UNAS Pro Custom Fan Control via systemd

The UNAS Pro’s stock fan control tends to run fans aggressively or inconsistently under modest loads. This documents replacing stock fan behaviour with a community-maintained PWM script that runs as a persistent systemd service, with temperature targets tuned for Seagate IronWolf drives.

Script source: hoxxep/UNAS-Pro-fan-control

I have also included a section on manual control, for cases where you are not filling all drive bays and want to set a static fan speed. Personally I use this method as the device sits in a normal room rather than a climate-controlled rack, and I am only running 2x HDD — it works well under those conditions.

This would not be advisable if you are putting the UNAS under heavy load, particularly the CPU.

Background

The UNAS Pro exposes fan PWM channels through the Linux hwmon subsystem at /sys/class/hwmon/hwmon0/. Each PWM channel has an _enable control knob that determines who owns fan speed authority:

The hoxxep script requires pwm*_enable to be set before it can assert control. Without releasing the channels first, writes to the PWM registers may be silently ignored by the firmware.

Reference: Linux kernel hwmon sysfs interface documentation

Configuration Values Applied

The following values were injected into the downloaded script via sed in-place substitution. Targets are tuned for Seagate IronWolf NAS drives, which Seagate specifies with an operating range of 0-70C and recommends keeping below 60C during sustained operation.

Seagate IronWolf operating temperature specification: 0-70C (Seagate IronWolf datasheet DS1904-3-2006US).

Deployment

Step 1: Release PWM Channels to OS Control

echo 2 | tee /sys/class/hwmon/hwmon0/pwm1_enable /sys/class/hwmon/hwmon0/pwm2_enable

Sets both fan PWM channels to automatic/OS-controlled mode. tee writes the value to both paths in a single command. This must run before the script is started, otherwise firmware retains authority and the script’s PWM writes have no effect.

Step 2: Download Script and Service Unit

wget -O /root/fan_control.sh https://raw.githubusercontent.com/hoxxep/UNAS-Pro-fan-control/refs/heads/main/fan_control.sh
wget -O /etc/systemd/system/fan_control.service https://raw.githubusercontent.com/hoxxep/UNAS-Pro-fan-control/refs/heads/main/fan_control.service

Step 3: Make the Script Executable

chmod +x /root/fan_control.sh

Required before systemd can execute the script as a service.

Step 4: Apply Custom Temperature Targets

sed -i 's/^MIN_FAN=.*/MIN_FAN=30/' /root/fan_control.sh
sed -i 's/^HDD_TGT=.*/HDD_TGT=55/' /root/fan_control.sh
sed -i 's/^HDD_MAX=.*/HDD_MAX=62/' /root/fan_control.sh
sed -i 's/^CPU_TGT=.*/CPU_TGT=75/' /root/fan_control.sh
sed -i 's/^CPU_MAX=.*/CPU_MAX=88/' /root/fan_control.sh

Each sed -i command performs an in-place substitution, replacing the entire line beginning with the variable name. The ^ anchor ensures only top-level variable declarations are matched and not comment lines or occurrences within the script body.

Step 5: Enable and Start the Service

systemctl daemon-reload
systemctl enable fan_control.service
systemctl restart fan_control.service

Full Deployment Block

All steps combined for clean execution:

# 1. Return fan control to the OS so the script can take over cleanly
echo 2 | tee /sys/class/hwmon/hwmon0/pwm1_enable /sys/class/hwmon/hwmon0/pwm2_enable

# 2. Download the fan control script and the system service file
wget -O /root/fan_control.sh https://raw.githubusercontent.com/hoxxep/UNAS-Pro-fan-control/refs/heads/main/fan_control.sh
wget -O /etc/systemd/system/fan_control.service https://raw.githubusercontent.com/hoxxep/UNAS-Pro-fan-control/refs/heads/main/fan_control.service

# 3. Make the script executable
chmod +x /root/fan_control.sh

# 4. Inject custom temperature targets
sed -i 's/^MIN_FAN=.*/MIN_FAN=30/' /root/fan_control.sh
sed -i 's/^HDD_TGT=.*/HDD_TGT=55/' /root/fan_control.sh
sed -i 's/^HDD_MAX=.*/HDD_MAX=62/' /root/fan_control.sh
sed -i 's/^CPU_TGT=.*/CPU_TGT=75/' /root/fan_control.sh
sed -i 's/^CPU_MAX=.*/CPU_MAX=88/' /root/fan_control.sh

