Synology NAS Lüftertausch: Was das Dezibelmessgerät nicht erfasst hat

I did a Synology NAS fan replacement on my DS1621xs+ because my family kept complaining about the noise in the living room. I could hear the rack from my bedroom, five metres away, through a closed door. The swap took 27 minutes of physical work and about 23 minutes of node downtime. Afterwards the room went silent, and when I compared the sound level meter readings before and after, the number had not dropped at all. It had gone up by 0.8 dB. This article is the full measured record of that Synology NAS fan replacement, including the part where the data contradicted the result, and the part where I admit I bought the wrong fan.

SynoPower Club Point: everyone shopping for quieter NAS fans compares one number, and that number is dB(A). I measured 48.2 dB(A) before and 49.0 dB(A) after. By that number the swap was a failure. By every other measure it was the best thing I have done to that machine all year. What actually changed was that the two fans stopped fighting each other.

What is a Synology NAS fan replacement, and when is it worth doing?

A Synology NAS fan replacement means removing the rear case fans your NAS shipped with and fitting aftermarket ones. On a six-bay unit like the DS1621xs+ there are two 92 mm fans on the back panel, held on with four screws each and plugged into a four-pin PWM header on the chassis. Mechanically it is a twenty minute job with one screwdriver.

The reason people do it is almost always noise. The reason it works is usually not the one they expect. My stock fans were Y.S.TECH FD129225 units rated DC12V 0.32A, and after five years they were caked in dust and no longer spinning at the same speed as each other. That second detail turned out to be the whole story.

It is worth doing if the noise bothers someone in the room. It is not worth doing for temperature, and as you will see below, it can easily make temperature worse.

Was the Synology NAS fan replacement about volume, or about pulsation?

Before starting the Synology NAS fan replacement I logged both fan speeds and the CPU temperature every five seconds, straight off the hardware monitor, and left it running all day. That gave me 420 samples of the old fans and more than 2,500 of the new ones. The number that mattered was not the speed itself. It was the gap between the two fans.

Measurement	Old fans	New fans
Average gap between fan 1 and fan 2	40.3 RPM	16.3 RPM
Resulting beat frequency	0.67 Hz	0.27 Hz
Samples	420	2,531

Two fans running at slightly different speeds produce sound waves that drift in and out of phase. The loudness rises and falls at the difference frequency. Forty RPM of mismatch works out to roughly 0.67 Hz, which is a swell about every 1.5 seconds. That is squarely inside the band of amplitude modulation that human hearing is most sensitive to, and low-frequency modulation passes through walls and doors far better than steady broadband noise does. Steady noise gets filed away as background. A sound that pulses does not.

How do you empty a NAS before you open it?

My DS1621xs+ is one of two nodes in a Virtual Machine Manager cluster, and it was hosting the VMs behind several live websites, a mail server and a reverse proxy. I could not simply power it off. VMM Pro protection plans let me snapshot each VM, replicate it to the second node, shut it down, switch over, and boot it on the other machine.

VM moved	Method	Downtime
Web and shop VM	VMM interface, by hand	303 seconds
Mail and reverse proxy VM	Scripted through the API	426 seconds
Web and shop VM, moved back	Scripted through the API	192 seconds

Three to seven minutes per VM, and the node was free. Worth being honest about what visitors experienced: pages served from the Cloudflare edge cache never went down at all, because the edge kept serving them. Anyone trying to log in, add to a cart or check out during those windows was stopped for the full few minutes. Cached browsing and live transactions are not the same thing, and only one of them was protected.

One thing I did not plan for: emptying the VMs does not empty the host. That NAS was also running Surveillance Station with eight cameras attached, and those eight cameras stopped recording for the entire 23 minutes the machine was off. Check what packages are running on the host, not just what VMs are on it.

Synology NAS fan replacement in 4 steps

What did the sound level meter actually record?

I took a sound level reading before and after the Synology NAS fan replacement with a phone app, 20 cm from the front of the chassis, before and after. These are not calibrated laboratory figures and I am not going to pretend otherwise. But they were taken the same way on the same day with the same phone, so the comparison between them is fair even if the absolute values are not.

Reading	Old fans	New fans
Average dB(A)	48.2	49.0
Sample length	13 minutes	1 min 23 sec
Room temperature	27.8 degrees Celsius, 56 percent humidity

The new fans measured 0.8 dB louder. Within the error of a phone app that means identical. And yet the subjective result went from two people complaining in the living room and the noise being audible five metres away through a door, to the rack being completely inaudible from the next room. If I had chosen fans by the spec sheet number alone, I would have concluded this swap did nothing.

Why did the CPU run 8 degrees hotter?

This is the part of the Synology NAS fan replacement that went wrong. I bought a Noctua NF-A9x14, which is the low profile 14 mm version. The fans I took out were 25 mm thick. The screw holes are identical, it fitted perfectly, and I did not think about it until I looked at the logs.

Load average	Old fans	New fans	Difference
0 to 1.5	1801 RPM / 63.7 C	1506 RPM / 72.1 C	plus 8.4 C
1.5 to 2.5	1903 RPM / 64.3 C	1529 RPM / 72.6 C	plus 8.3 C
2.5 to 3.5	2085 RPM / 65.8 C	1549 RPM / 73.0 C	plus 7.2 C

Both sets of readings were taken in the same DSM fan mode, binned by system load so I was comparing like with like. The penalty is remarkably consistent at around eight degrees across the whole load range. Drive temperatures stayed fine, between 34 and 40 degrees for the hard disks and 39 to 43 for the NVMe cache, and DSM still reported thermal status as normal. Nothing is in danger. But eight degrees of headroom is eight degrees I gave away for nothing.

