To Vent or Not to Vent: A DIY Kiln Vent

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.

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Enter the kiln vent.  These mighty constructs of fans and tubing can remove a significant portion the firing fumes from the indoor environment by transporting the gases outside where they mix with fresh air and are diluted to safe levels.  I have also read that using a kiln vent can even out the temperature in the kiln a bit (every kiln has its cold spots), as well as increase the life of the kiln elements (presumably since the vent can remove gases harmful to the elements).  The atmosphere inside the kiln is also positively impacted, and can lead to better results while firing. For a basic primer on kiln venting, check out this Ceramics Arts Daily article.

Vent-A-Fume updraft kiln vent by Vent-A-Kiln Corporation.

Skutt EnviroVent 2 downdraft kiln vent.

So why would anyone not vent their kiln?  Well, here’s my story (excuse?).  I have always housed my kilns on a screened back porch, and I have found that the floor to ceiling screened windows provide more than enough ventilation for the kilns. I prop the kiln lid and open all the peeps during the beginning phase of my firing, and leave the top peep open throughout the entire firing.  The fumes are naturally shunted to the outdoors, and I have never found that they leak inside the house.  For added protection, while the kilns are running I do not spend significant amounts of time on the porch.

This cross ventilation method has seemed to work well for years, but it was really only a stopgap solution.  A kiln vent was always on my wish list of supplies to purchase once I found myself with some extra cash.  With the promise of increased kiln element life (kiln elements are expensive, and I seem to go through them fairly quickly), as well as the flexibility to move my kilns safely indoors if the need ever arose, I have dreamed of a working kiln vent for years.

Here I should mention that my kilns did at one point have working vents.  They were these strange top venting affairs with a fan that sucked that fumes through a filter before releasing clean air back into the room.  This type of vent was only made by one company, which has since gone out of business, and I have never seen them sold since.  Also, both decided to stop working a few years ago. So, my kilns basically had these strange, bulky lid ornaments.

My old top-venting Freedom Kiln Vent

So what’s stopping me from just buying a kiln vent? Well, the price, of course.  And I am sure this deters many a starving artist from creating a safer studio.  All the commercial kiln vents run $350 to $450, money I just never seem to have on hand because it goes to supplies, or food, or other necessities.  But, health is just as necessary.  Even though I believe my kilns are safe enough on my porch to run without mechanical ventilation, I decided to solve my problem once and for all.  So, lacking the necessary funds for a commercial vent, I naturally asked, “Can I make one myself?”

Yeah for research!  I downloaded manuals for a couple commercial kiln vents in order to familiarize myself with their design and workings, and if you try your hand at creating your own kiln vent, I suggest you read them as well (L&L Vent-Sure manual, Skutt EnviroVent2 manual and Orton Vent Master manual).  I figured I could base my design off of those vents that, presumably, actually work, and I found that the vents on the market were all pretty similar.  I also found that there are one or two people who have made their own kiln vents a posted brief blurbs about them online.  This was heartening news, since now I knew for sure a DIY kiln vent was a possibility.

The Design

There a two main kiln vent designs on the market today.  The first is an updraft venting system that uses basically a fume hood above the kiln lid to remove the firing vapors.  The second is a downdraft system that pulls fumes out from the bottom of the kiln.  According to some sources, the downdraft system allows fewer gases to escape into the firing room, has a less complicated setup and also benefits the kiln’s atmosphere during firing.  Ultimately, I decided on the downdraft system since it appeared I could purchase the needed components from my local home improvement store.

Honestly, based on the commercial diagrams, the construction looked pretty straightforward. The downdraft kiln vent is comprised of three main sections: the bypass/collector cup, the vent tubing or ducting and a blower fan.  Holes are drilled into the kiln lid to allow air into the kiln.  Holes are also drilled in the bottom of the kiln to allow the gasses to pass out of the firing chamber.  The bypass cup is installed under the bottom of the kiln, covering the exit holes.  Flexible tubing connects the bypass cup to the blower fan, and the blower fan sucks the air and fumes out of the kiln and exhausts them outdoors.

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: 

For the bypass cup I decided on a 4” round duct elbow.  From the commercial diagrams, it appeared the elbow is of similar size to the bypass cup.  One end of the elbow would rest on the bottom of the kiln, while the other would easily attach to the vent tubing.  It was actually a tough decision between the duct elbow and a 6-inch to 4-inch duct reducer for the bypass cup.  I was thinking the duct reducer may have been a bit better, but it was too tall to fit under my kiln with the tubing coming out of the bottom.

