Airflow path


Can anyone describe the Bullet’s airflow path? From inside the drum it’s obvious the air moves through the exhaust tube, chaff collector, filter screen, fan, and out the exhaust. But what is the path to get into the drum? I see holes in the back of the drum that must be the inlet. Does the air flow in through the chaff spill slot below the front window, around the drum?
This is partly out of curiosity, but also to understand how much pre-heating the air gets before it enters the drum. From experience with the fan speed during roasting it seems to behave as if the inlet is cold (room temperature) air, so turning up the fan cools the beans.


Here’s part of your answer…

As I understand it, air enters at the lower rear of the Bullet via the perforations at the bottom of the rear shrouds (drum motor cover & chaff collector). It’s pulled in by the cooling fan at the bottom of the rear plate (the fan is the one visible when you remove the chaff collector). How it gets from the output side of that fan to the inside of the drum is more than I know!

The gap below the exit door seems to have no significant contribution to air flow since chaff keeps falling out even at high exhaust fan settings. It’s pretty much masked by the bean mass.

So… not a complete answer but a start!



Hi! I enjoyed this short description compliments of Dan on the Sweet Maria’s library site:


Some of the air is preheated under the drum before entering the drum.


Thanks for the diagram! Helps a lot.

I didn’t realize the gap beneath the door was part of an intentional air path below the drum. Pre-heating that air makes sense now that you point it out. My first thought leaves me scratching my head about chaff falling out during the roast as air enters. Does that mean there’s a chance for chaff to collect below the drum? Sounds like I better make a determined effort to vacuum that better!



Thanks for sharing… using the photo from sweet maria, what does the electronics fan do?


Yes and chaff do accumulate, so good idea to vacuum out the front after each session.

@andyvtec The electronic fan is part of the cooling for the induction electronics. It sucks air from under the roaster, it passes the induction coil and the IGBTs to keep it all cool.


@jacob wonder if it’s possible to restrict the ambient air coming into the drum from the back to encourage more air coming through the bottom which is preheated?

By the way, have you guys tested how much air is coming from the back versus the bottom which is preheated? Like 70% back / 30% bottom preheated?


I don’t recommend modifying the airflow or blocking the air intake as it could cause overheating of other parts. The shaft and belt pulley are cooled by the back air intake.
We have not measured the exact airflow as this is technically quite difficult to accurately do.