That is the question (though methinks I have misquoted). This is a weighty question; a question whose answer can lead to easy breathing, or health issues down the line; a question that can affect glazes and kiln hardware. And, it’s a question, I admit, I have struggled with over the past couple years. In theory, I am resoundingly in favor of venting my kilns, primarily for safety reasons. When clay and glazes are fired, toxic fumes are produced, including carbon monoxide, sulfur compounds and metal vapors. And if the kiln is located in a living or working area, the unmitigated fumes can pose health risks to residents.
Once I had the basic design down, it was off shopping! I marched straight to the HVAC section of the home improvement store, and stood in front of the shelves pondering for quite some time. I knew that I would not be able to find the exact replica components of the commercial vents, since it looked from the pictures that at least the bypass cup was custom made. So, it was up to me to decide what sections of steel HVAC piping would be a good alternatives. After much hemming and hawing, I decided on the following materials:
Moving on, I grabbed an 8-foot length of flexible dryer 4-inch diameter tubing for the vent ducting.
And the grand total (drum roll, please) is about $64, with a savings of $300 to $400 off a commercial vent! Well worth it, I’d say!
- Electric Drill
- 1/4-inch Drill Bit
- Duct Tape
- Safety Glasses
- Dust Mask
- 4-inch Duct Elbow
- 4-inch by 8-foot dryer vent ducting
- Blower fan (about 140 CFM capacity)
- 2) 4-inch ring clamps
- Duct Connector (optional)
- Duct Reducer (optional)
Finally, I thought it might be easier to install the vent if I had unrestricted access to the bottom of the kiln. Therefore, I disassembled the kiln. This is not necessary, bit it did make things a bit simpler.
Small holes are drilled in the lid of the kiln to allow air to be drawn from the room to replace the air and fumes removed from the kiln. The Skutt and Orton vent manuals base the number and position of the lid holes on the volume of the kiln. However, the L&L manual states that since there are small, natural gaps in the kiln walls (around the peeps, lid, element holes, etc.) additional lid holes are not absolutely needed to allow for proper air flow. In addition to peep holes, I have small holes drilled in each level of the kiln for pyrometers. There is also a ½” diameter hole in the center of the lid from the last kiln vent, which I close with a peep plug. Since I have an L&L kiln, I decided not to drill any further holes in the lid of my kiln.
The bypass boxes for the commercial vents have holes in the sides. These holes serve to draw outside air into the bypass box, modifying the amount of “pull” the system has on the kiln gases. The bypass box also allows the cooler outside air to mix with the hot air from the kiln so that the high temperature of the kiln gases does not damage the vent fan.
Make sure to check the air flow direction of your fan – the air flow should be moving away from the ducting. On the model of fan I bought, the end of the fan did not have an easy way to connect to the ducting. Therefore, I attached a duct connector to the fan, and sealed the junction with duct tape.
Since I purchased a 6-inch blower fan, and the rest of my system is 4-inch in diameter, I attached a 6-inch to 4-inch Duct Reducer to the duct connector on the inline fan. Of course, this step is not necessary if you purchase a 4-inch fan. Unfortunately, these were not available when I did my shopping.
♦ Using the other 4-inch Ring Clamp, attach the other end of Flexible Dryer Vent Duct to the prepped inline blower fan.
Speaking from experience, the foil of the flexible ducting can be easily ripped by protruding metal when connecting it to the fan and elbow, so a bit of care is needed.
♦ Support the bottom of the bypass box so that the bypass box is pressed against the bottom of the kiln.
And here is where I ran into some problems. The Orton and Skutt vents have a stand for the bypass cup that keeps the cup pressed against the bottom of the kiln without actually attaching it to the kiln floor. I thought that this would be the way to go, and shouldn’t be too difficult to assemble.
Ha! Famous last words! First, I attempted a stand like the Orton and Skutt models by using a lag bolt, nut and washer. Since the duct elbow I used for the bypass box is curved at the “bottom”, this method did not work. I next tried to construct a stand out of sheet metal, but again ran into difficulties due to the curve of the elbow.
And I was right. I attached little brackets of scrap sheet metal to my bypass box, and then attempted to connect the other ends of the brackets to the kiln floor. After some effort, I realized that this was not going to work at all. The kiln brick was just too soft and all the screws easily pulled out with the slightest bit of torque. I ended up with a bunch of screw holes in the floor of my kiln which now require patching.
♦ Install the vent cap on the outside wall over the hole.
♦ Slip the fan end of the ducting through the hole and secure if needed. Insulate around the vent.
I was able to skip this step. Since my porch is screened, I just placed the end of the ducting against the window. Done and done.
Once my kiln vent was assembled and at long last attached to the kiln, my first question was, of course, “Does it work?” The commercial manuals say that you can test the venting properties of your kiln by lighting a match and holding it over one of the holes in the lid of your kiln while the kiln vent is running. If the vent is working, the flame should be drawn into the hole.
Now, this was probably not the best idea, but I decided to smell the exhaust from the vent while the kiln was running to determine if the vent was working. And, low and behold, the exhaust smelled strongly of kiln fumes. Success!