There are 2 routers. Very similar load (20-30% CPU). One of them never activates fan, and the other never turns it off 
That’s the problem. I don’t understand why?
I also have another problem: the third router. It’s not connected to the network, it’s idle on my desk now, and his fan is constantly at 10000-11000 RPMs! Why? Why 3 different routers works in different way?
[admin@MikroTik] > /system health print
fan-mode: auto
use-fan: main
active-fan: main
voltage: 12V
current: 744mA
fan-speed: 10526RPM <-------------
temperature: 34C
cpu-temperature: 32C
power-consumption: 8.9W
[admin@MikroTik] > /system resource print
uptime: 1h2m50s
version: 5.12
free-memory: 1525256KiB
total-memory: 1555212KiB
cpu: e500v2
cpu-count: 2
cpu-frequency: 1066MHz
cpu-load: 0%
free-hdd-space: 24932KiB
total-hdd-space: 41768KiB
architecture-name: powerpc
board-name: RB1100AHx2
platform: MikroTik
Edit: yeah, I can hear that 10krpm fan. It’s very very loud.