Exhaust Fan sucks in more air than it expells

When running the Chamber Fan aka Exhaust Fan, the fan is sucking in more air than it is expelling. It behaves more like an intake fan than an exhaust fan.

Try this test. Turn on your Chamber Fan and place a tissue against the Exhaust Fan rear grate . The tissue will get sucked up the grate. This is not how an exhaust s is supposed to behave. I don’t see how there’s any benefit to the carbon filter that’s behind the fan. I hope that Bambu Lab can come up with a fix that can correct this issue.

Mine does this also, in fact I came here looking for this issue.


We had a few tickets regarding this question. Please note that the main role of the Chamber Fan is to filter the air from the enclosure through the carbon filter and then regulate the temperature inside it for filaments like PLA.

The fan is pulling air through the carbon filter, but the filter is adding some drag and limits the airflow. The back grill also changes the airflow which can disturb the normal flow of air.

If you need more air to be pulled out from the printer and exhausted (maybe for PLA) you can simply remove the carbon filter.

Exhausting the air only benefits filaments like PLA and TPU for example, as they require a low temperature for the enclosure. But for the rest of the filaments, like ABS/ASA/PA-CF you don’t want to expel the hot air of the enclosure outside the printer but you still want to filter it through the carbon filter.

I hope this makes it clearer


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Come on it is a bit of ridiculous dysfunctional design.

You don’t turn the fan on when printing ABS with its styrene fumes (or whatever) but you do turn it on for PLA and others when the plate temperature is high, you also recommend removing the lid or opening the door - what is the point of that carbon filter then?

Pull the carbon filter and try the paper test again.

If you want it to cool the chamber pull the carbon filter. If you want it to filter printing fumes then actually turn it on and in that case what speed? What speed is required so the only air leaving the chamber is through the filter? It doesn’t need to be any faster.

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Bambu Lab describes this as an exhaust fan and customers are treating it as such.The presence of the carbon filter seems to support that intention. I’ve seen a few YouTube Videos where owners have connected a duct at that fan to expel the exhaust outside. Normally that’s a great idea, but in this case it’s a false sense of safety. That fan is boxed in by that plastic wall and the carbon filter. There’s no room behind the fan to get any airflow to it.

I suspect that dumping the sponge and screwing the fan directly to the interior back panel might leave enough room behind the fan for it to to blow air out instead of creating a vacuum effect.
The screw holes are already there as if it was intended to be mounted there in the first place.

I also suspect that the lack of an effective exhaust may cause the other fans to be less effective.

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Good point. Removing the filter may allow enough air to the rear of the fan. It might be even better if the fan were mounted directly to rear panel with a thin gasket seal in between instead of the thick sponge.

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If you think about it, the exhaust fan is blowing out exactly as much as it is sucking in. How could it exhaust any more than that? The air inside is pushed out, lowering the pressure inside, and new air moves in to replace it. There is no air being created here. The fans just move it around.

I tried to sense the vacuum using a small piece of paper and sure enough it does produce a vacuum when put to close to the rear vents. I think the vacuum is created only when the paper is too close to the exhaust vents and my theory is the intake vent holes inside of the chamber allow less volume of air vs the exhaust vent holes. The air in this instance will take the easiest and shortest path and that is why the vacuum is present?

Using a wind meter, the air flow is traveling outside of the chamber when held aprox. 2" from the exhaust vents and no vacuum is present. Like someone suggested earlier, removing the carbon filter will increase the air flow ,but not as much if you were to run with the intake cover off.

Intake Cover Out and Filter Out: 3.4 MPH
Intake Cover In and Filter Out: 1.9MPH
Intake Cover In and Filter In: 1.3MPH
Note: Chamber Fan Speed at 100% for all readings

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It’s a good thing my car exhaust doesn’t do that. :skull_and_crossbones::joy:

If the tissue is pressing against the fan while it’s running then there’s more air moving in than out. That’s the definition of vacuum.Moving the meter away from the fan and getting a different reading doesn’t change that. The meter could be picking up air draft. The reading at the fan is most important.

