Air vs liquid cooling for ASICs: where air hits its limit (a breakdown in numbers)

Air cooling runs out at around 13.5 J/TH – that is the best air-cooled Antminer, the S21 XP. Liquid-cooled (hydro) machines of the S23 series reach 9.5 J/TH, roughly 30 % less energy per terahash, and deliver up to 1.16 PH/s from a single chassis. For 1–5 machines at home, however, air remains the right choice. Below is a breakdown in numbers: where air hits its physical ceiling, why water breaks through it, and when paying extra for liquid isn't worth it.

Summary

• Air has a low heat capacity – the only way to remove more heat is more air, meaning faster and louder fans.
• Best air-cooled machine: S21 XP, 270 TH/s, 13.5 J/TH. Best hydro: S23 Hyd, 9.5 J/TH.
• A 4 J/TH difference means 96 kWh per day for every PH/s – at 1.13 CZK/kWh about 108 CZK per day.
• Hydro needs three-phase power, a CDU and circuit maintenance. At home and at small scale it doesn't pay off.

Every six months, mining chats bury air cooling. And every six months someone deploys a fresh batch of air-cooled S21s and quietly makes money with them. The truth isn't in the slogan but in the numbers – and in exactly where the line runs between “air still pays” and “from here on, only water”.

Why air cooling has a ceiling

The job of cooling is always the same – remove heat from the chip faster than the chip produces it. Air manages this as long as the heat flux is moderate. But it has a low heat capacity, so the only way to remove more heat is to push more air – that is, to spin the fans faster.

And this is where physics kicks in, and no marketing gets around it: noise grows faster than heat removal. An air-cooled S21 at 200 TH/s already roars at 70–80 dB – a vacuum cleaner next to your ear, twenty-four hours a day. For fans to remove 5 kW or more from a single machine, they would have to run at speeds that turn the miner into a turbine: unacceptable because of noise, bearing wear and the energy the fans themselves consume.

Water removes the same 5 kW and more from a single chassis without the acoustic penalty, because its heat capacity is several times higher. That is the whole trick – not a “hydro fashion”, but the different physical limits of two media.

Where exactly the line runs: efficiency in J/TH

The key metric in mining isn't hashrate but efficiency: how many joules go into one terahash (J/TH). It determines what share of your electricity bill turns into bitcoin and what share into heat. Lower is better – more detail in the article ASIC energy consumption.

Note the boundary. The best that air offers today is the S21 XP at 13.5 J/TH. The best hydro is the S23 at 9.5 J/TH. The difference is roughly 30 % in efficiency. That doesn't mean air is bad: 13.5 J/TH is an excellent result that was out of reach only a few years ago. Air simply can't physically “pull” any further – water can.

Tellingly, air and hydro versions often use the same chip. The S21 XP and S21 XP Hyd are the same generation of silicon. The liquid circuit, however, keeps the chip at a stable temperature and lets you get more out of it – 473 TH/s instead of 270 TH/s, and with better efficiency. Same chip, different ceiling.

Density: where air loses several times over

Efficiency is the electricity bill. But hydro has a second trump card that is often more important in practice: density.

The flagship S23 Hyd 3U delivers 1.16 PH/s from a single 3U chassis. To put that in perspective: it replaces roughly 12 S19 machines (95 TH/s), about 4 air-cooled S21 XPs, 2.5 units of the S21 XP Hyd or two separate S23 Hyds at 580 TH/s.

And it's not just about space. Twelve machines mean twelve Ethernet connections, twelve power feeds, twelve places where something can go wrong and twelve devices to monitor. One S23 Hyd 3U is one connection, one feed and one line on the dashboard. Real outages at farms happen precisely at cables and connectors. Consolidation therefore saves above all nerves and maintenance hours.

Where air still wins

Now the half that promotional copy usually cuts out. Hydro doesn't always win, and there are situations where switching to water is money thrown away.

