The Fan Speed Percentage function locks your miner’s cooling fans to a fixed duty cycle—a set percentage of full speed—instead of letting the firmware vary them automatically with temperature. It lives in the fan-control section of the stock Antminer web interface as a checkbox with a value field. In almost every case you should leave it unchecked: the firmware’s automatic, temperature-driven control is both safer and more efficient than any number you can type in. This guide explains exactly what the setting does, how to use it correctly on the rare occasions it helps, and the failure modes it invites when misused.
How ASIC fan control actually works
Antminer-class machines cool their hashboards with 4-wire fans. Two wires carry the 12 V supply and ground, a third carries a PWM (pulse-width modulation) control signal that sets fan speed, and the fourth is a tachometer line that reports actual RPM back to the controller. The control board generates the PWM duty cycle on dedicated channels and reads the tach feedback continuously, so the firmware always knows both what speed it commanded and what speed the fan is really turning.
In the default automatic mode, that feedback drives a closed control loop. On-die thermal diodes inside the ASIC chips and I2C sensors (typically LM75/TMP75-class parts) on each hashboard feed temperature into a controller that continuously trims fan PWM to hold the boards near a target temperature. When chips warm up, the fans spin faster; when they cool, the fans back off. Above a defined hot threshold the firmware forces the fans to 100%, and above a dangerous threshold it triggers an emergency shutdown to protect the silicon. This is the fan curve the manufacturer tuned for the hardware, and it reacts to conditions far faster and more precisely than a fixed number can.
What the Fan Speed Percentage setting changes
Checking the box switches that hashboard from closed-loop temperature control to fixed-duty (manual) mode. The value you enter—say 80—commands the fans to a constant 80% PWM duty cycle regardless of chip temperature. The loop no longer speeds the fans up as the boards heat, nor slows them as the boards cool; the duty cycle just sits where you set it.
Two behaviors surprise people here:
- There is a low-speed floor. The firmware will not let a percentage command drive the fans below roughly 1000 RPM, because stalled or near-stalled fans provide effectively no cooling and can trip fan-fail logic. Setting a very low percentage clamps at that floor rather than going lower.
- Commanded percent is not the same as RPM. PWM duty is an open-loop command in manual mode—actual RPM depends on the specific fan, its bearing wear, supply voltage, and static pressure in the chassis. Expect the tachometer to read somewhat above or below a naive “percent of max RPM” estimate.
How to enable it (and confirm it took effect)
- Log into the miner’s web interface on your LAN and open the fan-control or miner-configuration section.
- Enter the desired percentage in the field, then tick the checkbox next to it—both are required, the value alone does nothing.
- Click Save, then reboot the miner so the setting is applied to the control loop.
- After it comes back up, open the status/monitoring page and watch the per-fan RPM and per-board temperatures for 15–30 minutes under full hashrate. Confirm the fans hold your commanded speed and that board and chip temps stabilize below the firmware’s hot threshold—not creeping upward.
To revert, uncheck the box, save, and reboot; the miner returns to automatic temperature-driven control.
When a fixed percentage is legitimately useful
This is a niche tool, not a tuning knob for daily use. Reasonable cases include:
- Diagnostics. Forcing fans to a known speed while you watch temperatures helps isolate whether a thermal problem is airflow-related or heat-generation-related.
- Acoustic or airflow management in a fixed environment. In a climate-controlled room with a stable, low ambient temperature, a knowledgeable operator may cap fans to trim noise—accepting that the safety margin shrinks.
- Bench and repair work. On a test bench you may want a predictable, constant airflow rather than a loop that ramps as the load changes.
Outside situations like these, the automatic curve wins. It is worth stressing that a miner does not need a manual fan setting to “run cooler”—lowering hashrate or power target, improving intake air temperature, and cleaning airflow paths all do more for temperature than overriding the fans ever will.
The risks of a fixed fan speed
A fixed percentage removes the firmware’s ability to react, and that cuts both ways:
- Set too low, the miner overheats. Ambient temperature climbs, dust accumulates on the boards and heatsinks, or a fan degrades—and the fixed duty cycle does not compensate. Boards drift toward the hot threshold, and you invite thermal throttling, accelerated silicon aging, or a protective shutdown. Sustained high temperatures are one of the most common root causes of hashboard failure.
- Set too high, you waste power and wear the fans. Running fans harder than the boards need burns extra watts at the wall and shortens fan bearing life for no hashrate benefit.
- Fixed speed makes the machine fragile to change. Because it cannot track shifting ambient conditions, a setting that is safe on a cool night can be dangerous on a hot afternoon. The automatic loop absorbs that swing; a hardcoded percentage does not.
Note that fan-fail and over-temperature protection still run in manual mode—if a fan drops below the expected RPM or a board crosses the dangerous threshold, the firmware will still alarm, throttle, or shut down. Manual mode removes proactive control, not the last-ditch safeties.
Common mistakes
- Typing a percentage but forgetting to tick the checkbox—nothing changes and the operator assumes it did.
- Setting a low percentage to quiet a noisy machine whose real problem is a failing fan or clogged heatsink. Fix the airflow, don’t mask it.
- Confusing commanded percent with guaranteed RPM and under-provisioning cooling as a result.
- Leaving a manual setting in place after moving the miner to a warmer room or a different season.
When to stop and get it checked
If your machine only stays within temperature at a high manual fan percentage—or throttles and shuts down on automatic—the fans, heatsinks, or thermal interface are telling you something. Overriding the fan curve to keep it hashing is treating a symptom. Inspect for dust-clogged fins, dried-out or missing thermal paste, seized fans, and blocked intake before you trust the numbers.
Related: Walk the airflow and temperature checks in the ASIC fault finder, pull the correct settings screens from the miner manuals, source replacement fans and heatsink hardware from ASIC repair parts, or hand a persistently overheating machine to the bench via start a repair.
