Air, hydro or immersion: what a miner's cooling means, and how to choose between them

Every miner turns nearly all its power into heat, and air, hydro and immersion are three ways of getting that heat out. The surprise for a first-time buyer is that the cooling does not set a machine's efficiency: the Antminer S21 XP is 13.5 J/TH in air and in immersion alike.

Every miner is a heater, and the cooling is how the heat leaves

A bitcoin miner turns almost every watt it draws into heat. The air-cooled Antminer S21 XP draws 3,645 watts, which is more than two large electric room heaters running flat out, day and night. If the heat stays in, the chips fail.

Cooling is the way that heat is carried out of the machine. There are three, and the word on a machine's listing, air, hydro or immersion, is the first thing you meet. It decides what the machine needs around it, and that matters more to a first-time buyer than it sounds.

The cooling decides where a machine can live. It does not decide how well it mines.

Air: fans push the heat into the room

An air-cooled machine is the plain version. Fans pull air through metal fins on the chips and blow the hot air out the back. You plug it in and it runs, which is why most machines are sold this way: 75 of the 140 industrial bitcoin machines in stock in our store on 7 October 2026.

The price of that simplicity is noise and heat. The S21 XP is rated at 76 dB, about as loud as a vacuum cleaner beside you, and the building has to move a great deal of air in and a great deal of hot air out. That is a job for an industrial building, not a garage, and it is part of why hosting works the way it does.

Hydro: water carries the heat away from the chips

A hydro machine swaps the big fans for water. Water is pumped through plates that sit on the chips, picks up the heat and carries it to a cooler outside the building, where it is released before the water comes round again.

Water moves heat better than air, so a hydro machine can be smaller for its size of output. The store lists the S21j XP Hyd at 410 × 170 × 209 mm for 495 TH/s, against 400 × 195 × 290 mm for the 270 TH/s air-cooled S21 XP. It is also quieter: the S21e XP Hyd 3U is listed at 50 dB.

The cost is the loop. Pipes, pumps, a cooler and someone who looks after the water all have to exist before the machine can run, and a hydro machine is no use in a room that does not have them.

Immersion: the whole machine sits in a bath of fluid

An immersion machine is lowered into a tank of dielectric fluid, a liquid that does not conduct electricity, which MARA's explainer describes as typically oils or synthetic liquids. The fluid takes the heat straight from the machine because, in the page's words, "liquids have a higher thermal conductivity than air". It is then circulated through a heat exchanger or radiator, where it gives the heat up.

The same page names the trade-offs: a more involved set-up and a higher cost at the start, and a fluid to look after. Like hydro, immersion needs a site built for it. The store lists the S21 XP Immersion at 300 TH/s for 4,050 W, and it lives where tanks already exist.

Hydro needs a water loop. Immersion needs a tank. Hosting means you rent the room that already has one.

The cooling does not set the efficiency

A first-time buyer often assumes liquid cooling means a better machine. Look at one machine family. The S21 XP in air is 270 TH/s for 3,645 W, which is 13.5 J/TH. The S21 XP Immersion is 300 TH/s for 4,050 W, which is also 13.5 J/TH. Same efficiency, different cooling. The spec sheet guide shows how J/TH is worked out.

Across the store, the cooling label tells you little. Of the machines in stock on 7 October 2026, the most efficient air machine is 12.0 J/TH, the most efficient immersion machine 13.5 and the most efficient hydro machine 8.9. Yet the middle hydro machine is 18.5 J/TH, behind the middle air machine at 15.5 and the middle immersion machine at 17.0. Hydro holds the best machine in stock and some of the least efficient, because it holds old machines and new ones.

Efficiency turns into money in one step. Each J/TH is 0.024 kWh of electricity per terahash per day. At 6¢ a kilowatt-hour a 13.5 J/TH machine spends about 1.9¢ per terahash per day, the 12.0 J/TH S21j XP Hyd about 1.7¢, and the 8.9 J/TH S23 XP Hyd about 1.3¢, or 0.8¢ at 3.9¢.

Against that, a terahash earned 4.1¢ a day on 6 October 2026, the hashprice of $40.85 per petahash on our hashprice chart. All three clear their electricity with room to spare, and the more efficient the machine, the more of the day's earnings it keeps. The hashprice guide has the full story.

Compare the J/TH, not the cooling. The cooling only decides where the machine can run.

Chart: What one petahash earned each day this year, in dollars. A machine's cooling does not move this line; its efficiency decides how much of it is left after electricity. Open the chart

How to choose, in four steps

Work through these in order and the cooling mostly chooses itself.

Step 1: start from the site. The site sets the power rate and the uptime, and it limits the cooling. On our sites page on 7 October 2026, Argentina takes air and hydro, the UAE hydro and immersion, and Nigeria air and hydro, at 6¢, 6¢ and 3.9¢ per kilowatt-hour. Air machines start in Argentina, hydro and immersion machines in the UAE, and a machine can later cross to cheap power once. What uptime costs is worth working out before you pick.

Step 2: compare efficiency at that site's rate. Divide the watts by the TH/s, multiply by 0.024 for kWh per terahash per day, then multiply by the rate. Do it for every machine you are weighing, and the one with the lower figure keeps more of each day's earnings. How much electricity a miner uses works through it.

Step 3: compare the whole price, not the box. A hydro or immersion machine can look cheap per terahash partly because fewer buyers have a room that can run one. With hosting that room already exists, so what matters is the all-in price, which covers freight, duty and setup as well as the machine.

Step 4: ask the host who looks after the cooling. Ask what the site uses, who maintains the loop or the tank, and what happens when a machine needs a repair. A fault is a line on a bill, not the end of the plan, as the repair guide explains. A host that answers all three plainly is easy to trust.

What it means for a miner

The cheapest coin we can show comes from a hydro machine, and the reason is not the water. On 7 October 2026 the most efficient machine in stock, the S23 XP Hyd at 8.9 J/TH, at the cheapest site that can run it, Nigeria at 3.9¢ per kilowatt-hour, comes to about $18k to make one bitcoin from electricity, against $86k to buy one. That is electricity only, at today's hashprice, before the machine and the pool fee.

What earns that figure is the 8.9 J/TH and the 3.9¢. The cooling is what lets the machine run at that efficiency, and the host's building is what supplies it. Pick the efficiency and the site first, and let the cooling follow.

Mining output depends on bitcoin's price, network difficulty and uptime, and is not guaranteed. Nothing here is investment advice.

Sources and further reading

Immersion cooled bitcoin mining explained — MARA, 27 Feb 2023

Disclosure

Firsthand Bitcoin sells and hosts mining hardware, including this machine. No manufacturer, distributor or affiliate programme paid for or reviewed this page and we take no commission on the links above. Historical figures are computed from daily bitcoin price and network hashprice, each day valued at its own prices. Nothing here is investment advice.