Bitmain Antminer S21e Hyd (288Th)

The Antminer S21e Hyd (288Th) is Bitmain’s liquid-cooled SHA-256 miner built on the BM1368 5 nm chip. It is factory-rated at 288 TH/s for 4,896 W (17 J/TH), released April 2025, and designed for water-cooled Hashcenter deployments rather than the living room. Below is the full engineering picture, grounded in our own teardown and firmware research.

Chip and hashboard architecture

At the heart of the S21e Hyd is the BM1368, Bitmain’s fifth-generation SHA-256 ASIC fabricated on a TSMC 5 nm process (the same node family as the S21 and T21, not 3 nm as some spec sheets claim). Each BM1368 die packs roughly 1,280 hashing cores (a mix of 80 large and 16 small SHA engines) in a QFN package, delivering on the order of 600–750 GH/s per chip depending on how hard it is clocked.

The unit carries three hashboards wired as series chains, with about 108 BM1368 chips per chain (~324 chips in total). The chips on each board are not powered individually. Instead they are grouped into 12 voltage domains of roughly 9 chips each, with every domain regulated at approximately 1.2 V. This is the single most misunderstood point about modern Antminers: voltage is controlled per domain, never per chip. The chips inside a domain sit in series, so the domain voltage equals the per-chip voltage multiplied by the number of chips in that group, and any tuning you do moves a whole domain at once.

The S21 generation, including this hydro variant, is a “no-PIC” design — Bitmain removed the PIC microcontroller that policed voltage on S9-through-S19 boards. On the BM1368 platform, domain voltage and board telemetry are instead handled through TAS5782M devices addressed over I2C, repurposed from audio-DAC silicon. There is no separate PIC heartbeat to satisfy, which changes both the failure modes and the repair approach versus older Antminers.

Control board

The S21e Hyd runs on an Amlogic A113D (AXG) control board — a quad-core ARM Cortex-A53 SoC with 256 MB of DDR3, originally an audio chip, with no FPGA. All three hashboard chains are driven from a single UART (/dev/ttyS2) that ramps up to 3 Mbaud for mining, with GPIO lines selecting and resetting each board. Because there is no FPGA fabric, the control board talks to the ASICs directly in software — simpler hardware, but it means firmware does the heavy lifting the FPGA used to do.

Real-world power and efficiency

The 4,896 W on the label is the board-and-PSU figure at the factory turbo point. At the wall, plan for more: PSU conversion losses add several percent, and a hydro miner is never alone — the external pump and dry-cooler fans that reject its heat draw their own power. Budget your circuit and your facility’s heat-rejection loop accordingly. At 4,896 W this is a 240 V-class machine drawing north of 20 A, not something to share with a household circuit.

Where the S21e Hyd really shines is its tuning headroom. Liquid cooling pulls heat off the dies far more effectively than air, so the same BM1368 silicon holds higher sustained clocks — or, run the other direction, far better efficiency. Our firmware research catalogues 80 distinct power profiles for this model across three modes, the richest profile set of any SHA-256 miner we have documented:

The takeaway: the factory 288 TH/17 J/TH point is essentially turbo. Dial the unit into a balanced “normal” profile and it settles at a flat 16.5 J/TH; push into low-power and it reaches 14.8 J/TH — the single best efficiency figure across every SHA-256 machine we have profiled — by trading away roughly 40% of peak hashrate. Conversely, the performance curve runs all the way to ~482 TH, but efficiency degrades steeply past the factory point, so overclocking buys hashrate at a real cost in J/TH. Explore the full curve on our ASIC power-profiles database before you commit a fleet to a setpoint.

Firmware compatibility

The S21e Hyd ships with Bitmain’s stock firmware, which is competent but deliberately limits how far you can tune voltage and frequency. The BM1368 generation is well supported by the major third-party stacks — BraiinsOS+, VNish and LuxOS all run on this silicon — which is where the 80-profile tuning range above comes from. A few honest caveats:

• Firmware locks. Amlogic-based units shipped after March 2024 carry signed-firmware locks. Loading third-party firmware on a recent S21e Hyd can require an unlock step, and getting it wrong can brick the control board. This is not a casual flash.
• Stratum V2. If protocol-level sovereignty matters to you, note that BraiinsOS+ is the only firmware that natively supports Stratum V2 today; the others remain Stratum V1.
• Autotuning is calculated, not preset. The per-domain frequency and voltage targets a good autotuner lands on are computed at runtime for your specific boards and ambient conditions — they are not a fixed table baked into the image. Two “identical” S21e Hyds can settle at different setpoints, and that is correct behaviour.

