Cooling ASIC miners

01How much heat a miner returns

Almost all the electrical power ends up as heat. The miner does no mechanical work, it heats air. A 3,5 kW ASIC gives the room those same 3,5 kW back, roughly three electric kettles running without a break.

One consequence follows: the thermal load of a room is the sum of the nameplate power of every miner in it, with no coefficients and no corrections. Four machines at 3,5 kW are 14 kW of heat, day and night.

02How much air has to move

Air carries heat away by warming up itself. The more you let it warm, the less of it you need. The sum works out like this: cubic metres per hour are roughly 3000 multiplied by the kilowatts and divided by the temperature rise you allow, in degrees.

Ten degrees is a sensible working point. But if it is 35 outside, the outgoing air lands around 45 and the miner starts dropping frequency. For a Spanish summer at 3,5 kW, work closer to the five degree column, about 2000 cubic metres per hour per machine.

The miner fans do part of the job. What they move is air through the case, not through the room. If the room is not ventilated, all they do is stir already hot air.

03Intake and extraction

Up to three miners an open window for intake and an extractor fan for the exhaust usually suffice, provided they sit on opposite sides. From four machines the flows have to be separated deliberately.

The room splits into two zones. The cold aisle is the side the ASICs take air from, and that is where the intake from outside arrives. The hot aisle is the exhaust side, and from there the air leaves the building. Miners are lined up facing the same way, so that one machine does not blow into the intake of the next.

• Intake and extraction go on different walls, better still at different heights
• The exhaust is not aimed at a wall closer than two metres, or the air comes back
• Leave a gangway between rows so a machine can be reached from both sides
• Make extraction slightly stronger than intake, then dust is not pulled in through the gaps

04Why air conditioning does not work here

A domestic 3,5 kW split unit covers exactly one miner and draws about a kilowatt of its own. You end up paying for electricity three times: for the miner, for the split, and for heating the street.

Four machines need 14 kW of cooling capacity. That is industrial equipment, its price and servicing come close to the cost of the miners themselves, and it adds not one terahash.

Ventilation costs a fraction of that. Air conditioning earns its place in one case: when outside air is so hot that even a large airflow cannot keep the inlet temperature inside the manufacturer range. In Spain that is a few weeks a year, and it is usually handled by running at night.

05Dust and filters

Tens of thousands of cubic metres of air pass through an ASIC every day, and all the dust they carry settles on the heatsinks. A clogged heatsink raises chip temperature, the miner drops frequency, and the hashrate falls without a single error in the log.

Fit a G4 coarse filter on the intake and change it by how dirty it is, not by the calendar. Blow the miners out with compressed air every three to six months, more often in a dusty room. Do it with the machine off and outdoors, otherwise the dust simply moves inside the case.

06Noise

The 75 dB on the datasheet of most air cooled ASICs is vacuum cleaner territory, measured in a lab. In a real room, with walls sending the sound back, it comes out higher.

The scale is logarithmic: every three decibels double the sound energy. That is why two miners sound considerably worse than one, rather than twice as loud as the numbers suggest.

What helps: move the machines to a separate room, seal the door with a heavy strip, hang absorbing panels on the exhaust side wall. What does not help: a box around the miner with no calculated airflow. It muffles the sound and kills the cooling.

07Frequent questions