I was really surprised to find out that HP does not ship any fan control utility with Z800 (I have 2 x X5675).
The problem with my system was that BIOS relies on CPU sensors which are located on the mainboard and in my case they were showing 20 C less than actual core temps (HWMonitor). This have resulted in a CPU fan rpm to be very low because system didn't know that core temps were actually around 80 C as motherbpard CPU sensors have indicacted 59C.
Luckily, there is a solution and it's called SpeedFan utility. It's not easy to set it up, but I can post my setup screenshots.
You can control both CPU fans and I think that the third one are memory fans.
However, I do not reccomend using it unless you don't know what you're doing.
Cheers...