We acquired an old electric kiln at an estate sale. It had been sitting in the basement, waiting it's turn to be fixed up and played with (in addition to a second hand pottery wheel that was acquired along the way). When we last had an electrician out, we took the opportunity to have a 220V outlet installed by the kiln in the basement - so the only remaining item to get operational was to figure out a venting solution.

Unless you're going to fire a kiln outdoors, it needs to be vented. During the firing cycle, the kiln will emit all sorts of nasty gasses and fumes from the clay being fired, and this definitely does not need to be inhaled. Ever.

I did some research, and preferred the downdraft venting approach vs. a hood type venting system. Downdraft venting seems to work better, and is less bulky than having a large hood over the kiln. In addition, supposedly, a downdraft ventilation system also contributes to a more uniform firing inside the kiln.

Either system, if purchased, will set you back a few hundred dollars. Downdraft vents start at around $400.

So I decided to make my own. A fun project, straightforward concept, and pretty inexpensive vs. the retail versions.

Here is a (very) rough sketch of the downdraft venting system, listing the main components.

Step 1: Things You'll Need

Parts List

Galvanized metal sheet (I had some already, but you can get one from HD for about $10 in the duct/HVAC aisle)
Sheet metal screws
4" Duct Collar
6" Duct Fan (from hardware store)
6"- 4" Duct reducer (x2) (from hardware store)
4" Flexible semi-rigid Dryer Duct (from hardware store)
High heat Silicone (I already had some leftover from a smoker mod)
Duct hose clamps (I had some spares from last dryer install)
Dryer vent hood
Duct tape and small self-tapping sheet metal screws

Tools

Tin snips/metal cutting scissors

Sheet metal folding tool (probably could do without, but oh so much easier with)
Drill/drill bits
Screwdriver

Step 2: Technical Considerations

tldr:

The vent system will depend somewhat of the type and size of kiln you have. The larger the kiln, the more fumes to be extracted, and so you will need to make sure you have enough suction (cfm (cubic feet per minute) rating on the duct fan) and the appropriate amount/sizing/placement of the holes in the top and floor of the kiln. And adjustable dampers on the plenum/bypass box to fine tune the amount of outside air to draw into and mix with the hot kiln fumes.

I researched a few different commercial models, which seem to use either 3" or 2" duct hose. I wanted to use 'standard' sizes that were easily available, so opted for 4" (dryer vent) flexible duct. Because of the larger size duct, I wanted to make sure the duct fan would be powerful enough (measured by cfm) to pull the fume-air mix through the duct and maintain positive pressure to vent to the outside. I needed about 12' of duct to vent to the outside of my basement. I looked up the cfm needed for the commercial versions, and knew I probably needed one with similar power - around 150-200 cfm would work just fine.

Fast-forward a bit here. The venting system will be working correctly when it pulls just enough air through the kiln to remove the fumes and expel them to the outside via the ducting. Too much and it affects kiln performance and withdraws too much heat from the kiln. Not enough, and the kiln will leak fumes into the room. When properly set up, very little air/fumes are being drawn out of the kiln, and it won't affect firing times.

With the vent fan on, this is tested by holding a lit match (or lighter) close to a vent hole in the kiln lid - You want the flame to be gently pulled into the hole. Not too much so that the flame goes out, but not too weak a pull either. (Side Note: the Skutt envirovent tells you to drill small hole(s) in the kiln lid to draw in room air. The L&L version says a lid hole is unnecessary because there are enough cracks where air can be drawn in. I decided to test it first without a hole, and then add a hole(s) if necessary.)

The flow strength through the kiln is adjusted by opening/closing the dampers on the plenum/bypass box. The fan is pulling a constant, so basically the load is made up of the sum of room air + fumes coming into the plenum box. More room air (opening the dampers) means less coming through the kiln, and vice versa.

Step 3: Plenum/Bypass Box - the Prototype

It's really all about the bypass box or plenum cup, as the rest of it is simply flexible ducting and a fan - no harder than installing a dryer vent.

A plenum space in HVAC terms is simply an enclosure that mixes air from different sources - a junction box sort of thing. For this purpose, the plenum will mix gasses/fumes from the kiln (that are super hot) with room temp. air pulled in from the surroundings. This cooler mix is then sucked out via flexible dryer vent duct, incorporating an in-line duct fan, and vented to the outside.

The plenum/bypass box needs to have adjustable damper(s) that provide control over how much air the vent pulls from the kiln, and how much cool air is also pulled in, to mix with and cool the fumes coming from the kiln.

I hunted for something I could adapt and really couldn't think of anything that I could use, so decided that I would fabricate one out of metal sheet.

First though, I made a cardboard template - I played with the dimensions and this then served as the model for the metal sheet. The box needs to be large enough to accommodate the 4" duct flange opening, so 5" or 6" sides. Also make sure it's small enough to fit under the kiln. I have added notes to the pics to explain the process.

Step 4: Use Cardboard Template Outline to Cut the Metal Sheet

Next I used the cardboard template to copy the pattern onto the metal sheet. I also drew dashed lines to indicate where the metal would be folded to form the box. On two of the sides I allowed for a lip on each side. This overlap would be folded inwards and allow me to hold the sides together with a couple of sheet metal screws.

