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CRS328-24P-4S+RM installation of fan3 and fan4

Posted: Thu Jul 04, 2019 12:39 pm
by paulkmoore
Hi all,

First post here, so please feel free to redirect if required :)

I've just received my CRS328-24P-4S+RM which looks excellent; however, as others have noted here the "typical" fan noise is not "house" friendly. I'm therefore planning to replace the existing two fans with Noctua NF-A4x20 PWMs.

The installed fans are Sunford AD4028V12BLBGF's. I've not been able to find the exact specs for this fan, but from what I can deduce from the Sunford AD4028GF series specs the closest match is the AD4028V12_LWGF with 11.61 CFM airflow and 12.15 mmH2O static pressure. By comparison, the Noctua NF-A4x20 PWMs have 5.5 CFM airflow and (only) 2.26 mmH2O static pressure.
The static pressure is my main concern here, as my PoE usage will be limited and with low wattage requirements. Accordingly, I'm planning to replace the existing Sunfords and install two further fans (4 in total) in the fan3 and fan4 slot.

My question regards the direction (push or pull) that I should install fan3 and fan4. fan1 and fan2 as currently configured are operated in pull mode (air pulled from inside the case to outside). My inclination is therefore to install fan3 and fan4 in push mode, so that they draw cool air into the case, and in general the airflow is maintained right to left across the unit.

Does anybody know in which direction fan3 and fan4 are intended to operate?

Thanks, Paul

Re: CRS328-24P-4S+RM installation of fan3 and fan4

Posted: Thu Jul 04, 2019 2:29 pm
by mkx
My inclination is therefore to install fan3 and fan4 in push mode, so that they draw cool air into the case, and in general the airflow is maintained right to left across the unit.

If you do that, you could create a 'circling' path for the air: fan3 and fan4 suck air in, it passes from right ot left, fan1 and fan2 push it outside, where (part of it) passes from left to right and fan3 and fan4 suck it in (again) ...
The problem can become serious if space behind the RB is not really open (e.g. inside some smaller equipment rack or if router is mounted between longer servers) which makes harder for hot air to move away from the back side.

Most of gear I've seen maintains single direction of air across the device and usually that's front-to-back (most of rack installations assume cold zone in front and hot zone at the back). So if you find additional fans necessary, I'd go for the same air flow direction (front to back).

Re: CRS328-24P-4S+RM installation of fan3 and fan4

Posted: Thu Jul 11, 2019 6:44 pm
by paulkmoore
Hi Metod,

Thanks for the reply. The fans in the CRS328-24P-4S+RM are located on each side of the unit, so the airflow is right to left (when looking at the front of the switch). There are no rear fans or vents, so no "circling" I think.

I had a response from Mikrotik support which corroborates my intent;
"you should put them in "reverse" mode. Since 1&2 push air from the case, 3&4 fans should push air into the case."
Images of the before and after here:

Before
After

And all seems to be well - note the switch is running slightly warm as it's in a cupboard :?
> /system health print
     cpu-temperature: 49C
   power-consumption: 10.5W
  board-temperature1: 35C
        psu1-voltage: 26.4V
        psu2-voltage: 52.7V
        psu1-current: 0A
        psu2-current: 0.2A
          fan1-speed: 1590RPM
          fan2-speed: 1605RPM
          fan3-speed: 1530RPM
          fan4-speed: 1545RPM