An air-cooled miner pushes room air through its own heatsinks with fans; a hydro-cooled miner has no fans and passes its heat to a liquid loop through cold plates. Air is simpler to install and needs no plumbing, but it is loud and exposes the electronics to whatever is in the air. Hydro needs pumps, pipework, a heat exchanger and a three-phase supply, and in return packs more hashrate into each unit, runs quietly and tolerates hot, dusty sites. The choice is mostly decided by your site, not by the machine.
Key points
- Hydro machines are not standalone. Budget for the cooling loop (pumps, manifolds, heat exchanger, dry cooler) and for three-phase power before comparing prices per terahash.
- Air cooling suits small deployments, cool and clean air, and sites where you can move a lot of air and noise is not a constraint.
- Hydro suits hot or dusty locations, noise-sensitive sites, dense container builds and any project that wants to reuse the heat as warm water.
- The cooling method does not set efficiency by itself. Compare J/TH model by model, then add the power your cooling plant draws.
- Mains and three-phase work must be designed and installed by a qualified electrician to local code.
How each one removes heat
Essentially all the electrical power a miner draws becomes heat (1 kW ≈ 3,412 BTU/h). The two designs differ in what carries that heat away.
Air. Heatsinks are bonded to the chips and fans force air through the chassis from front to back. The air leaves hotter than it arrived and has to be removed from the room, or the machine re-breathes its own exhaust. Air is a poor carrier of heat: a cubic metre holds roughly 1.2 kJ per degree, so the volumes are large.
Hydro. A cold plate (a metal plate with internal channels) sits against the chips, and coolant pumped through it picks up the heat. The warm coolant goes to a heat exchanger and then to an outdoor dry cooler or cooling tower. Water holds roughly 4,180 kJ per cubic metre per degree, about 3,500 times more than air, so a thin hose replaces a large duct. The loop is covered in Hydro Mining Explained.
Bitmain's published figures show the scale. Its listing for the air-cooled Antminer S21 XP states a maximum airflow of 550 CFM, which is about 15,600 litres of air per minute (550 × 28.32). Its specification for the S21 XP Hyd. asks for 8–10 litres of coolant per minute.
A matched pair from the same generation
The figures below are manufacturer-published by Bitmain for two machines of the S21 XP generation, taken from its specification articles and current shop listings; where these differ slightly (the support article gives 449 mm for the air unit's length), the newer listing is used. Hashrate, power and efficiency are typical values with a stated tolerance of ±3% on hashrate and ±5% on power.
| Antminer S21 XP (air) | Antminer S21 XP Hyd. (hydro) | |
|---|---|---|
| Hashrate | 270 TH/s | 473 TH/s |
| Power at the wall | 3,645 W | 5,676 W |
| Efficiency | 13.5 J/TH | 12.0 J/TH |
| Rating condition | 25 °C inlet air | 35 °C inlet coolant |
| AC input | 220–277 V single-phase, 20 A | 380–415 V three-phase, 12 A max |
| Size | 450 × 219 × 293 mm | 339 × 173 × 207 mm |
| Net weight | 18.7 kg | 13.8 kg |
| Noise | 76 dB(A) with fans at maximum speed | no figure published |
| Thermal limits | −20 to 45 °C operating temperature | 20–50 °C inlet coolant, 8–10 L/min, ≤ 3.5 bar |
Three things follow from the arithmetic:
- Power per unit. 5,676 ÷ 3,645 = 1.56, so each hydro unit carries 56% more electrical load and delivers 473 ÷ 270 = 1.75 times the hashrate.
- Power density. The chassis volumes are 450 × 219 × 293 mm = 28.88 L and 339 × 173 × 207 mm = 12.14 L. That is 3,645 ÷ 28.88 ≈ 126 W per litre for the air unit against 5,676 ÷ 12.14 ≈ 468 W per litre for the hydro unit, about 3.7 times denser.
- Heat to reject. 3.645 kW × 3,412 ≈ 12,400 BTU/h per air unit; 5.676 kW × 3,412 ≈ 19,400 BTU/h per hydro unit.
The hydro unit in this pair is also the more efficient one, but that is a property of these two models. Check J/TH for the models you are comparing in the comparison tool; there are hydro models that are less efficient than current air models.
