Coolant for hydro miners – manufacturer specifications and what works in practice

The coolant has three jobs: transport heat, protect the aluminium of the cold plates and not freeze in winter. Tap water does only the first.

Why not tap water

It contains limescale, chloride and dissolved oxygen. Limescale clogs the narrow channels of the cold plates, chloride attacks aluminium, and without an inhibitor there is no protection against that.

What Bitmain specifies

Antifreeze, pure water or deionised water are approved; Bitmain rules out tap water. For the pH value, Bitmain gives a separate range for each coolant:

• Antifreeze: 7.0 to 9.0
• Pure water: 6.5 to 7.5
• Deionised water: 8.5 to 9.5

From a conductivity of 100 µS/cm, Bitmain requires the coolant to be changed; at first fill it should be below 20 µS/cm. However, the manual for the S21 XP Hyd gives the 20 µS/cm as a guide value and points out that inhibitors raise the conductivity considerably.

What MicroBT specifies

For the Whatsminer hydro units, the coolant consists of three parts: pure or distilled water, a corrosion inhibitor and antifreeze. Ethylene glycol or propylene glycol at 20 to 50% is specified as antifreeze, chosen according to the freezing point you need. For the pH value, MicroBT gives 7.0 to 8.7. A unit of the M63 series itself holds about one litre.

What works in practice

Both specifications describe the same thing: very pure water, glycol, inhibitor. That is exactly what standard radiator antifreeze for engines with aluminium parts is. You can get it at any petrol station and from car accessory shops.

Here is what you need to watch out for:

1. Buy concentrate and mix it yourself with distilled or deionised water. Premixed products work too, but cost more.
2. Approved for aluminium. Modern radiator antifreezes are; it says so on the canister.
3. Do not mix. Different inhibitor types can flocculate when combined. Stick to one product and note it on the system.
4. Distilled water alone is not enough. Without an inhibitor it is aggressive towards aluminium.

Whether the manufacturer accepts such a product in a warranty case is the manufacturer's decision. If the unit is running within the warranty period, stick to the list in your model's manual or ask.

Ethylene glycol or propylene glycol

• Ethylene glycol transports heat slightly better and is less viscous. It is harmful to health and must not come into contact with drinking water.
• Propylene glycol is considered not harmful to health and is therefore the right choice when the heat exchanger borders on drinking water or a swimming pool.

The concentration

As little glycol as possible, as much as frost protection requires. Glycol transports heat less well than water and makes the coolant more viscous, so the pump has to work harder. If the system is in a frost-free location, 20% is enough. If pipes or the dry cooler are outdoors, the concentration depends on the lowest temperature at the site. 20% is also the lowest level that MicroBT specifies. For ethylene glycol, MicroBT gives a freezing point of −10 °C at 20%, −15 °C at 30% and −24 °C at 40%.

Filling

1. Flush the circuit with distilled water until it runs clear.
2. Mix the coolant completely outside the system.
3. Fill slowly from the bottom and bleed at all high points.
4. Let the pump run without the miner and bleed again.
5. Check pressure and flow rate, and only then start the miner.

Checking and changing

• Every few months: pH value and appearance. Clear and within the target range is fine.
• Freezing point before every winter with a refractometer.
• Change it as soon as the pH value leaves the range or the coolant turns cloudy. Topping up with water does not replace a depleted inhibitor.

Sources

• Bitmain, specifications with coolant requirements: S19 XP Hyd, S21 XP Hyd
• Bitmain: notes on hydro miners, no tap water
• Bitmain: S21 XP Hyd, user manual with guide value for conductivity (PDF)
• MicroBT: operating instructions for hydro miners, coolant and freezing points
• Tyforop: Tyfocor L, technical information on propylene glycol (PDF)
• Dow: Dowtherm SR-1, specification for ethylene glycol (PDF)

As of October 2026. Your model's manual is what counts; in a warranty case, the manufacturer's specification is what matters.