At home and at small scale (1–5 machines). Here air is the practical choice. Building a liquid circuit with a pump, radiator and coolant maintenance for five units makes no economic sense. An air-cooled S21 with good ventilation does its job, and noise can be reduced with, for example, a soundproof box.

A standard electrical supply. Air-cooled machines run on single-phase 220–277 V. Flagship hydro machines require three-phase industrial power at 380–415 V. If you don't have such a supply, that is a separate construction project before the first hash.

Upfront investment and speed of deployment. An air-cooled machine is cheaper and you switch it on straight away: power, internet – and it runs. Hydro requires a cooling circuit that sometimes costs more than the miner itself.

A short horizon. If you want to get in quickly, earn back the investment and possibly exit, air pays back faster thanks to the lower entry cost.

The honest conclusion: for most home miners with 1–5 machines, air with proper ventilation remains the right choice.

Where water wins: post-halving economics

Beyond that lies hydro territory – and it is defined not by fashion but by the electricity bill.

The difference between the air ceiling (13.5 J/TH) and the hydro flagship (9.5 J/TH) is 4 J/TH, i.e. 4 kW for every PH/s. Per day that is 96 kWh per PH/s. At a price of 1,13 Kč/kWh (the cheapest hosting as of 18 October 2026) that comes to about 108 CZK per day and around 39,600 CZK per year for every PH/s that an air-cooled farm pays extra for the same mined bitcoin. For a 10 PH/s operation that is over 390,000 CZK a year.

After the halving the block reward is cut in half, and that changes everything. Less efficient machines are the first to slip into loss when difficulty rises and the reward doesn't. A 17 J/TH machine that was profitable last year may fall below the profitability threshold after the next difficulty increase. A 9.5 J/TH machine survives that rise with room to spare.

On top of that, hydro offers stable temperatures without thermal cycling (less wear, longer lifespan), noise of around 50 dB instead of 70–80 dB, and the option of dense installation. For a serious long-term operation, water comes out ahead overall.

What hydro really requires

Hydro is not “set it and forget it”:

• CDU (Coolant Distribution Unit) – a central unit with a pump, reservoir, filtration and monitoring that supplies coolant to dozens of machines. You have to buy it, house it and maintain it.
• Three-phase power at 380–415 V and the right connectors (LP34 on flagship models).
• Circuit maintenance – check the coolant level monthly, test its quality quarterly, replace the coolant every 2–3 years and the filters every 6–12 months. The mixture is usually water with propylene glycol.
• Protection against leaks and loss of flow – flow, temperature and leak sensors. The firmware must throttle or shut down the machine if cooling fails.
• System incompatibility – Bitmain (Antminer Hyd) and MicroBT (Whatsminer Hydro) circuits are not interchangeable: different dimensions, connectors, fittings and CDUs.

That is why, for many operators, deciding on the cooling system is harder than choosing the miner itself. You'll find components in the water circuits category; we cover farm cooling in general in the article Mining farm cooling.

So where does the line run?

• 1–5 machines, at home, standard supply, short horizon → air. The S21 XP at 13.5 J/TH is an excellent machine and you don't need water.
• A farm, industrial power, long horizon, expensive electricity, density and noise matter → hydro. A 30 % efficiency gap and consolidating up to 12 machines into one chassis pay for the infrastructure.

“Air is dead” isn't true. The truth is that air has a clear physical ceiling of around 13.5 J/TH, and everything beyond it begins where air ends and water begins.

How it's handled in practice

The main barrier to hydro isn't the miner itself but the infrastructure: three-phase power, a CDU, the circuit and coolant maintenance. Hosting takes that on. In our hosting data centres, hydro machines such as the S23 Hyd run in a ready-made environment – with three-phase power, industrial cooling and monitoring. You get 9.5 J/TH without building a pumping station or a three-phase supply at your place.

What next

• ASIC miner hosting – locations, electricity prices and availability of slots for hydro machines.