Billet Labs’ Fanless 9800X3D/RTX 5080 Build Shows Limits of Pure Passive Water Cooling

Announcement

Billet Labs unveiled a completely fan‑less gaming rig built around an AMD Ryzen 7 9800X3D CPU and an Nvidia GeForce RTX 5080 GPU. The system relies on a triple‑stacked copper‑to‑aluminum radiator “chimney” to move heat upward by natural convection. The build was demonstrated in a detailed video on the Billet Labs YouTube channel, where Felix explains the motivation, construction steps, and thermal results.

{{IMAGE:2}}

Technical specifications

Component	Specification	Relevance
CPU	AMD Ryzen 7 9800X3D, 8 cores / 16 threads, 4.9 GHz boost, 105 W TDP	7 nm Zen 4 die; high per‑core performance makes it a good test for passive cooling
GPU	Nvidia GeForce RTX 5080, 16 GB GDDR6, 350 W TGP	5 nm Ada Lovelace chip; top‑tier rasterization and ray‑tracing loads the water loop heavily
Motherboard	Gigabyte AORUS Pro B850 (AM5)	Provides robust VRM cooling; motherboard fan disabled for the test
Radiators	Three stacked units: 360 mm × 120 mm, 480 mm × 120 mm, and 560 mm × 120 mm (all copper‑core with aluminum fins)	Total surface area ≈ 3.4 m², creating a tall vertical column that encourages buoyant airflow
Pump	Billet Labs custom‑machined centrifugal pump, rated 70 W, max water temperature 60 °C	Operated at 80 % speed to reduce cavitation; water flow ≈ 1.8 L/min
Coolant	Distilled water with 10 % propylene glycol	Low viscosity for maximal heat transfer
Chassis	8 mm aluminum plate base, no side panels, open‑air layout	Eliminates obstruction to rising warm air

How the chimney effect works

When the radiators heat the water, the warmed coolant rises through the stacked radiators. As it ascends, it transfers heat to the surrounding air. Because the radiators are arranged vertically, the hot air forms a natural draft that pulls cooler ambient air from the bottom of the chassis upward. This buoyancy‑driven flow replaces the need for mechanical fans, similar to how a solar chimney ventilates a building.

Measured performance

Test	Power draw (W)	CPU temp (°C)	GPU temp (°C)	Water inlet (°C)	Water outlet (°C)
Idle (30 min)	45	48	38	22	27
Light game (Peggle)	120	62	55	24	31
Cyberpunk 2077 (1080p, Medium)	310	88	71	28	38
Cinebench R23 (single‑core)	180	92	62	26	34
Stress (Cinebench R23 + FurMark)	450	95+ (throttling)	78 (no throttling)	33	60 (pump limit)

The GPU never hit its thermal throttling point (≈ 85 °C), but the CPU crossed 95 °C during the combined stress test, triggering a brief frequency drop. The water temperature plateaued at 60 °C, which is the pump’s maximum safe rating; any higher and the pump’s bearings would risk premature wear.

Market implications

1. Thermal headroom on current silicon – Both the 9800X3D and RTX 5080 are built on advanced 7 nm/5 nm nodes that push per‑core power density above 2 W/mm². Even with a massive radiator area, passive convection cannot keep the CPU below its 95 °C throttling threshold under sustained >400 W loads. This suggests that, for the foreseeable future, high‑performance desktop silicon will still require at least one active airflow source.
2. Supply‑chain pressure on premium cooling components – The build reuses copper‑core radiators that are in short supply due to the surge in custom water‑cooling projects following the 2023 GPU launch cycle. Manufacturers such as EK and Alphacool have reported lead times of 8‑12 weeks for large‑format radiators, indicating that hobbyist‑level passive designs may be constrained by component availability.
3. Potential niche for silent‑gaming PCs – The experiment demonstrates that a quiet, fan‑less chassis is technically feasible for moderate workloads (e.g., office tasks, media playback). For users willing to accept occasional CPU throttling during extreme gaming sessions, the trade‑off could be acceptable, especially in living‑room environments where acoustic comfort is a priority.
4. Design lessons for OEMs – The chimney effect could be integrated into future small‑form‑factor cases. By aligning radiator stacks vertically and providing unobstructed intake at the base, manufacturers could reduce fan count by 1‑2 units, cutting cost and acoustic output. However, the data also warns that thermal margins are thin; a single 120 mm fan placed at the top of the stack, as Billet Labs plans for a follow‑up video, would likely bring CPU temps below 85 °C across all tested workloads.

Outlook

Billet Labs’ fanless prototype is a compelling proof‑of‑concept that pushes the envelope of passive water cooling. The results confirm that pure convection can handle GPU heat but struggles with modern high‑TDP CPUs. A modest addition of active airflow—perhaps a single 120 mm fan at the chimney’s apex—could close the thermal gap without compromising the silent aesthetic. As component lead times normalize and next‑generation 3 nm CPUs arrive with lower power envelopes, the market may see more hybrid passive‑active designs targeting the quiet‑gaming segment.

---

For a full temperature log and the Reddit discussion, see the original post linked in the video description.