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Hi everyone,
Has anyone made a home made downdraft venting system? I have seen several good manufacturer systems but they are a little pricey to get down here in NZ so I thought I could make one up at a reasonable price. My kiln is a top loading Evenheat GTS2541 at 7 cubic feet.

I know the draw needs to be gentle via a couple of holes in the lid and two in the bottom. Does anyone have any idea how big to make the holes? I'm fairly sure I can make up the rest at a pinch.

Any help, feedback or thoughts would be appreciated.
Cheers
Monty

Monty,
Making the holes are really technical... Put the holes opposite each other... top and bottom, or front to back. Drill holes where the elements are not. If you elements are on the top and too close you can have the holes front and back. I suggest to drill from the inside out.. ie thru the brick and then the metal. This way you will know that you will miss the elements....

Oh for the really technical part..... ............ there is none. There is nothing to this.

Orton suggests two 1/4" holes in the top and also in the bottom.
You can make up a manifold (box) to cover the two bottom holes under the kiln and hook that up with a flexible hose or duct to a small fan. I found that the fan directly connected to the kiln (as Orton recommended) resulted in too much vibration.

I put my inlet holes in the walls horizontally about an inch below the lid so that the cold incoming air isn't directed at hot glass.

ch

Marty wrote:Orton suggests two 1/4" holes in the top and also in the bottom.
You can make up a manifold (box) to cover the two bottom holes under the kiln and hook that up with a flexible hose or duct to a small fan. I found that the fan directly connected to the kiln (as Orton recommended) resulted in too much vibration.

Marty, how many fans have you melted?

Seriously, do you use a high temperature squirrel cage fan with this setup? And do you turn it on during cooldown or just use it to vent on the way up?

Tony

Tony- Actually, I got the Orton under-the-kiln vent with my first "real" kiln, the Evenheat oval, convinced that I would die of toxic waste fumes if I didn't. The damned thing vibrated so much that the tack-fused stuff I was attempting would move all over the place. I found it easier to just prop the lid open 1/4" on the way up to get rid of the glue and binder fumes. I stuffed the holes with fiberpaper and for all I know, the fan is still sitting under the kiln- I haven't looked.
I'm not using vent fans- the only time I vent is to speed cooldown from 250 to RT.

Hi Marty,

I rigged my Orton Kiln vent to a dimmer switch and keep the fan speed low enough that vibrations are not a problem.

Hi Tony,

I only vent on initial heating, not during cooling. I turn it off at 850F ish. Orton says that the vent can be turned on during cooling to speed up cooling. I notice on one kiln that, when the vent was left on at temperatures above 1000F, a darker (cooler?) spot appeared on the glass. Always the same relative place on the kiln shelf. Some pieces that were being slumped had thermally shocked and the cracks seemed to go through the "cool" spot. I fixed this by using the dimmer switch to slow the speedof the fan. I would be concerned that using the vent during cools might cause thermal shock. Perhaps if the fan was used during annealing it might create a heat differential.Just some observations and a guess at the implications.

Cheers,

Bob

Marty wrote:I'm not using vent fans- the only time I vent is to speed cooldown from 250 to RT.

Well, that's my intent axactly... that's why I was interested to know if you actually used it to cool. Naturally I assumed you also used it to vent your kiln... little did I know that it was just an expensive accoutrement.

Certainly the thermal shock issues are the major concern as Bob mentioned, but I think there must be ways to slowly cool the air in the kiln without opening the lid.... hmmmmmm
Tony

Thanks everyone, very helpful indeed. It's nice to get up in the morning and find all this great feed back.

Thanks for the specifics Marty that will give me a good place to start.

Hey Charlie,
The idea of not drilling directly in the overhead lid seems to make sense to keep the cool air off the glass. I'll give that one some thought.

As for venting on the way up and on the way down...I was thinking that venting on the way up to get rid off fumes etc was only required. For the rest of the process I thought a slow turning internal fan would help keep the heat nice and even thoughtout the Kiln...but I'm still thinking about the best method for that approach.

Thanks Heaps
Monty

Tony- build another kiln. You'll be too busy loading the 2nd one to bother with slow cooling in the first (or 3rd).
I get away with a half inch opening from 250 to 200, an inch from there to 130-ish, and 2" to RT. But it involves blood sacrifice to the kiln goddesses.

Tony,

Why don't you want to slow the cooling down by propping the lid with a small kiln post? When I do this the kiln still cools at a rate way slower than what the recommended cooling rates in various charts (assuming projects are less than 3/8" thick).

Cheers,

Bob

Bob wrote:Tony,

Why don't you want to slow the cooling down by propping the lid with a small kiln post? When I do this the kiln still cools at a rate way slower than what the recommended cooling rates in various charts (assuming projects are less than 3/8" thick).

Cheers,

Bob

Bob- Que cosa "recommended rates"? The kiln will definitely have to be controlled down to 700- I'm assuming that's not the issue- and most kilns will take themselves down to 250 or 300 quite nicely without help. It's that maddening crawl towards room temp that we're dithering about. (I think).
Marty

and does the period go inside the quotation marks or outside, as above?

Hi Marty,

"Recommended Rates" were from the Bullseye firing guide. For the most part they are faster than the cooling rate of the kiln so propping the lid open shouldn't really be a problem. Obviously size and thickness do matter.

Cheers,

Bob

One elegant solution I have seen:
Stainless steel tubes thru the kiln. Notmally closed at upper end, and when cooling they are left open.

The cold ambient air never enters the kiln, only heat is transferred from kiln air intu tubes, where it warms up and
while expanding has an chimney effect of increased draught.

Easy to do and easy to oprate.

-lauri

Feel like a heretic, but I never vent coming up to disperse burn offs from glue or thin fire. The glass marinates in this swirling miasma, and seems not to mind. Maybe its the secret rites at midnight with black candles...