External Ventilation for ABS: Using Cooling Mode at 55 °C

Bambu Lab H2 Series
1

I was able to open the rear vent while printing ABS by switching the printer to cooling mode and setting the chamber fan to about 10%, while still heating the chamber to 55 °C.

I typically print ABS at 50–55 °C. For very large parts this temperature may not be sufficient, but it works well for my use case.

What I Changed
I increased the air-duct mode temperature threshold to 60 °C in the machine G-code.
This means heating mode (top and rear vents closed) now close at 60 °C, instead of the default 40 °C.

Below 60 °C, the printer remains in cooling mode, allowing external venting.
I then set the ABS filament custom chamber temperature to 55 °C.

As a result, the printer heats the chamber to 55 °C while staying in cooling mode, which keeps the rear vent open for external exhaust.

Modified G-code (Set Airduct Mode)
This modification allows manual control of the chamber exhaust fan from the front panel.
Note: You can switch to heating mode manually, but once you do, all vents will close and you won’t be able to switch back to cooling mode.

;==== set airduct mode ====

{if (overall_chamber_temperature >= 60)}

M145 P1  ; set airduct mode to heating
M106 P2 S0     ; turn off auxiliary fan
M106 P3 S0     ; turn off chamber fan

{else}
M145 P0 ; set airduct mode to cooling
M106 P2 S0 ; turn off auxiliary fan
M106 P3 S26 ; turn on chamber fan at 10%

Modified Chamber Temperature Logic

;===== set chamber temperature ==========
{if (overall_chamber_temperature >= 60)}
M145 P1 ; set airduct mode to heating
M141 S[overall_chamber_temperature] ; start chamber heating
{endif}
;===== set chamber temperature ==========

Important Notes
You must lower the ABS chamber temperature to 55 °C in the filament settings.
Use this modification at your own risk.
This setup is intended for users with external ventilation who want to print ABS without internal filtration.
If your printer is located in a cold garage, the internal heater and fan may struggle to reach 55 °C. In that case, you can try 50 °C, but large parts may warp.
I’m considering printing a cover for the top vent to reduce air loss while the rear vent is open, but for now 55 °C is sufficient for my ABS prints.

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2

Sorry for being an idiot, but why would you not use the internal filtration for ABS?

3

I did, but every time I print ABS, I notice a strange smell. I run 6–8-hour print jobs daily, and they’re all in the same room where I work.

ABS

Acrylonitrile exposure can cause acute issues like severe skin/eye/respiratory irritation, headaches, dizziness, nausea, weakness, and even convulsions or death at high levels, with symptoms resembling cyanide poisoning. Long-term risks include potential cancer (Group 1 Carcinogen by IARC), neurological damage, and developmental problems, though occupational cancer risks are less clear in humans than in animal studies, which show brain tumors and stomach issues. It’s toxic due to metabolism into cyanide and reactivity with proteins.

Butadiene - Acute high exposures may cause damage to the central nervous system or cause symptoms such as distorted blurred vision, vertigo, general tiredness, decreased blood pressure, headache, nausea, decreased pulse rate, and fainting. Chronic effects caused by exposure to 1,3-butadiene are controversial.

Styrene exposure primarily affects the nervous system, causing headaches, tiredness, dizziness, confusion, slowed reactions, and concentration problems, often described as feeling “drunk”. It also irritates the eyes, nose, throat, and skin. Long-term exposure in occupational settings has linked to potential hearing loss, altered color vision, and, though inconclusive, possible associations with leukemia/lymphoma and potential genetic damage

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4

Small final code modification that allows you to print ABS in cooling mode at any chamber temperature defined in your filament settings — with all vents open. I typically print ABS at 50–55 °C.

I had to comment out a few lines of code because the printer was turning the vent fan on to 70% while the chamber heater was warming up (just before bed leveling), which was extremely counter-productive.

Now the fan still runs, but it stays at a consistent 10%. You can raise it to any value you want from the control panel.

Previously, chamber heating was only possible in heating mode (with all vents closed). That limitation is now gone

If you want to reduce how much fresh air enters the printer, you can print a “top vent filter” and make it denser — it very effectively limits airflow and drafts.

==== set airduct mode ====

{if (overall_chamber_temperature >= 60)}

M145 P1 ; set airduct mode to heating mode for heating

M106 P2 S0 ; turn off auxiliary fan

M106 P3 S0 ; turn off chamber fan

{else}

M145 P0 ; set airduct mode to cooling

M106 P2 S0 ; turn off auxiliary fan for cooling

M106 P3 S26 ; turn on chamber fan for cooling

M1002 gcode_claim_action : 29

M191 S0 ; wait for chamber temp

M106 P2 S0 ; turn off auxiliary fan

{if (min_vitrification_temperature <= 40)}

{if (nozzle_diameter == 0.2)}

; M142 P1 R30 S35 T40 U0.3 V0.5 W0.8 O40 ; set PLA/TPU ND0.2 chamber autocooling

{else}

; M142 P1 R30 S40 T45 U0.3 V0.5 W0.8 O45; set PLA/TPU ND0.4 chamber autocooling

{endif}

{else}

{if (!is_all_bbl_filament)}

; M142 P1 R35 S40 T45 U0.3 V0.5 W0.8 O45 L1 ; set third-party PETG chamber autocooling

