The Bitmain Antminer S21 Hydro is the water-cooled version of the S21 generation, presented in September 2023 and shipping since spring 2024. Like every Bitcoin miner of the series it runs SHA-256, which covers Bitcoin and Bitcoin Cash. We offer it at 319 or 335 TH/s, in both cases at 16.0 joules per terahash. Instead of fans, a water loop cools the boards; the unit weighs 12.3 kilograms and hangs on a three-phase supply of 380 to 415 volts.
Performance levels of the Antminer S21 Hydro
- S21 Hydro 319 TH/s at 5,104 watts (BTM-S21HYD-319TH)
- S21 Hydro 335 TH/s at 5,360 watts (BTM-S21HYD-335TH)
Both levels share the same efficiency; the larger one simply delivers more hashrate at correspondingly higher draw. Bitmain measures the figures at 35 degrees Celsius inlet water and quotes three percent variation for hashrate and five percent for power and efficiency.
Design, power connection and cooling of the S21 Hydro
Without fans the S21 Hydro measures 339 × 163 × 207 mm, packed 570 × 316 × 430 mm at 13.6 kilograms. Noise is around 50 dB. Like almost all hydro miners, the unit has no water pump of its own: the external cooler has to push the water through the miner, so the S21 Hydro only runs on such a loop.
Bitmain sets clear requirements for that loop: a flow of 8 to 10 litres per minute, no more than 3.5 bar, DN10 connectors, deionised or pure water with a pH between 8.5 and 9.5. Conductivity should be below 20 microsiemens per centimetre at first start-up, and the water has to be replaced from 100 upwards. Normal mode assumes 35 degrees inlet water; in high energy mode the boards also run at 45 or 50 degrees, then at 5,890 or 5,960 watts respectively and with correspondingly worse efficiency. Below 20 degrees water temperature the miner will not start. If temperatures drop below freezing where the miner is operated, antifreeze must go into the loop, no exceptions.
The built-in power supply draws three-phase power from 380 to 415 volts at 50 to 60 hertz, with up to 10 amps per conductor. A wrong voltage damages the unit, Bitmain is explicit about that. The network link is Ethernet, 10/100 Mbit on an RJ45 socket.
Custom firmware exists for the S21 Hydro as well: Braiins OS has supported it fully since version 25.03, and Vnish maintains its own build. This allows the power per hashboard to be adjusted, either more frugal or with more hashrate at slightly worse efficiency. If you want it, we load the firmware before dispatch at 29.90 euros net per unit; the service is limited to miners in your own operation and excludes units in our hosting. The warranty lapses with it, at Bitmain as with us.
As external cooling for a single machine we carry the Lian Li cooling unit, which already brings pump, valve and hoses.
As standard, Bitmain gives a 365-day factory warranty on the machine, counted from the shipping date; our standard price always includes our own warranty.
Comparison with the previous and next generation of the S21 Hydro
Its predecessor among Bitmain's water-cooled units is the Antminer S19 Pro Hydro; the S21 Hydro was superseded by the Antminer S21 XP Hydro, the strongest hydro model of the S21 series. Above that already stands the Antminer S23 Hydro. The four units by efficiency:
- Antminer S19 Pro Hydro: 27.5 J/TH (184 TH/s at 5,060 W)
- Antminer S21 Hydro: 16.0 J/TH (319 to 335 TH/s at 5,104 to 5,360 W)
- Antminer S21 XP Hydro: 12.0 J/TH (473 TH/s at 5,676 W)
- Antminer S23 Hydro: 9.5 J/TH (563 TH/s at 5,349 W)
From the S19 Pro Hydro to the S21 Hydro the electricity needed per terahash drops by more than a third. At 12.0 joules per terahash the S21 XP Hydro is another quarter more frugal than the S21 Hydro, and the S23 Hydro brings the figure below 10 joules. Within the same series the Antminer S21+ Hydro sits in between at 15.0 J/TH. The miner comparison ranks the S21 Hydro by daily yield against everything else we sell.
Verdict: Bitcoin mining with the Antminer S21 Hydro
The S21 Hydro is a solid miner that shows practically no failures with us and with our customers, provided it is operated sensibly. Then the unit runs through without interruption; we hardly ever see problems.
That is exactly where its strength lies: anyone with very cheap electricity looking for a unit that runs stably and without attention is right with a hydro miner, and the S21 Hydro is an extremely good choice for that. It equally suits operators with cheap power who want a lot of hashrate at a low price.
For stable continuous operation on cheap electricity the S21 Hydro is a clear recommendation. With your electricity price and the level you have in mind, the Bitcoin mining calculator delivers the numbers for the S21 Hydro. It uses the Bitcoin network hashrate and the price from the cockpit, where the Bitcoin mining pools are also listed with share and fees.
