I have recently inherited two Duos one of which runs the fan at maximum speed. If I swop the disks over between the Duos the problem one starts the fan off slowly but gradually gets faster until maxed out. Fan Recalibrate has no effect. The drive temps remain around 25C.
The box now with the drives from the misbehaving Duo initially runs the fan at maximum speed but at least will respond to a recalibrate and will drop the fan speed to 1600 ish. The same thing will happen if I swop the drives back over between Duos. The fan in the dodgy Duo starts slow but maxes out after 20 minutes or so.
I have read a few posts regarding fan speed problems and tried installing previous firmwares (currently 4.1.16) but cannot find the referenced 4.1.15(T3) Beta. I suspect that the add-on EnableFanMinRPMOverride wont help with maximum fan speed?
I have never had a DuoV1 and I don't currently have a disassembled NV (which I assume has a similar sensor since it uses the same firmware). But all of the NASes I have looking inside of have a very clear thermal sensor that looks like an RTD. I just looked at one on a DuoV2 board, and it's not labeled with any part number and is held to the board by a soldered wire holddown and also appears to have some thermal epoxy (or maybe more solder) underneath. So even if you could find the part number and buy one in single quantities, you'd need the proper equipment and skill to replace it.
If the NAS is kept where temperature stays pretty constant, your best bet is probably a fan speed reducer. The fixed value ones are essentially a cable with a bump, which would fit. The variable ones could give you trouble finding a spot unless you run the wires outside the case.
Being a bit handy wth this sort of stuff I took the boards out to see if the sensor was replaceable but couldn't find a sensor similar to that described. The speed reduction adapters I found advertised had 24 ohm or 47 ohm resistors in so I tried those values first and got down to 3125 rpm with 71 ohms (25 and 47 in series) but think sub 2000 rpm would be right based on my other Duo. Two thin fans in series plus 47 ohms got me 2343 rpm but the stacked fans made more noise than a single. I will try more resistance ( not futile in this case ) but may be tempted by a 2000 rpm Nanoxia.
