Rys Sommefeldt

Deciso DEC3862 fan swap

Wednesday, Feb 11, 2026 · 1102 words · approx 4 mins to read

My home router is a rack-mountable Deciso DEC3862 and, as is common with rack mount hardware at 1U height, it’s got a reliable yet noisy fan. The biggest issue isn’t the noise of the airflow, but rather the sound the bearing makes. It’s annoying enough when the fan is spinning at constant speed, but becomes very noticeable and irritating if the fan speed changes.

There’s not much heat for the whole cooling system to get rid because the main system-on-chip is a comparatively low-power embedded AMD EPYC Embedded 3000 SoC (the 8-core 3251 with a 55W TDP), so even though it’s only 40x40x25mm the fan doesn’t have to work too hard to keep the router cool here in my office.

After some back and forth with the folks at Deciso about them exposing fan control to software so that I could get some more control of it that way, they came to the conclusion that they have no plans to do that, but did give me the specs of the OEM fan and the wiring diagram so I could think about replacing it with something else.

The OEM fan is a Sunon HA40201V4-1000U-A99 with 2-pin connector, meaning fan speed control happens via adjusting the voltage. Page 30 of the linked PDF gives us the key specs needed when thinking about a replacement: 5.5ft³ per minute maximum airflow, and 0.10 inH₂0 static pressure.

Quick aside: please take a second to marvel at the amazing non-SI unit for the static pressure. Inches of water.

Any replacement should ideally have very similar static pressure and airflow characteristics, with noise being the thing you want to adjust while keeping the rest of the fan’s operating properties the same. That’s because Deciso took care to design a custom fan shroud to control the cooling system’s airflow and optimise its cooling performance, so I didn’t want to compromise that with a poor choice of replacement fan.

Thankfully Noctua have the NF-A4x20 FLX which has a very comparable static pressure of 22.16Pa and a peak airflow of 9.4m³ per hour, and a 3-pin power connector. The 3rd wire is for speed sensing, so not having it connected in this application is fine. I leave research into the weird units used in the Sunon spec sheet and how to convert to the SI units I quote for the Noctua as an exercise for you, dear reader.

The stated noise level of the Noctua is quite similar to the Sunon (in fact the Sunon is supposedly quieter), so the gamble was that the overall acoustics of the Noctua’s bearing are much better than the Sunon’s, and that gamble paid off.

Swapping the fan was simple. First you undo the screws keeping the black top cover to the rest of the chassis, then you slide the top cover away from the rear and then lift it up to take it off, taking care not to damage the ground wire connected to that top cover piece. That reveals the fan shroud at the back right, holding the Sunon in place:

You can see the orange Deciso logo on top of the shroud, and the original Sunon fan is held into that with pressure from the shroud with no screws, which I really appreciate. That means removing it to fit a replacement is easy: gently pull the Sunon out and push the replacement in, making sure to get orientation for its airflow right. Air leaves out of the back of the unit, so make sure you push the new fan into the shroud in an orientation that respects that:

Then it’s a simple matter of placing the shroud back where it was, locating the small clips on the shroud that hold it to the back of the chassis into their small holes.

Put the top cover back into place and screw it down and you’re done. There’s a warranty void sticker over one of the screws but depending on where you live consumer protection laws might allow you to disregard that.

The end result was exactly what I hoped for: the router works fine with the new fan in place and its bearing is near-silent, meaning I can’t hear it at all now over the noise that some other things in the rack make.

Temperatures are slightly higher given I’ve also chosen to use the low-noise adapter that comes with the fan to quieten it down even further. It reduces peak airflow and static pressure, but because the EPYC 3251 gives off so little heat (Deciso offer a completely passively cooled model based on the same SoC, even) and the airflow from the fan isn’t necessary for cooling any other components on the NetBoard-A30 mainboard the DEC3862 is built from, it doesn’t cause any operational issues.

Absolute temperature is roughly 3C higher than with the Sunon, which is totally fine for my operating environment and the router stays well within thermal limits.

Hopefully this simple guide and recommendation for the Noctua NF-A4x20 FLX as a viable replacement helps someone else out.