4-inch Round Duct Elbow to be used as the bypass cup for the kiln vent.

​Moving on, I grabbed an 8-foot length of flexible dryer 4-inch diameter tubing for the vent ducting. 

A compressed 8-foot length of 4-inch round dryer vent ducting.

The store had a couple inline vent fans, so I decided on the smallest size: a 6-inch Suncourt.  When looking through the specs of the commercial kiln vents, the blower fan capacities were listed as around 140 CFM.  The Suncourt inline vent fan is rated at a max of 250 CFM, so it should be powerful enough to run the kiln vent.

Suncourt 6-inch inline duct fan to be used as the kiln vent blower. This fan did not come with a cord attached, so I just wired on an extension cord.

Since my tubing was 4-inches in diameter, I also needed a 6-inch to 4-inch duct reducer so that I could attach the fan to the tubing.  Along with the reducer, I also purchased a 6-inch duct connector in order to connect the fan to the ducting.  [One side of the fan allowed for easy connection to ducting, but the other did not.  And, of course, for the correct air flow direction, the side that was not easily connected to the ducting, had to be connected to the ducting.  Hence the need for a duct connector.]

6-inch to 4-inch duct reducer and 6-inch duct connector used to connect the 6-inch inline duct fan to the 4-inch dryer ducting.

Finally, I needed two 4-inch ring clamps to connect the vent ducting to the 4-inch elbow bypass box and the blower fan.

4-inch worm clamp to connect ducting to bypass box and blower fan.

Since my kilns are on a porch, my plan was to just place the end of the fan against the screened window for venting purposes.  However, if the kiln were located in an enclosed room, a hole would have to be drilled through the exterior wall for the venting to go through to the outdoors.  In this case, a vent cover would be a good idea to prevent water and pests from getting into the fan and venting.

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!

Project:

DIY Kiln Vent

Materials:

• 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)

Prepping the Kiln for Vent Installation

Before starting my kiln adventure (ha, ha, get it?), I had to prep my kiln a bit.  For one thing, the kiln stand is only a mere 6 inches high.  (My kiln is about 30 years old at this point, and it seems L&L now makes stands that are a bit taller.)  Anyway, 6 inches is not much room to work with, and I knew that any kiln vent I constructed probably would not fit under the kiln at that height.  At some point I will purchase a new kiln stand, but for the time being, I decided to prop up the kiln stand on some bricks I had lying around.  I hear shocked gasps.  So, my methods are unconventional, but never fear, I made sure that the kiln is sturdy and level – it will not be going anywhere.  The bricks increased the height a couple inches, enough to install a vent.

My steadfast L&L J18X kiln, complete with old, defunct top-sitting kiln vent, and supper low kiln stand.

​I also wanted to remove my old, defunct kiln vent from the lid of my kiln.  This was easy enough (just a matter of unscrewing four screws), but it did leave a ½-inch hole in the kiln lid.  Ah, well.  Easily plugged with a peep plug until I can seal it more permanently.
 
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.

Step 1: Drill the Lid Holes

♦ Based on your kiln’s volume, drill the requisite number of holes in the lid of the kiln (refer to commercial kiln vent manuals for the size and number of holes needed).
 
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.

Tip: It is inadvisable to breathe in the dust from the kiln brick. Wear a mask to prevent inhalation of the dust, especially if using an electric drill to create the holes.

The kiln brick is so soft, that I can easily twist a 1/4-inch drill bit in my fingers to create the vent hole in the floor of the kiln.

Step 2: Drill the Bottom Holes

♦ Based on your kiln’s volume, drill the requisite number of holes in the floor of the kiln (refer to commercial kiln vent manuals for the size and number of holes needed).

Small holes are drilled in the floor of the kiln to allow the gases to be drawn out of the kiln.  This creates a downdraft in the kiln as air from the room flows in through the holes in the kiln, down through the kiln and out of the kiln floor.  Again, the various commercial vent manuals specify the number and position of the floor holes.  Since I have an L&L EconoKiln, I decided to base the number of floor holes on the L&L recommendations – with a small modification.  For my volume of kiln (3.9 cubic feet), the manual specifies one ¼-inch hole to be drilled in the center of the kiln floor.  I decided to drill two ¼-inch holes instead of just one.  Since I fire mainly tiles, I can (and do) pack most of my firings pretty tightly and figured I could use the extra exhaust capabilities.  The other manuals suggest two holes for kiln volumes of 4-8 cubic feet, and since my kiln is just shy of 4 cubic feet, I thought two holes would be okay.

