Is reverse osmosis water any different from distilled?

Not meaningfully for this purpose. Reverse osmosis removes virtually all minerals, so it behaves the same way and needs the same treatment before use.

What about just once, to flush the machine?

A single flush is not the concern. The problem is running demineralised water continuously as the standing water inside a heated metal boiler.

BrewWater

Does distilled water damage an espresso machine?

Short answer

Yes, it can. The World Health Organization states that demineralised waters are highly aggressive and must be stabilised, and that unstabilised soft water strongly tends to corrode metal surfaces and pipes. Distilled water also tastes flat. Remineralise it before use rather than running it neat.

Measured values on this page, with sources
Measure	Value	Unit	Source
Target total alkalinity for brewing water	40	mg/L as CaCO3	Specialty Coffee Association of America · verified 2026-07-25
Upper bound of soft water; above this, water is at least moderately hard	60	mg/L as CaCO3	World Health Organization · verified 2026-07-25

What does the WHO actually say?

Its hardness background document states that demineralised waters produced by nanofiltration, reverse osmosis or thermal desalination are highly aggressive and must be stabilised before distribution, typically by adding lime and alkalinity. Separately it states that soft water which has not been stabilised has a great tendency to cause corrosion of metal surfaces and pipes.

Why is this the opposite of what people expect?

Because the reasoning usually stops at scale. Scale is visible, well known and descalable, so avoiding it feels like the whole problem. Corrosion is invisible until it is not, and it is not reversible. Removing every mineral solves the first problem and creates the second.

So what should you actually do?

Use distilled or reverse osmosis water as a base and add minerals back to reach roughly forty milligrams per litre of alkalinity. That gives you both the repeatability of a known starting point and enough buffering that the water is not chemically aggressive to the boiler.

Common questions

Is reverse osmosis water any different from distilled?: Not meaningfully for this purpose. Reverse osmosis removes virtually all minerals, so it behaves the same way and needs the same treatment before use.
What about just once, to flush the machine?: A single flush is not the concern. The problem is running demineralised water continuously as the standing water inside a heated metal boiler.