A downdraft vent comes in several brands. It is used on ceramic and glass kilns to prevent fumes from escaping the kiln into the firing room.
The downdraft vent removes just enough air to create negative pressure inside the kiln. This prevents fumes from leaking out. On most kilns the vent is positioned between the kiln bottom and kiln stand. (The fan on the new Orton Vent Master mounts away from the kiln.)
If you can still detect odors during vent operation, check the following:
1) Make sure you can hear the fan spinning. This is basic but easy to overlook in a busy studio.
2) The holes that draw air out of the kiln must be free of obstructions. If the exit air holes are in the bottom of your kiln, the bottom shelf should be positioned 1” above the kiln floor. The shelf must not block the holes.
3) The intake and exit vent holes must be of the correct number, size, and location in your kiln. See the vent instruction manual to be sure, or call the manufacturer. (Orton downdraft vents: 614-895-2663.) The vent housing must cover the air exit holes. Otherwise hot particles can fall onto the floor from inside the kiln.
4) Insert all the peephole plugs and keep the lid/door fully closed throughout the firing.
5) During firing, hold a match above a vent hole in the lid. The negative air pressure inside the kiln should be strong enough to try to pull the flame into the hole.
6) Check the vent duct for leaks. A leak somewhere inside the wall or ceiling can release fumes.
7) Make sure the duct is free of obstructions and that the vent flapper on the outside of the building can open freely. Go outside. You should feel warm air coming out of the vent.
If you fire ware that contains many impurities, you may need to fire smaller loads. If the exit point for the vent system is near a door outside, you may have to move that exit point. If you still smell fumes after checking the points above, you may need to drill another air exit hole in the kiln.