Smoke not venting completely

I am setting up 2 roasters in a shed, and am trying to vent the smoke passively. However when I am roasting to second crack, not all of the smoke is getting captured into the exhaust duct.
It could be that I don’t have adequate make up air in the room, and the cooling fans are depressurizing the space enough to draw smoke in from the exhaust pipes.

Does anyone know how many cfm the cooling fan will push through a 4” duct?

Knowing how many cfm both bullets are moving will help me know how to provide adequate make up air.

This may not answer your question directly, but I think if you have a window/door open it should be enough for make up air. I roast in my kitchen under a 900 CFM range hood vent. I only have the cooling tray fan connected to a semi-flex hose where the other end is propped up close to the range hood. Make up air for my house is provided by a “fan in the can” in the basement that draws air from outside when the range hood vent is turned on but I also open my kitchen window just a little crack (in the winter) or fully open in the summer - more than enough for me. But then I only run 1 roaster.

Beautiful setup you have there, by the way :slight_smile:

Happy Roasting.



I have a 4” ducted opening with a damper in it, but it appears not to be enough when it is fully open. I can put a 6” or larger duct in its place, but I want to try to size it before I start cutting more holes in the walls!

Opening the door isn’t ideal for keeping bugs out and controlling the temperature.

You could run a small duct from the outside (with a screen on it to keep the bugs out) and have the exit air from the outside near the Bullet. You won’t be able to control that ambient air temperature by adding a duct though…

It looks like you have put ducting on the exhaust from your colling bowl and fan. Nice!

Beautiful setup.

I have a 4" duct through the wall with a screen for keeping bugs out for make-up air, but it seems to be inadequate.

In the future I may use a MUAH (electrically heated in-line duct make up air fan) but for immediate needs I will just have a passive duct and deal with the cold air. I just need to know how many CFM the Bullets are pushing out so I provide adequate make up air.


I understand better now.

I think that this will be difficult to determine because we don’t end up with just one fan speed. At F2 we will have minimal cfm and at F6 much higher cfm. Also when you drop and the Bullet goes into cooling mode, the bean bowl cooler pushes out a lot of air. Very high CFMs. It is using a 24VDC fan

i have a feeling that judging by your setup that you have already done a lot of research on this.

I think that the majority of the people here are using fans to draw the air out. Not going passive.

It is the cooling fan I am primarily concerned with. Have you ever measured it with an anemometer?

I used an inline fan in flex duct to exhaust the smoke for about 8 months but it is not easy to clean, so I want to try a passive setup with short ducts that are much easier to clean.

I have an anemometer, but haven’t tried it for that reason. I was using it to check to see how much the chaff was blocking the airflow. It was substantial.

I agree about the cleaning. I just find that aesthetically, the dryer vent just doesn’t look “clean”

I was thinking about using PVC and then I would be able to use Cafiza and a brush setup like is used for cleaning the barrel of a gun.

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I have definitely noticed chaff buildup affecting airflow. I hold a lighter up to the trier if I suspect it isn’t drawing & increase airflow if the flame isn’t getting sucked in.

I am not a HVAC expert, but have some degree of knowledge on the subject based on experience in nit o my coffee, but cooking, and outdoor smoker usage.

Passive venting requires very short runs of exhaust. One reason is air volume you mention, and the other is the effect of Venturi caused by exhaust heat. Venturi really works well when there is one path, intake and one intake and exhaust. Rising heat also creates suction. You effectively have two intake and one exhaust, but on,y until cold air is pushed into the one exhaust. Now you have more happening than that pipe can handle, and the cold air acts differently in the pipe. Because air knows nothing about where to go, I would would think the pressure caused but the cooling tray fan may even be finding its way back into your roaster, and pushing smoke into the room , and make up air isn’t as much of an issue as this.

I sort of understand the why behind passive venting, but venture to say that what you are trying to do, because of constantly changing variables (roaster variable fan speeds, combined with on or off cooling tray) may not be effectively manageable with a flap or damper in-line.

The solution, is more complicated than the problem, effectively. If it was me, I would be more likely to combine the two roaster vents into one exhaust, and then the two cooling tray exhausts into one exhaust pipe. From an engineering standpoint it would be mush easier to design…although it won’t be near a clean look like you have now. Even then you may run ito a problem with the two roasters effectively competing for room in the same pipe, pushing air back into the roasters at worst, but at the very minimum effecting the roasts in each roaster so that consistent roast results are impossible.

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