# 5. Reload systemd, enable on boot, start immediately
systemctl daemon-reload
systemctl enable fan_control.service
systemctl restart fan_control.service

Manual Fan Control (Override Mode)

The commands below allow direct PWM control without the script running. Useful for testing fan behaviour, diagnosing noise issues, or forcing a fixed speed during maintenance. This does not require the systemd service to be active.

Enter Manual Mode and Set a Fixed Speed

# 1. Disable automatic fan control and switch to manual mode
echo 1 | tee /sys/class/hwmon/hwmon0/pwm1_enable /sys/class/hwmon/hwmon0/pwm2_enable

# 2. Set fans to a specific raw PWM value (0 = off, 255 = full speed)
echo 75 | tee /sys/class/hwmon/hwmon0/pwm1 /sys/class/hwmon/hwmon0/pwm2

# 3. Verify fan RPMs and check temperatures
sensors

The raw PWM value 75 is an approximate starting point for ~1800 RPM, but the exact value that hits your target RPM will vary depending on the specific fans fitted. Identify your precise value by incrementing the number and checking sensors output until the reported RPM matches your target. Document the confirmed value here once found.

Note: pwm*_enable=1 (manual mode) persists only until reboot. If the system restarts while the service is enabled, systemd will bring the service back up and restore automatic control.

Return to Automatic Mode

Run this single command to hand control back to the OS or to the custom script:

# Return fan control to automatic mode
echo 2 | tee /sys/class/hwmon/hwmon0/pwm1_enable /sys/class/hwmon/hwmon0/pwm2_enable

This is the same command used in Step 1 of the deployment. After running it, restart the service if it is not already running:

systemctl restart fan_control.service

Verification

Check Service Status

systemctl status fan_control.service

Expected output includes Active: active (running).

View Live Logs

journalctl -u fan_control.service -f

The -f flag follows log output in real time, useful for confirming the script is reading temperatures and adjusting PWM values as expected.

Read Current PWM Values

cat /sys/class/hwmon/hwmon0/pwm1
cat /sys/class/hwmon/hwmon0/pwm2

Values range from 0 (off) to 255 (full speed). At idle with drives around 40C, a MIN_FAN=30 setting corresponds to a duty cycle of approximately 30/255 (~12%), which is in the 1800 RPM range on typical fans.

Read Current Temperatures

cat /sys/class/hwmon/hwmon0/temp1_input

Values are reported in millidegrees Celsius. For example, 42000 = 42C.

Persistence After Reboot

systemctl enable creates a symlink in the appropriate wants directory so the service starts during the boot sequence. Confirm this with:

systemctl is-enabled fan_control.service

Expected output: enabled

The pwm*_enable values written in Step 1 are not persistent across reboots by default. The fan_control.service unit from the hoxxep repository includes a pre-start step that re-applies those values before the script runs. Confirm this is present in the service file if behaviour after reboot is unexpected:

cat /etc/systemd/system/fan_control.service

Look for an ExecStartPre line writing to the pwm*_enable sysfs paths. If absent, add the following to the [Service] block:

ExecStartPre=/bin/sh -c 'echo 2 | tee /sys/class/hwmon/hwmon0/pwm1_enable /sys/class/hwmon/hwmon0/pwm2_enable'

Then reload and restart:

systemctl daemon-reload
systemctl restart fan_control.service

References

• hoxxep/UNAS-Pro-fan-control (GitHub): https://github.com/hoxxep/UNAS-Pro-fan-control
• Linux kernel hwmon sysfs interface: https://www.kernel.org/doc/Documentation/hwmon/sysfs-interface
• systemd.service man page: https://www.freedesktop.org/software/systemd/man/systemd.service.html
• Seagate IronWolf datasheet (DS1904-3-2006US): https://www.seagate.com/files/www-content/datasheets/pdfs/ironwolf-12tb-DS1904-3-2006US-en_US.pdf