With the door closed and the lid on, as stated, the exhaust fan is creating a vacuum inside the print chamber.

Crack the door open 1/8" and run your wind tests again. Let some fresh air get in to replace what was exhausted.

Your Kestrel should have a baro sensor on it. Might be able to see the pressure change with the fan on and door / lid closed.

I think you’re right about there being a vacuum. Even if some air is expelling from that fan the force of the vacuum is stronger than what’s being expelled making it ineffective as an exhaust. When printing with filaments like ASA or ABS I wouldn’t want to keep the door open while printing. Even with the filter removed and the door open the exhaust still seems really weak compared to other chambers with exhaust fans that I’ve encountered. Seems like very little payoff for such a noisy fan. I don’t think it helps that the fan is in such a tight spot that’s mostly closed off by the wall of plastic in the back of the printer.

…When printing with filaments like ASA or ABS I wouldn’t want to keep the door open while printing. Even with the filter removed and the door open the exhaust still seems really weak compared to other chambers with exhaust fans that I’ve encountered. …

Don’t have to leave it open, just give it a little air coming in. Might be that all the other grills and vent holes are enough to let air in to circulate. Too much and you chamber temps will drop, and you certainly don’t want to crack the top glass open (heat, toxic heat rises).

Where do you expect this air to come from? The fan is pushing air out of the closed box. This lowers the pressure inside. If you don’t want low pressure inside, then you have to open the box.

Test results were when door and top were closed.

No change to air speed with doors open or glass top off and this is because the “filament poop chute” is the intake that allows fresh air into the machine. The hole size of the chute is large enough to keep up with the CFM of the chamber fan with our without the carbon filter installed.

Covering up a fan completely changes the airflow within and around the fan, making it utterly useless judging its sucking or blowing capacity at that time.

If you want a more scientific explanation, covering up a fan gives you 0 air speed. What happens then it because it is still rotating, it will start accelerating the air in a circle inside the fan itself. This makes the air speed at the place of coverup higher than the air speed at the other side of the cloth where there is no fan spinning (speed parallel to the cloth). Some guy Bernouilli once stated that pressure goes down when airspeed goes up, maybe not in those words (also this is what makes a venturi work).
Because axial airflow is completely or almost at zero (by covering the fan) the little pressure difference created between fanside of the clots and outside of the cloth can be enough to make it stick to the fan.

This means that the airspeed of a fan cannot be measured right in front of it because it disturbs the working of the fan itsself severely. And now we’re getting to a point where we can start understanding why the fans actual exhaust speed out of “the box” is so low. Look at that massive hole punched steel sheet in front of it.

Cut out the hole punched piece and you’ll get the sucking and blowing action you crave for. (absolutely no disrespect here, I just could not withstand making this pun :smiley: )
But I think we al know no one is going to take the grinder to their beloved X1C (my wife calls it my second wife…)

as a sidenote, this is not at all the case for all fans in all configurations. The airflows around the fans have sooooo many variables that you say can my explanation above here is pure guesswork, and it certainly is but hey I’m trying to help here :wink:


He and I were not covering the fan completely. Dangle a flimsy tissue in front of the fan and it gets sucked towards the grill and sticks, predominantly in the middle.

The carbon filter is so restrictive there is very little air flow through it. The majority of the air the fan blows out around the periphery where the blades mostly are is being sucked in through the grill around the middle where the blades are not.

The design is crappy. If it is meant to purge the chamber through the filter it should be a blower with better pressure for much less noise. If it is meant to cool the chamber it should not have flow grossly restricted by the filter. Currently it does a crappy noisy job of both things it might be intended to do.

Personally I have pulled the filter and run the fan slowly if the chamber temperature is looking high for PLA (which so far is about never, but, the room is fairly cold).

How dare you sir! :rage: JK I do like the pun :sweat_smile:

I have an old desktop PC"s with a sealed case that manages it… It’s a much smaller box as well. I’m not looking for a windstorm, but maybe a filter that isn’t as restrictive could help. Show me another exhaust fan anywhere that sucks air in.

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It’s not the tissue, It also sucks in smoke/vapor. I just tested it with a wooden match.