{else}

{if (nozzle_diameter == 0.2)}

; M142 P1 R35 S45 T50 U0.3 V0.5 W0.8 O50 L1 ; set PETG ND0.2 chamber autocooling

{else}

; M142 P1 R35 S50 T55 U0.3 V0.5 W0.8 O55 L1 ; set PETG ND0.4 chamber autocooling

{endif}

{endif}

{endif}

{endif}

;==== set airduct mode ====

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5

Interesting.
I am trying to get where my H2S can run the rear exhaust fan but keep the top vent closed when printing longer ABS jobs. Just want to exhaust some fumes through an external filter.
I do not need active heating as the chamber easily reaches 60c.
If I can keep the top vent closed I can run the rear fan at 20% and the chamber temp will hold around 60c.
Also, keeping the top vent closed will mostly eliminate air currents which are a main cause of warping/lifting off the bed.

I don’t need to start cooling mode or heating mode just want to be able to activate the rear fan and set it to a low percent for specific jobs.
I’ve tried adding specific gcode to the filament start section but so far nothing is working.
Doesn’t seem it should be so hard to just be able to turn the fan on or off!

6

I had the same request and intention, but it turns out the rear fan does not open unless the front vent is open.

What I did instead was print this filter holder:

along with this filter:

However, I did not use the holes included in the design. I printed it as a solid, fully closed piece by removing the infill modifier. This way, there is no draft. The filter opens, the fan is set to 10–20%, and only a minimal amount of air flows through.

I print ABS and other materials, and I vent everything outside, including PLA.

2 Likes
7

Thank you, I will take a look at this and give it a try.
Currently I am trying heated mode and set the temp to 40 to see if it will filter through the internal filter without constantly running the heater.

Currently the chamber temp is only 42 when normally with everything off it reaches about 60.

I wonder if it is cooling along with heating to maintain the temp set?

We’ll see if the temp gets up there. I don’t like the idea of using the heater for ABS as the chamber gets plenty hot and also not to waste the power.

8

Hi Sebo, unfortunately any sort of non-cycling filtration simply won’t work.

What you really need is another enclosure+cycled internal filtration, or an active ventilation/extraction (though, worse option) system. You can easily make a new enclosure with a large cardbox, and the filtration would be easy you just need a normal small air purifier that has activated carbon inside the enclosure.

9

If the chamber temperature naturally reaches 60 °C, the chamber heater does not activate. It only maintains the set temperature, ensuring no unnecessary power consumption. All of this is controlled by temperature sensors.

10

I’m printing ABS as we speak and venting outside—my workaround works 100%. I’m seating 1.5m from the printer and can’t smell anything.

11

Thanks.

When on heat mode does it do any cooling to maintain temp? Or does it just try to maintain a minimum temp?

Here is what I am seeing. First time using heat. I set it at 40 in hopes it wouldn’t run much and the bed would do the rest and get things to its usual 60 or so.
Temp set at 40c.

Print running for 2 hours and it has reached 48 in the chamber.

Normally with heat off, the chamber would be at 60 within 30 minutes or less just from the bed.
Could it be the circulating air that is slowing the temp rise in the chamber from the heatbed?

Losing heat from the circulation fan and filter?

12

Yes if it’s venting outside then it works

13

You are attempting to print at 60 °C, but drawing conclusions from prints done at 40 °C is not valid, as the conditions do not translate—especially since you appear to be printing in a hot room.

14

No, my question is why is the printer able to reach a chamber temp of 60 when the active heater is off/disabled, radiant heat from the heatbed only source heating the chamber, but can only reach 48 when the heater is active?
Same print, settings etc. except one scenario the heater is on and the other it is off.

Just curios.

15

Hi all,

It’s most likely me who’s incompetent or I’m missing something… my question is:

Can I print ABS or ASA this way with the H2S?

  1. The top air intake is closed.

  2. The rear extraction fan is turned off, and therefore the rear motorized flaps are closed.

  3. The internal heater and its fan are both turned off.

The printer chamber would heat passively thanks to the bed temperature set to 100°C,

just like I did with the P1S, and that would be fine with me.

16

Yes, that is how I print ABS. My chamber temp will reach about 60c in those conditions.
The only drawback is there is no venting of the fumes so whatever room the printer is in will stink after a little while.
I find using the heater set at 40c the chamber temp does not get as high as without the heater.

I also find suing the ventilation, rear fan and top vent, anything over 10% for the fan will remove a lot of heat and possibly cause warping.

17

In my case, I think that when the internal heating is on, the smoke and odor (despite the activated carbon filter) are blown out of the print chamber.

If, however, all the fans were turned off and all the vents were closed, there would be less odor and harmful substances in the room.

To extract the fumes from the room, I’m thinking of a solution similar to a kitchen extractor hood.

18

Then I’ll try to do as you say. Thanks.

19

With the heat on and internal circulation fan on for filtering I think the smell in the room is less than with everything off.
If I print with all fans off then the smell in the room is quite bad and strong.

20

It’s a little crude but maybe it works…

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