The Story of the Fans in a Dell Poweredge T710

Recently I got an old Dell Poweredge T710 to play with. It is a very sturdy machine and a great virtualization environment despite its age. It can be used as a 5U rackmount or a standalone tower. Having the tower option would seem to imply that it can sit quietly in a corner of the office and do its stuff, which would be mostly true except (unsurprisingly) for the cooling fans. Indeed, the machine features four big honking high power fans in a redundant setup. I believe that they happen to be Delta FFC0912DE; I might be wrong on the model, but they are surely rated for some 110 CFM at 4800 RPM and 55 dBA. Put four of those together and you got a machine that surely gets noticed.

Granted, under Dell's power management the fans will spin at about one third of the rated RPM and so ultimately they are not horribly loud; in fact they are more silent than I expected for the air they move. The noise is however noticeable and still makes the machine unsuitable for an office setting. I therefore started looking for replacement fans. Unsurprisingly enough I am not the only one. The fan that is universally quoted as a good replacement is the Arctic F9 PWM, which is rated for 43 CFM at 1800 RPM and 23.5 dBA. That's mighty quiet alright, but it has certain disadvantages, as follows.

For one thing, the T710 was not designed for this airflow. The four fans are redundant, meaning that two of them are enough to provide adequate airflow, meaning in turn that the machine was designed for an airflow of 220 CFM at full load. If one does not care about redundancy then the same airflow can be provided by four fans, which makes each of these fans responsible for at least 55 CFM. The Arctic F9 falls short of that by about 20%. In practice I would like to maintain at least one redundant fan and so I would like 70 CFM per fan. The reason is that the T710 does not have separate fans on the CPU coolers; the four fans cool down everything, including the hard disks, the RAM, and of course the CPUs. The Arctic F9 would be below requirements by about 40% for such a mildly redundant setup.

The above notwithstanding, I guess that the F9 would provide a decent alternative as long as the loads are kept on the low side. With the stock fans the CPU temperature hovers around 38C at idle, which offers ample room for reducing the air flow. However, the Arctic F9 is a low RPM fan which tends to panic the iDRAC controller. Indeed, the iDRAC controller's threshold for determining if a fan is working is 1000 RPM. At idle, the same controller spins the fans at about one third, which for the Arctic fans means 600 RPM. This being below the threshold causes the iDRAC to signal the failures of all four fans, try desperately to spin all of them fully up, recovering from failure, spinning the fans back down, and so on in an endless cycle. Ways around this include hacking or otherwise misleading the iDRAC in software. A hardware solution also exists and provides a fake PWM signal back to the iDRAC. I am not a big fan (see what I did here?) of any of these solutions (except perhaps the iDRAC hacking), on the principle that if you have the iDRAC you may as well use it rather than trick it into believing that all is A-OK no matter what, so that it can keep providing feedback in the event of a real failure.

For all of the above reasons I regard the Arctic F9 as a bad replacement for the fans in the T710. Fortunately, I also have a solution: I found a great replacement in the Bgears b-PWM 90. This is a solid 2-ball fan rated for 88 CFM at 4000 RPM and 37 dBA. That's plenty of air flow for maintaining a one-fan redundancy. The fan is noisier than the Arctic F9, but in practice the noise level is more than acceptable in an office environment. In fact the fans are nearly silent spun up one third (which is the most common mode). In addition, the fans will not go below the iDRAC threshold so they are a drop-in replacement with no hacks needed.

Are we out of the woods yet? Yes and no. For one thing, replacing the fans (with either the Bgears or the Arctic ones) does require soldering, as the Dell fan connectors are proprietary. Secondly, the server is pretty much silent as long as the ambient temperature is reasonable and the power draw is below about 400W (with a dual power supply, the threshold may be substantially lower for a single PSU setup). Once either condition is exceeded, even if the system fans remain silent, the PSU fans spin up. They are 40mm (or so) fans and are pretty loud at high RPM. I found no reasonable replacement for those, so for the time being I just live with the occasional higher fan noise. I am still investigating though. In the meantime, enjoy your (mostly) silent big honking server!