The NL-ACF1 is an optional 80mm fan, sold separately, that clips onto the pump's magnetic faceplate in place of the Noctua logo badge, directing airflow across nearby VRMs, RAM, and M.2 drives. It uses the same impeller as Noctua's NF-A8 fan in a custom frame that leverages the Coandă effect for better flow distribution, spinning up to 2200 RPM.

Noctua's own testing (Ryzen 9 7950X, MSI X670E Carbon, NL-LC1-36 top-mounted) found the fan genuinely useful, though rarely essential:

Case/radiator fan speed	VRM temp, fan off	VRM temp, NL-ACF1 fullspeed
Case fans off	31.54°C	19.00°C
Case fans at 40%	31.08°C	18.75°C
Case fans at full speed	19.14°C	16.95°C

In practice, this means the NL-ACF1 lets you turn other case fans down to 40% (a meaningful noise reduction) while keeping VRMs just as cool as running everything at 100% without it — and even with case fans off entirely, the single NL-ACF1 matched the VRM temperature that full-speed case fans alone achieved. Noctua's own conclusion is that it's rarely strictly necessary — mainly relevant if you have a hot-running VRM design, are pushing an aggressive overclock, live somewhere with high ambient temperatures, or have generally poor case airflow — but it's a cheap, quiet way to buy some thermal margin or trade fan noise elsewhere in the system.