Side-by-side comparison
| Criterion | Air-cooled | Hydro-cooled |
|---|---|---|
| Heat carrier | Room air moved by fans in each machine | Coolant moved by central pumps |
| Moving parts in the machine | Fans | None; pumps and cooler fans are in the plant |
| Noise at the machine | High. Bitmain publishes 76 dB(A) for the S21 XP | Low at the machine. Pumps and dry-cooler fans make the noise, usually outdoors |
| Ambient air sensitivity | Direct. Hot intake air cuts headroom; thin air at altitude cools less | Indirect. The outdoor cooler must still reject heat, but the allowed coolant inlet range is wide |
| Dust, salt, humidity | Drawn across the boards | Electronics are not in the airstream |
| Infrastructure | Racks, intake filtration, exhaust fans or ducting | Racks or cabinets, manifolds, pumps, plate heat exchanger, dry cooler or tower, coolant |
| Electrical supply | Single-phase per unit (220–277 V on the S21 XP) | Three-phase per unit (380–415 V on the S21 XP Hyd.) |
| Commissioning | Plug in, configure pool | Flush, fill, bleed, leak-test and balance the loop first |
| Routine work | Dust cleaning, fan replacement | Coolant testing, filters, leak checks, pump service |
| Heat reuse | Warm air, hard to move far | Warm water, easy to pipe to a heat load |
| Starts as a single unit | Yes | Only with a cooling loop sized for it |
Sensitivity to site conditions
An air machine is rated against its intake air. Bitmain's S21 XP specification gives −20 to 45 °C operating temperature and adds that between 900 m and 2,000 m altitude the maximum operating temperature falls by 1 °C for every 300 m. Its general guidance for Antminers also recommends a low-dust environment, humidity below 65%, and distance from coastal salt air. Each of those is a constraint on where an air farm can sit and how much filtration it needs.
A hydro machine is rated against its coolant. Bitmain's S21 XP Hyd. specification allows 20–50 °C inlet coolant and notes the miner cannot start below 20 °C. That wide upper limit is what lets a dry cooler work in hot climates without evaporating water: the same specification says that with Bitmain's external dry cooler, water leaving the cooler at 35 °C corresponds to a maximum ambient of 30 °C, 45 °C to 40 °C and 50 °C to 45 °C. The lower limit matters in cold climates, where the loop needs a way to keep supply temperature up and antifreeze to protect it when stopped.
Where the cost goes
With air, most of the money outside the machines goes on the building or container envelope, air handling (intake filters, exhaust fans, hot-aisle separation) and power distribution. The cooling is cheap to buy and the running cost is fan power plus cleaning labour.
With hydro, the plant is the project. You buy pumps (normally duty and standby), stainless pipework and manifolds, quick connectors, a heat exchanger, a dry cooler or cooling tower, an expansion tank, relief valves, filtration, controls and the coolant itself. A prefabricated hydro container bundles those items. You also pay for commissioning skill. In return you fit more load into the same footprint and spend less on the envelope. Compare the two on total installed cost per kilowatt of miner load, not on machine price alone; a project quote can be requested through farm solutions.
Electrical differences
The pair above shows the usual pattern: the air unit takes single-phase power and the hydro unit takes three-phase. A hydro deployment therefore needs three-phase distribution to every position, with breakers and cabling sized for a continuous load. See Three-Phase Mining Equipment for the formulas. Have a qualified electrician design and install the supply to local code, and include the pumps and cooler fans in the load calculation.
Maintenance profile
Air machines fail in predictable ways: dust blocks heatsinks and fans wear out. Both are simple to deal with and both need regular labour. Bitmain recommends cleaning Antminers about once a month.
Hydro machines have no fans to replace and no dust on the boards, but the loop needs chemistry discipline. Bitmain's S21 XP Hyd. specification says the fluid must be replaced if conductivity reaches 100 μS/cm, and manufacturers set pH ranges and coolant types that you must keep to. Leaks, air in the loop, fouled filters and freezing are the failure modes to plan for. A fault in a pump or cooler affects every machine on that loop at once, so redundancy matters more than it does with air.
Which conditions favour each
Choose air when you are installing a small number of units, have cool and reasonably clean air for most of the year, can exhaust large air volumes, and noise is acceptable. Start at air-cooled miners.
Choose hydro when ambient temperatures are high, the air is dusty, sandy or salty, noise limits are tight, land or container space is limited, or you have a use for warm water. Start at hydro miners.
If you are weighing immersion or evaporative cooling as well, ASIC Cooling Methods Compared puts all four on one framework.
Sources
- Bitmain Support, "S21 XP Specifications", https://support.bitmain.com/hc/en-us/articles/35383015643673-S21-XP-Specifications, checked 2026-10-01.
- Bitmain Support, "S21 XP Hyd Specification", https://support.bitmain.com/hc/en-us/articles/34523540504857-S21-XP-Hyd-Specification, checked 2026-10-01.
- Bitmain Support, "Do's and Don'ts of a New ANTMINER", https://support.bitmain.com/hc/en-us/articles/360001836994-Do-s-and-Don-ts-of-a-New-ANTMINER, checked 2026-10-01.
- Bitmain Support, "How to clean and dust the ANTMINER?", https://support.bitmain.com/hc/en-us/articles/115004520173-How-to-clean-and-dust-the-ANTMINER, checked 2026-10-01.