vtraudt

Obviously, hard tap water is not good.
Probably: bottle water from store?
Or soft well water (water softener)?

Not readily on hand, but probably best: the fancy water from grocery store my wife uses for her ironing iron?

How about reverse osmosis water. It sure has no minerals. But might be aggressive to the coolant system metals?

DIEDE

Bottle water from store is almost the same as tap water: The problem is minerals it has.
Soft water is the same as one that is used on ironing iron, and is exactly what you need for coolant: Deionized water.

About reverse osmosis, as far as I know, the result product is the same as in ion exchange method.
I don't know about your country, but here (Brazil), the reverse osmosis is not so common.

dalek453

Distilled Water is the mineral free water used in irons and is also free of charge if you have a dehumidifier running in your home. Water is removed from the air and therefore has no minerals in it which is what you want.

joaquinnparker

I use filtered water for coolant 50/50 ratio. Using tap water will cause some residue to your system and most likely corrosion. I use tap water into my 1.8 and after couple of years my water pump suddenly stop pumping liquid.

N_Jay

Distilled Water ONLY.

Filtered does NOT remove minerals (which is the issue)

vtraudt

Reverse Osmosis (drinking water processing systems installed in homes) have no minerals. BUT are known to be aggressive (use plastic lines instead of copper for that reason).

Any verifiable comment on using reverse osmosis water?

N_Jay

I guess it depends on the quality of the RO system.

In theory if the membrane is good, and no metal lines are used post the RO membrane, the water should be well enough deionized.

Don't use home softened water. It has a lot of sodium. It is NOT like distilled (or RO, or Deionized). It simply trades "soft" ions (sodium) for "hard" ions (calcium).