My wife needs a CPAP machine for sleep apnea. One of the things her model does is humidify the air. To do this it evaporates about 1 cup (237 mL) of water every night.

One is strongly advised to use distilled (i.e. deionised) water as, if one uses the hard tap water, limescale quickly builds up.

Now while distilled water is not expensive (though it does add up), I am wondering if I can get it for free from one of the devices in my house.

I have noticed we have a few machines that produce a fair bit of water:

  • dehumidifier - My notion is since it is pulling water vapour our of the air, the fact that it is vapour means it can't have much impurities dissolved in it. Though vapour isn't a gas so maybe it can.

  • condensing clothes drier - After drying a load of washing it needs at least 2 cups (473 mL) of water to be drained. If all that water is being evaporated (rather than spun) out of the cloths then it should be pure.

  • window vacuum - You use this to wipe/suck condensation off of windows. It doesn't make much water but it does make some.

Maybe other devices too, like fridges?

The water doesn't need to be perfect, and certainly doesn't need to be food grade. But I am wondering is any of my household see devices are producing distilled water?

  • 4
    "certainly doesn't need to be food grade" - yes it does. Your wife will be breathing this water in, it MUST be clean.
    – Hobbes
    Nov 19, 2018 at 15:33

1 Answer 1


Great question.

I think you're right in that the water from all of these sources should contain little-or-no scale chemicals - but I'd be reluctant to use it for this purpose, purely because of the risk of bacteria/viruses that could be present and would get breathed directly into your wife's lungs.

A couple of better sources come to mind:

1) A standard filter-jug. Having used one of these for many years, I can verify that scale build-up in the kettle is negligible. Clearly the filter is absorbing the scale very effectively.

2) Water softener. I assume you don't have one, but if you have a friend or neighbour who has one, maybe you could ask them to let you have one or two litres per week, given that it's for health purposes? You would need to keep the stock refrigerated though - otherwise you'll have the same potential for contamination that I mentioned at the beginning.

  • Agreed, the water from all those sources will be contaminated.
    – Hobbes
    Nov 19, 2018 at 15:29

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