For operators who want to keep their hardware fully under their own control, D-Central’s in-house DCENT_OS has driven BM1368 silicon in our lab and the chip sits on our firmware roadmap. DCENT_OS is GPL-3.0 and currently in closed beta — we would rather under-promise here than oversell a build that is still being hardened. Whatever firmware you choose, we treat it as one more layer of decentralization on top of hardware the manufacturers got right.

Common faults and troubleshooting

Most S21e Hyd problems fall into two buckets: the BM1368 board faults common to the whole S21 family, and hydro-specific cooling-loop issues.

On the board side, the failure to understand is the voltage domain. Because chips are grouped 9-to-a-domain in series, a single shorted chip pulls its whole domain’s voltage low and the boost circuit compensates; an open circuit pushes the domain voltage high. The control board reads back a low ASIC count or an abnormal domain voltage and the chain stops hashing. Typical symptoms include a board reporting 0 or a reduced ASIC count, “abnormal” temperature or voltage readings, or one of three boards dropping out while the other two keep mining. Diagnosis means metering each domain’s test points (labelled around VDD1V2 / VDD0V8) to localise the fault to a specific domain and LDO rather than guessing at a chip.

On the hydro side, watch coolant flow above everything: a weak or failed pump, air trapped in the loop, a clogged cold plate, or an undersized dry cooler will spike die temperatures and trip the miner long before an air-cooled unit would complain. Inspect every fitting — a slow leak around a quick-connect is the classic hydro failure — and keep coolant chemistry and flow rate within spec.

Work symptoms backwards with our ASIC fault finder, which maps error codes and behaviours (chain detection failures, temperature-protection trips, low hashrate, fan/flow faults) to likely causes and the right next test.

Repair and longevity

A liquid-cooled miner is a long-term asset, and the BM1368 platform is repairable at the board level. D-Central has run in-house ASIC repair in Laval, Quebec since 2016 — chip-level reflow and replacement, voltage-domain diagnosis, cold-plate and water-block servicing, control-board recovery, and PSU work. The no-PIC architecture actually simplifies some repairs (there is no PIC handshake to defeat), but it demands proper domain-level metering and reflow gear rather than parts-swapping. Rather than writing off a hydro unit over one dead domain or a leaking fitting, send it to our ASIC repair service — a single recovered hashboard is usually a fraction of replacement cost, and well-maintained S21-class hardware has years of useful life ahead of it.

Who it is for, and buying

With a home-mining score of 44/100, the S21e Hyd is squarely facility and Hashcenter hardware, not a bedroom miner. It cannot run plug-and-play: it needs an external water loop — pump, dry cooler or cooling-distribution unit, and the plumbing to match — plus a 240 V circuit sized for ~5 kW. In return you get a near-silent ~50 dB unit (the real noise moves outdoors to the dry cooler) and roughly 16,705 BTU/h of recoverable heat in a captured, water-borne form that is far easier to duct or reuse than hot air. For an operator already running liquid cooling and chasing the best J/TH the S21 generation offers, it is one of the strongest options Bitmain shipped before the BM1370 machines.

If a hydro deployment is more than you need right now, browse our full ASIC miner catalogue for air-cooled S21-family alternatives, or talk to us about a build-to-order configuration. We are mining hackers, not a faceless reseller — we would rather point you at the right machine than the most expensive one.

Generational context

The S21e Hyd belongs to Bitmain’s S21 generation, the 5 nm BM1368/BM1370 family that delivered a step-change in efficiency over the 7 nm S19 era (which sat near 30 J/TH). Within the lineup, the BM1368-based S21 and S21e Hyd land around 16–17.5 J/TH, while the newer BM1370 machines — the S21+ Hydro and S21 XP Hydro — push to roughly 12–15 J/TH. The “e” hydro variant carves out a specific niche: a profile-rich, liquid-cooled BM1368 unit tuned for exceptional efficiency in normal and low-power modes rather than raw top-end hashrate. It is a refinement of a chip Bitmain got right — one we are happy to keep alive on the bench long after its warranty ends.