I have a CRS310-8G+2S+IN which runs SwOS v2.18 in L2 managed mode, basically can only manage via web, I have replaced the internal fan with a Noctua NF-A4x20 PWM as the stock foxconn fan was very noisy.
With the replacement fan the temperature of the entire switch is warm/hot to touch vs prevoius foxconn fan even if I set the Fan Target Temp (C) to 35C:
Thank You
Is the direction of the new fan equal to the original fan?
I don't know what it originally was, so can't answer that.
Is the fan voltage the same as what was in there originally? what about the cubic feet per minute values?
It might pay to get the two model numbers so we can have a look if your not sure what your looking at.
As for the direction of the air flow, are there other fans in the case? You could try to revere it and see what happens.
The original fan is a Foxconn model: PIA040H12H, the replacement fan I installed is a Noctua NF-A4x20 PWM, my Mikrotik switch model is CRS310-8G+2S+IN has 1 fan only. The voltage is the same 12v for both fans. The original fan blows air external out of the unit, the replacement fan is also set to blow air external out of the unit.
My setup on this Mikrotik switch is running L2 VLANs only: 8 x 2.5gbe 1 x sfp+ rj45 module and 1 x sfp+ DAC so all ports are occupied working
Im using SwOS 2.18 which is webgui only.
Can I change a setting in SwOS 2.18 that makes the Nocua replacement fan go faster rpm? ive already tried setting the Health>Fan Target Temp (C) to: 35C and switch still gets hot.
Do I need to change from SwOS to RouterOS and only use the L2 features only which allow me better fan control settings to be set for the Nocua replacement fan?
Do I need to get a completely different fan which is quieter than the original Foxconn model: PIA040H12H but not as noisy?
Noctuas are quiet precisely because they have a lower CFM rating.
You could try reversing the fan direction so it blows directly over the circuit board instead of sucking air over it. Just make sure the intake on the back isn’t blocked.
Otherwise, you’ll want a different (louder) fan. Some of those SFP+ modules get pretty toasty.
Could try reversing fan so it takes external air from and blows into the board but to fully test that id have to plug all ports back in and the physical area of getting the switch in and out is a challenge.
Hoping to see what others have done in resolving this scenario?
There’s a previous thread … CRS310-8G+2S+IN fan replacment
Search the forum, you will get more hits. There was a issue about heatsink coming out, see whether yours have similar issue.
Myself upgraded to noctua, never faced this much temperature issue, but I was using ros.
link refers to fan clicking and fan locking up not tempreture
both heatsinks are seated ok, how many ports do you have plugged in and that are active? and what temps do you get?
Maybe you heatsink, not glued correctly ; it's a know problem on this hardware/model.
I've multiple devices and on one, the pad behind the mainboard (behind chip) was missing, i've added it and no more trouble.
Thank You A1phaM4trix,
Ideally id like to stay on SwitchOS webgui but as fan control is not possible in SwitchOS, and fan control only possible in RouterOS may have to ”switch” 
.
On possible solutions:
Stay on SwitchOS (webgui only) and get a different fan e.g ARCTIC S4028-6K - 40x40x28 mm fan, 250-6000 RPM, PWM regulated, 4-pin connector, 12 V DC – Black. – Would this sufficiently cool this fully port occupied switch?
As RouterOS is the only way to have fan control then question arises to keep the original Foxconn fan PIA040H12H vs Noctua NF-A4x20 PWM? Which fan would keep temperatures within thresholds with fully port occupied switch?
Can I back-up the config on SwitchOS (.swb backup file) and restore the .swb backup file in RouterOS OR would I need to rebuild the switch configs in RouterOS?
+1 
Generally speaking, blowing fresh air from the outside on the heatsinks should be (a little) more efficient than extracting air from the chassis, so, if I were you, I would try first thing reverting the fan blowing direction.
Here is a picture of your device:
The factory PIA040H12H-P17-AB is a sort of "turbo", should be running up to 18,000 RPM and deliver at least thrice (or more likely four times) what the Noctua NF-A4x20 PWM can, as it has has 9.4 mÂł/h at 5,000 RPM, using up to 0.46 W of power.
There are mid-range fans (not as loud as the Foxconn, and not as silent as the Noctua) that maybe are an option.
Which mid-range fans (not as loud as the Foxconn, and not as silent as the Noctua) would suit best which is one of my questions above? e.g ARCTIC S4028-6K - 40x40x28 mm fan, 250-6000 RPM, PWM regulated, 4-pin connector, 12 V DC – Black.
OR is Test FAN and ERROR (trial and error)? 
The left side heatsink in your picture is much smaller than the photo I posted, that comes from:
https://www.servethehome.com/mikrotik-crs310-8g-2s-in-review-8-port-2-5gbe-and-2-port-10gbe-switch/2/
Likely a different board revision/batch (but the L signs in your picture imply that the heatsink should be much larger than what you have)
This may be a good thing as the SFP cages should get more airflow on your version.
Judging from the thermal images here (the board is still the version with larger heatsink), the chip on the left doesn't heat much, so likely the original heatsink was way too large than needed:
The issue with fans is that it is not easy to search for them, and then you can't find the model for sale, anyway I was thinking of this one 0.447 m3/min i.e. 26.82 m3/h:
https://www.delta-fan.com/FFB0412VHN-TP03.html
or this one, 0.231 m3/min i.e. 13.86 m3/h
https://www.delta-fan.com/afb0412va-00.html
but no guarantee implied, and it has to be seen if any of those can be actually found.
The Arctic ARCTIC S4028-6K is 12.06 mÂł/h.
Your current Noctua has 9.4 m3/h, so the ARCTIC one has some 30% more "juice".
It sounds reasonable, but since in SwOS you have less freedom (actually none, if I understand correctly) with fan speed settings, I think you are tright that it might be a trial and error game.
Or run RouterOS on it. It takes a little getting used to the bridge/VLAN configuration, but it’s much more powerful/flexible.
And you can set more of the fan settings on RouterOS. I have a dozen of these deployed throughout my network, some indoors, most outdoors. (Of course, the outdoor version [NetFiber 9] has a big heat sink and no fan…)
