Hello,

Quick question regarding the two fans on 1100AHX2.
I have noticed that only one fan is working at any given time so I opened the case to investigate (brand new unit)
I see two fans, each with it's own plug on the router board.
If I unplug one fan, after one second the other fan will start.
If I unplug the other fan, the same thing.
So both fans work and both plugs work, but not at the same time.

As far as I can tell, routerOS sees one fan plug as main and the other as auxiliary.
Given that both stock fans are housed in the same air flow, why separated them like that?

I inserted both fan pins into a joining plug, so I can power both of them from the main plug on the router board.
This way, under heavy load the noise will be lower, both fans being better than one, air flow wise

Did I miss something?
Is this normal? Or my unit is miss behaving?

So, judging by the lack of replies, I am the only one in this boat?

It is normal behavior, if one fan failed, other one takes over.

Sent from my Nexus 7 using Tapatalk

Did I miss something?

Yes, you apparently missed the users's manual for the device in question, where on page 4 it says this:

Only one Fan in each pair will rotate, other will start when first fails.
Only one pair can be connected at the same time

So the second fan is designed to work as a spare. The language that hints at more than one "pair" being able to be connected is left over from the original RB1100, which had 4 fan headers: 2x 12v and 2x 3.3v. As of the AH mark 2, Hx2, and AHx2, 3.3v fan support was removed from the board. So only one "pair" can be connected (since there are only 2 headers now), and like the manual says, "only one fan in each pair will rotate, [and the] other will start when first fails."

As far as I can tell, routerOS sees one fan plug as main and the other as auxiliary.

That should have been a clue to you that the fans were operating as designed. If, instead of unplugging the rotating fan, you did "/system health set use-fan=auxiliary" you would see that the spinning fan would shut down and the spare fan would spin up. There is no way to make both of them rotate (at least without the Y-cable hack that you put in place on your board).

-- Nathan

IMHO it is better to use two fans at half speed than one fan at full speed. Fans will last much more and there will be noticeable less noise. Take it as idea for future design.

Thank you for the clarification.
Indeed, I missed the part about fans in the manual.
Nevertheless, I think it is not the best choice, given that one fan at full speed will fail faster than two fans at moderate speed. Not to mention the noise.
Then again, I am not a hardware designer/engineer, so I cannot be 100% sure.

Presumably a similar "fix" could be applied in firmware - always power both fans, but at half the rate you would have done a single fan. The advantage to this would be a) you could be notified about fan failure b) no messing with Y-cables.

Fans are really too noisy. Will try to replace them with SUNON MagLev-s, they've declared NL around 12dB only and they have 3-wire 40x20 model.

But fans mount in this particular model is crippled — there is a need to remove CPU heatsink plastic cover, in order to access but. I would suggest reconsider using long screws and mount fans on self-tapping screws like in a PC PSUs.