@RMB
Perhaps the 3rd party fan that you have retrofitted has a different airflow direction.
In the P1S, the fan is permanently installed behind a rear panel (see video).
It also needs to be clarified whether it was deliberately designed this way or whether it is actually wrong.
But many thanks for your efforts with the instructions.
No worries, I purchased my fan from Bambu Lab, along with the P1S rear panel (so I didn’t have to drop holes in my P1P panel). I can check the airflow in mine for you tomorrow.
@xsynatic
No problem, I assumed from the beginning that it would blow out.
I was just surprised that my plaster didn’t come off the wall with the blast. 
On the other hand, I’m relieved to hear that someone else has had the same experience.
However, the direction of flow cannot be changed in the slicer, only the strength from 0-100%!
Yeah it should blow outwards, as it was originally designed to regulate the chamber heat (although the P1S doesn’t have a temperature sensor…
). That is unless BBL decided to make it just circulate the heat around the chamber because it lacks the sensor, in which case it will blow air into the chamber.
@RMB
But then the question arises as to why Bambu Lab uses a carbon filter to filter the air that flows in from outside.
I had to go test this out again. I just printed and installed a hose mount over the fan to vent to the outside when I’m printing ASA, so I got a little paranoid that I was obstructing the flow if it is indeed pulling air in.
I did the flame test and saw that the flame was moving towards the machine, but then I noticed that it moves away and is even blown out when on the periphery of the fan vent. Did the same thing with tissue paper: the paper is pulled towards the fan and stays in place when in the center, but is blown away on the periphery. Pretty strong inward vortex but the overall net effect is air blowing out of the machine.
Even though the net effect appears to be outward flow, I’m just going to leave the hose off as the inward flow seems like it is not an insignificant amount.
It didn’t leave me in peace either and I brought the printer to my desk.
Once I had the front at the back, I took a closer look at the inside and outside without the top panel.
I also did the paper test on the outside. It held, but not in the same way as with PC fans, not very sucking, but it held. At the outer edge it swirled, strange air flow.
Now I opened the carbon compartment inside and took out the block and lo and behold, quite a lot of air came out the back.
Overall the airflow is pretty poor and caused all the fuss. Thank you for taking part.
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Now that’s a thought; removing the carbon filter improved the airflow significantly? I’d be happy to remove the carbon filter in favor of venting to the outside via a hose. I’ll have to test that.
Addendum: Just tried it. Removing the carbon filter makes a huge difference in air flow. Makes sense, I just didn’t anticipate it being such a big difference. Keeping the filter door off also makes a big difference but I’m not sure if that’s too much and might cause cooling. I decided to keep the filter off, filter panel on, hooked back up the hose system to vent outside.
Yes, it does. Now I feel stupid that I didn’t think of it myself. 
In my defense, I have to say that I had opened the box once before and thought “aha, here’s the active carbon” and closed it again.
I did some tests regarding the chamber fan.
Not only does removing the carbon filter change it significantly, so does removing the rear cover. The holes are too small and are creating backpressure on the fan.
See this post (sorry it’s FB):
I’m just catching up on this thread, I’m not fan expert, but it seems like there is just too much static pressure “resistance” for the fan to actually move much air at all. (Just confirming that I see the problem too. It sucks in the middle, and blows around the edges.) (same as you all said) But, what’s the fix? I’ll probably remove the carbon box, as suggested, but kinda want it there. (Especially for ASA) So, I’m wondering if a different fan would help? maybe an adapted blower fan, that can redirect air outward through a small duct? (would probably be louder, which I’m not a “fan” of. 
) I might cut out the grill, as that might be the most simplistic approach. But I hate to hack up the asthetics. I’m also wondering, where is the largest restriction? Grill, or carbon box?
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From my tests (see my videos on FaceBook, or I can try to upload them here also), it looks like the rear panel holes are the most restrictive.
Cutting it out would be easy, especially using a nibbling tool. I’d then print a fan guard (there are many online) or design your own. It could be held on with magnets, or screwed in as you choose.
If you didn’t like the results, a replacement rear panel is about $16, not too bad of a cost.
I’m hoping to get something soon to do a smoke test to see visually check the airflow. That rear panel is very restrictive.
My post mentions that it’s probably to help keep EMI emissions down to help it pass governmental standards and tests about EMI. I could be wrong, as I’m only guessing as to why those fan vent holes are so tiny.
I don’t have FB, maybe you can copy out the informative lines, please?
In the meantime I have also tested back and forth and have another filter that sits on the back of the metal plate. I came across a study that deals with fine dust emissions during 3D printing. 
In addition to the bad odors, fine dust emissions are now also filtered out. I left out the carbon package and inserted an air duct at this point instead. The air flow is now much better.
I think if the carbon package was closer to the fan, the airflow could be better. I also thought about constructing a carbon box for it, but it felt like you had to dismantle the whole printer to install it and I was too lazy to do that
My current solution is a compromise of these considerations and it works. I’m also torn between leaving the carbon box out permanently, as this box from BBL minimizes the odours, but doesn’t remove them completely.
In combination with my HEPA filter, no more odors come out the back. If you want to take a look, you can see my construction on Makerworld.
I won’t remove the mesh even if it is at the back.
I’ll get more info here when i finish some tests.
I have the same problem with my X1 Carbon.
It sucks the air into the printer. I have a fogger and you can clearly see that the air is being sucked into the printer from the outside. To be honest, I don’t find this funny at all, because it’s basically a really serious defect. Why would you buy a printer with a closed installation space and a carbon filter if it doesn’t filter the particles at all?
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@sikoprint
Hi,
As we have already established, air is only drawn in at the edges, i.e. the outer edge of the housing fan. The flow is therefore not optimal, but if you introduce smoke or steam inside the printer above the carbon filter, you will also see the gases being extracted.
If this is not enough for you, take a look at the Makerworld link I sent above.
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Hi All,
I’m considering purchasing the P1S and I’m finding out about the filters.
I therefore read this discussion and I ask myself: isn’t it correct for it to suck air from that fan (the one in the videos you published) and then exit from the grills where the activated carbons are?
I’m in the process of installing a duct to the outdoors through a window, and I had the same initial confusion this thread discusses. It looks like your HEPA filter attachment on Makerworld takes into account fixing an additional factor that I didn’t see specified above:
The grated vent that the fan blows through is too wide. It extends past the edge of the fan and exposes the areas where the screws are that hold the fan in place. Seeing that the carbon filter resists air flow, these gaps around the fan’s edges are allowing a bypass for the air to flow back in. The air blows out with the fan, and then is sucked back in where the screws are, flows around the edge of the fan, and then blows back out again. I believe this bypass loop decreases the overall flow of air out of the printer.
As I plan to run this through a duct outdoors instead of using a HEPA filter, I’ve put some electrical tape over the areas where it extends past the fan for now. Thanks for the discussion on here!
well, good thing you have the tool for fixing a new shroud right there on your desks!
fixing the airflow so it runs proprely, either by replacing the fan with something that has a higher static pressure or adjusting the flow ratios will probably reduce noise as well.
Could you send an image of what you did? I’m not sure if you put tape on the inside or outside of the grate.