CPAP manufacturers specify distilled water for the humidifier chamber, and reverse osmosis water is not distilled water. It is much closer than tap water, and many users report no problems, but it still carries a small residual load of dissolved solids that concentrates in the chamber as water evaporates. Follow the manual, and understand what you are trading if you do not.
Why the manual says distilled
A humidifier chamber evaporates water and leaves behind everything that was dissolved in it. With tap water that means visible scale, mineral film on the chamber walls and a shortened chamber life, plus deposits that are harder to clean out of the corners. Distilled water leaves essentially nothing, so the chamber stays clear. The instruction is about deposits and hygiene in a device you breathe through every night.
How reverse osmosis water differs
A reverse osmosis membrane rejects most dissolved solids under pressure but does not reject all of them, so the product water carries a small residual. Distillation leaves nearly everything nonvolatile behind in the boiler. In a chamber that concentrates whatever it holds, a small residual still accumulates over months, just far more slowly than with tap water. That is the whole of the practical difference, and it is why the two are not interchangeable in a spec sheet.
The decision, put plainly
If the manual says distilled, using anything else is your call against the manufacturer's instruction, and it is the manufacturer who decides warranty questions. Many people in areas with expensive or awkward distilled supply use reverse osmosis water and clean the chamber more often. What you should not do is use plain tap water, softened water, which swaps hardness for sodium and still evaporates dry, or filtered water from a carbon pitcher.
If your motivation is avoiding jugs
An under sink reverse osmosis system produces low mineral water on demand at a dedicated faucet, which is what makes it tempting for this job. Our reverse osmosis buying guide covers stage counts and tank sizes. If you go that way, keep the membrane and cartridges on schedule, because a neglected system passes more dissolved solids and the whole reason for choosing it disappears without anything visible changing.
Questions people ask about can i use reverse osmosis water in my cpap
Will RO water damage a CPAP humidifier?
It will not damage the machine, but it leaves a small residue over time because the membrane does not reject every dissolved solid the way a still does.
Can I use filtered tap water in a CPAP?
No. A carbon filter improves taste and odor but leaves the minerals in place, and those minerals scale the chamber as water evaporates.
How often should I clean the chamber if I use RO water?
Follow the manufacturer's cleaning schedule and inspect weekly for film or cloudiness, cleaning more often than a distilled water user would.
Is bottled drinking water acceptable?
Not usually. Most bottled drinking water is mineral bearing by design, so it scales the chamber much like tap water does.