Vent #1 will only be found at specialty houses. I found the 6" louvered vent used in Kiln Vent #1 at Lowe's but not at Home Depot.

Material List:
106CFM, 115V, AXIAL FAN UF12A12-BTH
X71-096 -- Johnstone Supply -- $22.04
6" louvered vent -- Lowe's -- $4.96

4" dia. x 8' flexible aluminum duct

optional cord for fan:
24" FAN CORD W/PLUG 07190-24
G31-199 -- Johnstone Supply -- [$4.29]

Update, 08/22/07: I've been using the little fan in the Material List for a long time in a ten-sided kiln with a 27" tall interior, and it works just fine. It is a little underpowered, and I actually prop the louvre vents open when I run the kiln, which helps a lot. A fan with a slightly higher CFM (cubic feet per minute) rating would be better, but the fan specified above is adequate.
The best cheap diffuser box is an un-painted, galvanized, "indoor" 6 x 6 x 6 electrical junction box. You will discard the lid.
Here is a link to some options for attaching the diffuser box to the bottom of the kiln:
Kiln Vent Diffuser Box, Attaching <<-- (click)

You don't want to actually screw or bolt the box to your kiln -- you need to allow for expansion and contraction of the kiln.
For the "room air" inlets, try putting a 1" diameter hole in all four sides of the box. You can always drill more holes later, or plug one or more holes if need be. It would really help if you could find someone's installed commercial system to see how the "diffuser box" functions and how to drill the inlet holes in your kiln.

It is really worth doing this yourself. You will save a few hundred dollars over a commercial system, with very little added labor on your part. Kiln vents are not rocket science. There is not a lot of engineering involved. It is very easy to adjust the amount of air movement by adding (by drilling) holes or subtracting (by duct-taping over or plugging) holes in the kiln and or diffuser box.