Potters & Sculptors - Making Rock from Mud
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.