Step 5: Cut Out the Hole and Attach the Duct Collar

I drilled a small hole inside the circle as a starting point, and used the tin snips to cut out the collar hole. Check to make sure the duct colar fits snugly through the hole.

Step 6: Fabricate Custom Dampers on the Box Walls

The plenum box needs to have holes in the sides - this is where fresh air will be drawn in and mixed with the super-hot fumes from the kiln.

How many holes, and how big? I knew I would have to play with it to get the mixture just right, and so I went with three fairly large adjustable holes/dampers in the plenum box. The final calibration will depend on the holes in the kiln, the holes in the plenum, the width (I used 4") and length of the flexible duct, and of course the cfm (cubic feet/minute of airflow) rating of the in-line fan.

Adjustable dampers allow you to tweak the room-temperature air intake until it draws just the right amount of air into the plenum box. Once calibrated and operational, clearly mark exactly how large each damper opening should be - it won't change once you're up and running, and you can periodically re-calibrate as well.

To make the adjustable dampers:

Mark and cut out rectangular holes (1" x 3") in three sides of the box (the 4th side has the duct collar hole), and fold the long edges over 1/4" on each side. This will leave a 1/2 to 5/8" opening. Measure it so you know how big to make the sliding damper.
I used an old chisel to stamp out the cuts in the metal sheet - easy, effective, and precise. Chisel survived, too.
I used a metal folding tool, and highly recommend one for making this part very easy. If you want to use tools already on hand, you could use pliars or vise/clamp etc. Doesn't have to be pretty.
For the sliding damper: On a piece of excess metal sheet, fold one edge over 1/4". (I did a long piece and then cut the dampers to size.) Or you can make them individually.
Now measure and cut the other side, width needs to factor in the fold-over parts, and allowing about 1/8" for the folds/turns. Fold over the opposite edge, and you should have a damper that slides over the damper hole, held in place by the folded sides of the damper hole.I predict your first attempt will not fit either, but my second attempt was a perfect fit :)
The damper length needs to be a bit longer than the damper hole, plus about 3/4" that you can fold up into a handle, which helps slide it back and forth.The pics illustrate/explain this much better.
So you end up with an adjustable hole on three sides of the plenum box.
This should give you plenty of room to calibrate up/down until it's just right.

Step 7: Assemble the Plenum Box

Now we are ready to assemble the plenum box:

Fold the tabs - the side tabs fold inwards, the top/flange tabs outwards, all 90 degrees.
Attach the duct flange - insert from the inside and attach with screws.
Fold up each side wall, and secure thin sides together with screws through the inside tabs.
I used a wood block lined up along the fold lines to get nice crisp folds. The beauty here is that it doesn't have to be neat and tidy, or airtight. If air comes in around the seams, you will just compensate for it when calibrating the dampers.
Trim the sharp corners of the top flange (where the side lips came together), and then lay a fat bead of high-temperature silicone all around the top flange.
Set the box upside down onto some plastic, and allow silicone to cure. Once dry, peel off the plastic and you should have a nice sturdy silicone gasket that will sit against the kiln floor. And prevent damage to the soft/brittle firebrick.

Step 8: Putting It All Together

Time to attach all the elements and go for a test drive :)

Starting with the plenum box:

Plenum box duct collar => flex duct => reducer => duct fan => reducer => flex duct => vent hood/outside

Flex duct should be secured with hose clamps on both ends, and the reducers were attached to the fan with screws and tape.

In most cases you will need to cut a hole in a wall to vent to the outside, unless there is a window or other hole handy. This is basically the same procedure as installing a dryer vent. I had to go through concrete block in the basement, and cut the holes by drilling small holes around the circumference and then knocking the center out with a mini sledge hammer. Same from the outside. The vent cap went in from the outside, and was attached to the flex hose on the inside. Job done.

I made a 1/4" hole in the center of the kiln floor, twisting the drill bit with my fingers. It cuts through the delicate firebrick easily. After trying without, I made two 3/16" holes in the kiln lid - being careful to position the holes away from the kiln sitter probe and peepholes. You can refer to the commercial model guidelines (like the Skutt Envirovent) for holes/spacing on different size kilns.

To hold the plenum box flush up against the kiln floor, centered over the small hole, I used a simple lever/fulcrum (like a seesaw): a rod with weight on one end, other flat end of the rod pressing up against the bottom of the plenum box. The axis in the middle is a coffee can filled with sand, with a bit of floor molding on top. It's surprisingly stable. It needs to be a bit flexible - the kiln floor with expand/contract as the temperature changes, and you want the plenum box to move with it, or the seal won't be tight throughout the firing process.

With the plenum box snugly pushed against the kiln floor, I plugged in the inline vent fan. First realization was that I needed to hold open the louvres on the exterior vent hood - there is not enough airflow to push them open. Did this with some wire to hold them open.

Next realization was that three large dampers on the plenum box was massive overkill. All it needed was one, and a small one at that. Ended up keeping two of them closed, with the third open about 3/4"). The match test on a lid hole confirmed it was working and allowed for fine tuning. You light a match and hold the lit tip at the entrance of a hole - the flame should be gently pulled into the hole. In the photo, you can see the flame on the underside of the match being drawn into the hole. This confirms that the system is running under negative pressure, and fumes won't be escaping into the room. Success.

Hope this instructable was helpful - it was a fun and easy project. And it will save you a few hundred dollars vs. the commercial version :)