Worth being precise about the mechanism: the new fan is not running at its limit. It is rated to 2200 RPM and it was only turning at around 1550. DSM drives fan PWM from temperature, and 73 degrees sits inside the range it considers acceptable, so it never asks for more. Switching DSM from Quiet mode to Cool mode bought only 60 RPM and 0.3 degrees. The eight degrees is a new thermal equilibrium, not a cooling failure.

How does fan desynchronisation make noise carry?

When two sound sources sit close together at almost the same frequency, the combined amplitude swells and fades at the difference between them. This is the same phenomenon a piano tuner listens for when two strings are slightly apart. With fans, the frequencies involved are the rotation rate and the blade passing rate, and the modulation you end up hearing follows the speed difference.

Fans do not leave the factory mismatched by 40 RPM. They drift apart as bearings wear and dust loads up unevenly, which means this problem gets worse gradually over years and you acclimatise to it. I had stopped noticing when it started. I only knew it had been happening once it stopped.

The practical consequence is that a pulsing 48 dB is far more intrusive than a steady 48 dB. Steady broadband noise masks into the ambience of a room within a few metres. A slow amplitude swell keeps drawing attention back to itself, and it loses much less energy passing through a wall. That is why the fix worked despite the meter saying nothing had changed.

Which fan should you buy for a Synology NAS fan replacement?

Not the one I bought. Before any Synology NAS fan replacement, measure the thickness of the fan you are removing first. If it is 25 mm, buy a 25 mm replacement.

Spezifikation	NF-A9x14 (what I fitted)	NF-A9 (what I should have fitted)
Thickness	14 mm	25 mm
Maximum speed	2200 rpm	2000 rpm
Maximum airflow	50.5 m3/h	78.9 m3/h
Maximum static pressure	0.92 mmH2O	about 2.28 mmH2O

Note that the slim fan spins faster yet moves 36 percent less air, and has well under half the static pressure. Static pressure is the figure that matters in a NAS, because the air has to be forced through a dense stack of drives rather than across an open space. That single specification explains the whole eight degrees.

The deeper point about this Synology NAS fan replacement is that the noise was never caused by the stock fans being loud. It was caused by two worn fans falling out of step. A fresh pair at the original thickness would have fixed the pulsing exactly the same way, kept the full airflow, and probably run cooler than the originals did, because the originals were filthy. The quiet was free. The eight degrees was a mistake I made at the checkout page.

Where can I find more official resources?

If you are planning a Synology NAS fan replacement, Noctua publishes full specification sheets for both the 14 mm and 25 mm versions of the 92 mm fan, including airflow and static pressure curves, which is the data you need before committing to a thickness. Synology documents the fan speed modes in DSM and what each one targets. Your own unit hardware installation guide will tell you how the rear panel comes off, which varies between chassis.

I filmed the swap itself, including the sound level readings taken the same way before and after. The audio is the original recording and is not normalised, so the pulsing in the first section is what my family was actually hearing across the room.

For the migration side, the Virtual Machine Manager documentation covers protection plans, replication and switchover, which is what made the maintenance window possible at all.

I recorded the migration itself in real time with nothing cut, so you can see exactly how long each stage of a protection plan switchover takes.

This second recording shows what the surviving node displays while the other one is powered off, including the protection plans turning red and the cluster reporting itself frozen.

What are the limits and risks of a Synology NAS fan replacement?

Opening the chassis will affect your warranty. Synology does not sell these fans as user-serviceable parts on most models, and if the unit is still under warranty the correct route is a support ticket, not a screwdriver. I accepted that trade on a five year old machine. You should make that decision deliberately rather than halfway through the job.

In any Synology NAS fan replacement the fan headers are four-pin PWM, and the speed signal matters. If DSM cannot read a tachometer from a fan it will raise a fan failure alert and may run the remaining fan at full speed. Fit fans that report speed properly, and check after booting that both are being reported.

Plan the downtime around everything on the host, not just the virtual machines. Mine cost eight security cameras 23 minutes of recording that I had not accounted for, and in a two-node cluster the surviving node freezes rather than failing over, because high availability needs at least three nodes. That is normal, but it looks alarming if you have not seen it before.

Do not assume a thinner fan is a free upgrade because it fits. The mounting pattern is the same at every thickness, so nothing will stop you, and the consequence only shows up in temperature logs days later.

Measure before and after, and log continuously rather than taking single readings. A single spot reading of fan speed tells you almost nothing because the controller moves constantly with load. The eight degree penalty only became visible once I binned thousands of samples by system load.

Finally, remember that ambient temperature sets the floor. These readings were taken at 27.8 degrees in a Taiwanese autumn. In a hotter room, or in summer, the same Synology NAS fan replacement would land eight degrees above a higher starting point, and the margin I am comfortable with today would be thinner.

Verweise

• Noctua NF-A9x14 PWM specifications for the 14 mm airflow and static pressure figures
• Noctua NF-A9 PWM specifications, the 25 mm version used for comparison
• Synology Knowledge Center on fan speed modes in DSM
• Virtual Machine Manager protection plans, used for the zero to seven minute migrations in this Synology NAS fan replacement
• Synology DS1621xs+ product page for the chassis specification

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