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Martin d6b775fa1d docs: document sustained-load thermal and PSU reality

Both cooling.md and power.md previously framed everything around
burst gaming workloads. After the 40 CU unlock landed, sustained
compute (llama-bench long runs, stable-diffusion batches) became
a real use case with different thermal and PSU behaviour.

- cooling.md: new Sustained Load Reality section with measured
  10-min llama-bench data on stock heatsink + dual P12 Max (GPU
  89.6 avg / 107 peak, CPU at TJmax, fan at ceiling, ~10% throughput
  drop). Notes that VRMs are nowhere near limit, the heatsink is
  the bottleneck. Two-option recommendation: 1500 MHz governor cap
  or better cooling.
- power.md: added a Sustained 40 CU compute @ 2 GHz row to the
  measured power table (220-230 W observed). New PSU Headroom
  Under Sustained Load subsection with the math (~280-300 W wall),
  flagging the 300 W LOP at 93% load as borderline for sustained
  full-CU compute and recommending 500 W FlexATX for that use case.

Both sections cross-link to docs/system/40cu-unlock.md where the
full thermal data table lives, to avoid duplicating numbers.

2026-05-31 00:16:56 +02:00

18 KiB

Raw Blame History

Cooling Solutions

The BC-250 requires active cooling for gaming and desktop use. This guide covers tested cooling solutions and best practices.

Stock Heatsink Limitations

Stock Configuration

Type: Passive aluminum fin stack heatsink
Fin Orientation: Vertical, front-to-back
Design Purpose: Rack-mounted passive or low-airflow cooling
Desktop Use: Inadequate without active airflow
Variants: Three heatsink variants exist (8-row and 9-row fins). Quick ID: a QR code next to the PCIe 8-pin connector indicates the 9-row variant. The variant with fewer, thicker-gauge aluminum fins may cool slightly better stock.

!!!warning "Active Cooling Required" The stock heatsink is designed for rack airflow, not desktop use. Add a fan for gaming workloads.

!!!info "Backplate VRAM Cooling Recommended" The BC-250 has VRAM chips on the backplate with no temperature sensor. Ensuring airflow over the backplate is recommended, especially for sustained gaming. Pixel artifacts during gaming can indicate VRAM overheating. Many community builds work fine with basic case airflow over the backplate — a dedicated backplate fan is ideal but not strictly required if your case has decent airflow.

Temperature Targets

Safe Operating Temperatures

Component	Idle	Light Load	Gaming	Maximum
GPU/APU Edge	40-50°C	50-60°C	65-80°C	90°C
CPU (Tctl)	45-55°C	55-65°C	70-85°C	95°C
Memory (underside)	40-55°C	50-65°C	55-70°C	80°C

!!!success "Ideal Gaming Temps" Aim for GPU temperatures of 70-80°C during gaming for optimal performance and longevity.

!!!warning "Thermal Throttling" Above 85°C GPU temperature, the system may throttle performance. Above 90°C, instability and crashes can occur.

Sustained Load Reality

The temperature targets above are for normal gaming, where load comes in bursts. Sustained compute (looped llama-bench, long stable-diffusion runs, anything that pegs the GPU for tens of minutes) is a different regime, especially after enabling the 40 CU unlock.

Measured on a BC-250 with the stock heatsink and dual Arctic P12 Max in push-pull, 10-minute sustained llama-bench at 40 CU / 2 GHz:

Metric	Average	Peak
GPU edge	89.6 °C	107 °C
Package power	136 W	223 W
CPU	96.7 °C	100 °C (TJmax)
VRM MOSFETs	57 °C	58.5 °C
Fan speed	~2950 RPM	2977 RPM (ceiling)

Sustained throughput drops ~10% over the run as the package throttles. Stock heatsink plus dual P12 Max is not enough headroom for sustained 40 CU at 2 GHz. VRMs are nowhere near their limit; the bottleneck is the heatsink shedding the heat. Two practical options:

Cap the governor at 1500 MHz (see GPU Governor). The 40 CU unlock still gives ~1.5x compute scaling at this frequency, with temps holding around 83 °C, which the dual P12 Max can sustain indefinitely.
Upgrade cooling. The stock heatsink is the limiter, not the fans. Bigger fin area, better paste, and case airflow over the backplate all help.

For 24 CU stock at gaming workloads the dual P12 Max is comfortable. The thermal reality only bites under sustained full-CU compute load.

Backplate VRAM Cooling Solutions

The VRAM chips on the backplate benefit from active cooling, especially under sustained load. If you see pixel artifacts during gaming, VRAM temps may be the cause.

