HP Z800 Loud Rear Case Fan

[Deluxecars41]

Hi all, I have a HP Z800 which I purchased not too long ago.  The machine itself averages 46-50 decibels of sound primarily due to the rear case chassis fans. I noticed that when I disconnected them from the board the system was as quiet as 20-35 decibels which is a significant improvement to the constant humming and buzzing of the memory and rear chassis fans together when I unplug the rear chassis fans I get error codes with 'press f1 to continue' prompt. I've replaced the rear chassis fans with one from a spare parts unit I have but the issue still persists. Should I change to liquid cooling or is there a software/BIOS fix that I can do. As a side note, the BIOS Thermal fan idle speed is on the minimum. The specifications of the system are as follows:

• 2x Intel Xeon X5680 3.33Ghz
• 24GB DDR3 10600R memory
• SAS 2TB 
• SATA SSD 500GB Samsung
• SATA 1tb HDD
• Nvidia GeForce GTX 1070 ASUS Turbo 8GB 
• 800W PSU option

The issue when I unplug or remove the rear chassis fans is that the system becomes warmer as it accumulates heat from the two processors, graphics card and memory.  If there's any solutions or suggestions to this issue please let me know.

Thanks 

[DGroves]

check the bios to see if the default fan speed has been changed to a higher setting 

replace the two rear fans with the exact HP part numbers (your current fans may have a failed speed sensor)

https://www.ebay.com/p/Wp02-HP-468760-001-Z800-Workstation-Dual-Rear-Fan-Assembly-Tested/1941972404

or try installing 4 pin voltage reducers on the fans

https://www.aliexpress.com/item/32798490255.html

replace the HP fans with noctua low noise fans

https://www.amazon.com/Noctua-NF-A8-PWM-Premium-Quiet/dp/B00NEMG62M

note that if all case fans (except for cpu fans) ramp up to full speed shortly after power on you most likely have a failed chassis ambient temp sensor which is part of the power cable and the cable needs replacement ( a z600 power cable can be used on a z800)

[Deluxecars41]

The fan idle speed is on the lowest setting possible. I've replaced the fans with a donor one from another Z800 which I have and it didn't change a thing. Could the system need a BIOS update, or should I use a Z600 rear case exhaust fan. I'll give the Z600 fans a try along with a BIOS update and I'll let you know. 

Thanks 

[DGroves]

the z800/z820 use a ambient temperature sensor that is located in the power cable this sensor can (and does fail) you can test for this by entering the z800 bios and looking at the shown temperature graph if zero or +128 the sensor or the sensor circuit on the motherboard has failed if it has failed the 2 rear and also the front case fans will run at full speed

you can use the power cable from a z600 if a z800 cable is not available, while a bit short it will fit

note that the z620/z820 power  cable does not fit the previous models