OK, so the server's pretty much built I'm just waiting for some more (4) HDDs to come. While I was waiting I run a 4 passes memtest and now try to configure the fans. Obviously it's not a proper test yet as I am missing my drives but it's a good time to play around.
I have 3 Noctua NF-A14 PWMs as case fans and an NF-F12 PWM on the CPU with a X9SRE-F mobo. I set the threshold according to manufacturer specs and also modified and connected a fan splitter as described in OP.
I run into two small problems :
1) when setting the fan mode in IMPI's webgui it just won't stick and defaults itself to full speed. It's the same when do it from the console. I managed to set it once and kept changing things and I could not set it again. Even a reboot didn't help. Then again after countless tries it's on OPTIMAL so I won't touch it ;)
[UPDATE] after using the ipmi raw command from shell there is no problem setting it
Code:
ipmitool raw 0x30 0x45 0x1 0x0 (Set Mode to Standard)
ipmitool raw 0x30 0x45 0x1 0x1 (Set Mode to Full)
ipmitool raw 0x30 0x45 0x1 0x2 (Set Mode to Optimal)
ipmitool raw 0x30 0x45 0x1 0x4 (Set Mode to Heavy I/O)
ipmitool raw 0x30 0x45 0x0 (Get Fan Mode)
2) I don't know if this related but all fans (incl. CPU) spin around 300-450 when the server idle except one which spins almost full speed (1,350) and nothing changes it.
[UPDATE] this behavior disappeared after a few reboots and reinstalls and after a couple months switch off ;)
Any suggestion?
I'll will disconnect it and swap headers, although not on the mobo (as that obviously only used for reporting RPM) but on the splitter itself so I'll see whether the other one will spin up and this down.