Recommended Methods

1. Thermal Pads on VRAM (Recommended):

Apply 2mm thermal pads directly on backplate VRAM chips
Preferred over thermal adhesive (better conductivity, removable)
Attach aluminum heatsink or plate if available
Works well when combined with rear airflow

2. Rear Case Airflow:

Install or open case vents behind backplate
Use positive case pressure to force air over VRAM
Position intake fans to direct airflow across backplate
Ensure no obstructions blocking backplate area

3. Secondary Fan (Most Effective):

Mount 80mm fan with spacers positioned to blow directly over backplate
Fan can be mounted inside case pointed at backplate
Ideal: 80mm fan at 50-100% speed continuously
Can use J4003 header or fan splitter

!!!success "Best Practice" Combine thermal pads + rear airflow for optimal backplate temperatures. At minimum, ensure active airflow over the backplate surface.

!!!warning "Conductive Materials Caution" When working with backplate cooling, avoid conductive thermal paste or pads that could short components. Use only non-conductive materials rated for electronics.

Heatsink Modifications

Fin Straightening

The stock heatsink often has bent fins that impede airflow. Carefully straightening bent fins can improve airflow.

Process:

Work systematically through fin stack
Be gentle - aluminum is very soft and bends easily
Avoid forcing fins apart with tools

Benefit: 5-10°C temperature improvement if many fins are bent

Fin Removal (Optional)

Some users remove center fins to improve fan contact with the heatsink.

!!!danger "High Risk Modification" Fin removal is IRREVERSIBLE and can damage your board if done incorrectly. Only attempt if absolutely necessary.

Recommended Method:

Manual removal by pulling/tearing: Fins can be cleanly pulled apart by hand
MUST remove heatsink from board first - never modify while attached!
Work slowly and carefully to avoid bending adjacent fins

NOT Recommended:

Dremel cutting (creates dangerous metal shavings)
Hacksaw cutting (imprecise, messy)
Any method that creates metal debris near the board

Alternative: Use a 3D printed fan shroud instead - no heatsink modification needed.

Temperature Impact: 10-15°C improvement, but similar gains possible with proper fan shroud

Thermal Paste Replacement

The thermal paste on used BC-250 boards is often dried out.

Recommended Thermal Paste:

Arctic MX-4 (good value)
Arctic MX-6 (newer formula)
Thermal Grizzly Kryonaut (premium)
Noctua NT-H1 (reliable)
Thermalright TFX (budget option)

Application Method:

Remove heatsink (4 screws)
Clean old paste with isopropyl alcohol
Apply small dot (pea-sized) of new paste to APU die
Remount heatsink with even pressure
Tighten screws in X pattern

Temperature Impact: 5-10°C improvement if old paste was dried

!!!tip "Use Quality Paste" Avoid cheap thermal paste. Quality paste lasts years. PTM7950 phase-change material is also popular.

Memory Thermal Pad Replacement

GDDR6 memory chips on the underside can run hot under sustained load.

Symptoms of hot memory:

System crashes during extended gaming
Instability after 30-60 minutes
Pixel artifacts

Solution:

Remove board from case
Remove old thermal pads (if present)
Apply new thermal pads (1.5mm on front of board, 2.0mm on back)
Attach aluminum plate or heatsink to underside
Optional: Add fan for active cooling

Thermal Pad Recommendations:

Thermalright Odyssey (high performance)
Arctic Thermal Pad (good value)
Gelid GP-Ultimate (premium)

Fan Mounting Options

Option 1: 3D Printed Shroud

Many community-designed fan shrouds are available on Printables:

Popular Designs (GitHub):

BC-250 Sleeve Adapter - 120mm fan adapter for desktop use
BC-250 Shell Case - Full enclosure design
BC-250 Custom Case - Alternative case design
BC250 Case by Jardon - Compact case design

Search for more on Printables:

Search "BC-250" on Printables - Community uploads new designs regularly

Advantages:

Custom fit for board
Integrated mounting for fans
Can include case design
No modification to heatsink needed

Printing Requirements:

PLA or PETG filament
0.2mm layer height
20-30% infill

Option 2: Direct Fan Mount

Mount fan directly using zip ties.

Zip Tie Method:

Position fan over heatsink center
Thread zip ties through fan mounting holes
Loop around heatsink fins or board mounting points
Tighten evenly
Trim excess zip tie length

!!!warning "Do Not Screw Into Heatsink" Do not drill holes in the heatsink fins to screw fans directly. The aluminum is soft and the fins are thin - this can damage the heatsink and reduce cooling efficiency. Use zip ties or a 3D printed shroud instead.

Option 3: Cardboard/Foam Shroud

Quick DIY solution using cardboard or foam board.

