Fan size too big for downdraft vent? and where to drill holes and mount outlet...

[Saxicola]

Hi everyone,

I am making a vent system for my L&L e18m-3 kiln, which is a small kiln, 3.2 cubit feet. I am making a downdraft vent similar setup to the L&L and Skutt vents (it was over my budget to buy one which is why I'm making my own).

The fans sold with the pre-made vent kits are about 130 CFM, but the fan I have is 230 CFM. Does anyone know if this would be too powerful for my small kiln? My understanding is that the amount of air bring drawn out of the kiln will be determined by the number of  holes in the kiln lid and floor, and that the amount of fresh air being brought in to mix with the air from the kiln will be determined by how far I have the draft slide open on the bypass box.  I am sure length of duct pipe between the kiln and the fan will also make a difference. I'm planning on about 6-8 feet.  

I was thinking of starting off drilling one hole in the base of the kiln and adding more if needed.

As for where the outlet vent goes, the easiest for me is to make a detachable board that can be placed over a window located about 8 feet away above the height of the kiln, rather than make a hole in the garage wall, but I will do the latter if needed. 

So, for those who may have built their own vents or know a little more about it, I have several questions!
- is the fan too powerful to the point where I can't reduce the airflow enough using the methods I mentioned?
- is it necessary to drill holes in both the lid and the base or just in the base?
- does it make a difference whether the outlet for the vent (a dryer vent in my case) is mounted above or below the kiln? 

Thanks!

 

Edited February 19, 2023 by Saxicola

[neilestrick]

Is the fan too powerful? Maybe. But I think that if you're using the L&L bypass box and start with just one 3/16" hole in the floor and open the flap all the way to start with there's a good chance you'll be able to dial it down to where it works. You could also put a speed control on the fan, or put a short length of 3" rigid duct coming off the bypass box and put a hole in that to further limit the draw from the kiln. Since you've already got the fan, hook it up and see if it works. You probably won't need a hole in the lid, but I would make a lid hole just to test the draw while you're getting it all figured out.

[Saxicola]

18 hours ago, neilestrick said:

Is the fan too powerful? Maybe. But I think that if you're using the L&L bypass box and start with just one 3/16" hole in the floor and open the flap all the way to start with there's a good chance you'll be able to dial it down to where it works. You could also put a speed control on the fan, or put a short length of 3" rigid duct coming off the bypass box and put a hole in that to further limit the draw from the kiln. Since you've already got the fan, hook it up and see if it works. You probably won't need a hole in the lid, but I would make a lid hole just to test the draw while you're getting it all figured out.

Thanks, I got the fan off Amazon and procrastinated on making the vent so I can't return it. I made the bypass box myself so I should be able to modify it. 

Aside from too large of a pull on a flame test, what would be signs that it's pulling too strongly? Would it just take longer to fire? I have a two zone kiln with two thermocouples.

[Dick White]

The flame test is the best way to see if there is a good draw or too much. The board in the window is fine, no need to make a hole in the wall if you don't have to.

One other consideration with kiln vents is providing sufficient make-up air from outside the garage. The fan system will create negative pressure in the space as well as in the kiln, so fresh air needs to come into the garage from somewhere, such as an open window or door or other vent. A complicating issue is the source of the make-up air (the door or window) needs to be some distance away (or around the corner) from the kiln vent exhaust. If the source of the make-up air is near the exhaust, the negative pressure in the garage will suck the stinkies right back inside It will be a vicious circle. Having the source of the make-up air at some distance from the exhaust will allow the stinkies to dissipate  before fresh air is pulled into to replace the air that was exhausted.