Tip: Since kiln brick is very soft, drilling the holes in the lid and bottom of the kiln is quite easy. I did not even have to use a drill to create the hole, and merely twisted a ¼-inch drill bit with my fingers through the kiln brick.

Tip: Vacuum out the kiln after drilling the lid and floor holes. Kiln dust on the elements may burn them out, and kiln dust in the firing atmosphere can mar glazes.

Step 3: Prep the Bypass Box

♦ Using the ¼-inch drill bit, drill 3 to 6 holes around the 4-inch Duct Elbow about 1.5 inches from the top of the elbow.
 
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. 

Drilling 1/4-inch holes in the bypass box (4-inch round duct elbow)

It was quite difficult to guess how many holes to drill into my bypass box, since the commercial diagrams did not specify the size or number of holes in their bypass boxes, nor the volume of the bypass boxes themselves.  So, I guessed.  I ended up drilling 6 holes around the duct elbow. ​

6 holes drilled in the "bypass box". When I finally had the whole system up and running, I realized that so many holes did not provide enough sucking power, so I ended up closing 4 of the holes with duct tape.

Tip: Drilling through the steel of the duct elbow creates little metal burrs. Be sure to wear safety glasses. Be careful of sharp edges when handling the drilled bypass box. Gloves are also a good idea in order to avoid cuts.

Step 4: Prep the Blower Fan

♦ If needed, attach the Duct Connector to the inline blower fan, and seal the connection with duct tape.
 
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.

♦ If needed, attach the Duct Reducer to the duct connector on the inline blower fan.  Seal 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. 

The duct connector and reducer are connected to the inline duct fan, and the joins are sealed with duct tape.

Step 5: Attach the Ducting

♦ Using one 4-inch Ring Clamp, attach one end of the 4-inch by 8-foot Flexible Dryer Vent Duct to the end of the duct elbow furthest from the drilled holes.
 
♦ 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.

Bypass box connected to the dryer ducting with a 4-inch worm clamp.

Step 6: Install the Assembled Kiln Vent

♦ Center the open end of the bypass box directly under the holes in the kiln floor.
 
♦ 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.

Failed Attachment Attempt #2: a small supporting box made of scrap sheet metal, strapped to the kiln vent with wire. This may have worked, but I could not get the height exactly right.

​After my failed attempts at the bypass cup stand, I decided to try to attach the vent directly to the bottom of the kiln.  Let me say first off, do not try this.  I was hesitant to undertake this myself, actually.  Since the Orton and Skutt models do not connect at all to the kiln, and the L&L model connects to the kiln stand instead of the kiln itself, I figured there was probably a very good reason not to connect the vent directly to the kiln floor. 
 
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.

Failed Attachment Attempt #3: Screw the kiln vent to the kiln floor directly. Do NOT try this!

​Grrr!  Needless to say, I was getting a bit frustrated.  So finally, I just made due with what I had on hand.  I took the open end of the bypass box, centered it over the holes in the kiln floor, and shoved a bunch of reject ceramic tiles I had lying around under the bend in the elbow so that it was supported against the kiln floor.  It may not be as classy or polished as the commercial stands, but it works, and that’s all I care about!

My unsophisticated yet functional kiln stand - a stack of reject tiles.

Close up of the bypass box pressed against the bottom of the kiln floor. My failed flanges are hanging off the bypass box (decided not to remove them because of the extra holes). Also notice the duct tape over the bypass holes, surprisingly unaffected by the heat of the kiln!

Step 7: Vent to the Outdoors

♦ Drill a hole through the outside wall slightly larger than the fan end of the ducting.
 
♦ 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.

The blower fan pointed out the screened porch window. No need for a vent cap here!

Step 8: Check the Vent

♦ With the kiln vent on, use a match to test the downdraft power of the kiln vent.
 
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. 

Since I had not drilled any new holes in my kiln, I used the larger hole from the previous vent to run the test.  While the flame was not drawn significantly into the hole, the smoke from the match was sucked into the kiln.  Though not perfect, I thought that this was a good sign.  I closed up a few of the holes in the bypass box to increase the suction a bit (as mentioned earlier), and decided to test the kiln vent on a load of tiles. 
 
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!

Installed kiln vent, shunting firing fumes successfully outdoors.