Materials:

Cardboard or foam core board
Hot glue or duct tape
Box cutter

Process:

Cut cardboard to create air duct from fan to heatsink
Glue/tape to create shroud around fan and heatsink
Ensure no air gaps
Mount fan to shroud

Pros: Free, fast, adjustable Cons: Not durable, not aesthetically pleasing

Fan Control

PWM Control with nct6687

The BC-250 uses the NCT6686D Super I/O chip. For PWM fan control, you need the nct6687 module (Fred78290/nct6687d). The in-kernel nct6683 module is read-only and cannot set fan speeds.

Driver Installation:

# Build and install nct6687 module
git clone https://github.com/Fred78290/nct6687d.git
cd nct6687d && make && sudo make install

# Blacklist nct6683 and enable nct6687
echo 'blacklist nct6683' | sudo tee /etc/modprobe.d/sensors.conf
echo 'options nct6687 force=true' | sudo tee -a /etc/modprobe.d/sensors.conf
echo 'nct6687' | sudo tee /etc/modules-load.d/99-sensors.conf

# Rebuild initramfs
sudo dracut --regenerate-all --force  # Fedora
sudo mkinitcpio -P  # Arch
sudo update-initramfs -u  # Debian/Ubuntu

# Reboot
sudo reboot

Verify:

sensors
# Should show nct6686-isa-0a20 with named fan speeds and temperatures

CoolerControl (GUI Fan Curves)

CoolerControl provides a GUI for creating custom fan curves.

Installation:

# Bazzite
ujust install-coolercontrol

# Fedora
sudo dnf copr enable codifryed/CoolerControl
sudo dnf install coolercontrol
sudo systemctl enable --now coolercontrold

# Arch
yay -S coolercontrol

Web UI available at https://localhost:11987 after installation.

Configuration:

Open CoolerControl web UI or launch the GUI app
Select the nct6686 device (BC-250's SuperIO chip)
Create custom curve on pwm2 (Pump Fan header) using k10temp Tctl as source
Apply and test

Systemd Fan Curve (Lightweight Alternative)

If you prefer a simple script without a GUI:

# Create fan curve script
sudo tee /usr/local/bin/bc250-fancurve << 'SCRIPT'
#!/bin/bash
HWMON_PWM=/sys/class/hwmon/hwmon1/pwm2
HWMON_ENABLE=/sys/class/hwmon/hwmon1/pwm2_enable
TEMP_INPUT=/sys/class/hwmon/hwmon3/temp1_input
echo 1 > $HWMON_ENABLE
while true; do
    TEMP=$(($(cat $TEMP_INPUT) / 1000))
    if [ $TEMP -le 40 ]; then PWM=60
    elif [ $TEMP -le 50 ]; then PWM=80
    elif [ $TEMP -le 60 ]; then PWM=120
    elif [ $TEMP -le 70 ]; then PWM=160
    elif [ $TEMP -le 80 ]; then PWM=200
    elif [ $TEMP -le 85 ]; then PWM=230
    else PWM=255; fi
    echo $PWM > $HWMON_PWM
    sleep 3
done
SCRIPT
sudo chmod +x /usr/local/bin/bc250-fancurve

!!!note "hwmon numbering" hwmon paths may change between kernel versions or reboots. Verify cat /sys/class/hwmon/hwmon*/name to find the correct hwmon for nct6686 (fan control) and k10temp (CPU temperature).

BIOS Fan Settings

The BIOS offers three fan modes:

1. Default Mode:

Targets high temperatures
Fans run at 40% minimum
Not recommended (inadequate cooling)

2. Full Speed Mode:

Fans at 100% constantly
Simplest and safest option
Noisy but effective

3. Customize Mode:

Set custom temperature thresholds
Define fan speeds at each threshold
More granular than Default
Can conflict with OS-level control

!!!warning "BIOS vs OS Control" Do not use both BIOS Customize mode and CoolerControl simultaneously. They will fight for control.

Manual Fan Control

Set fan speed manually (for testing):

# Find the hwmon for the nct6686 chip
HWMON=$(grep -l nct6686 /sys/class/hwmon/hwmon*/name | head -1 | xargs dirname)

# Enable manual PWM mode (required before writing PWM values)
# Main fan is on pwm2 (Pump Fan header)
echo 1 | sudo tee $HWMON/pwm2_enable

# Set fan to 80% speed (value 0-255)
echo 200 | sudo tee $HWMON/pwm2

# Set to 100% (255 = full speed)
echo 255 | sudo tee $HWMON/pwm2

!!!warning "Requires nct6687 Module" PWM fan control requires the nct6687 module. The in-kernel nct6683 module is read-only.

!!!info "Fan Header Mapping" The main cooling fan is typically connected to the Pump Fan header, which is fan2/pwm2 in sysfs. CPU Fan (fan1) and System Fan headers (fan3+) are usually unused.