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Distilled, filtered or tap?

The EPA advises low-mineral water such as distilled. That is unambiguous, and it is also a gallon a day you have to buy and carry, so it is worth knowing exactly what you get for it.

By Scooter M. · Published · Updated

A glass jug and a plastic bottle of water on a counter
Licensed stock photograph, for illustration. It is not a photograph of any product on this page and we have not handled any of them.

The EPA's guidance is not ambiguous: use water with low mineral content, such as distilled, and avoid tap water because ultrasonic and impeller humidifiers are very efficient at dispersing the minerals in it into the air. That is the recommendation and we are not going to soften it. What follows is what it actually buys you, and what it costs.

What each water type does

Water types against the four problems
WaterWhite dustScale in the tankBacterial growthCost
DistilledEliminatedEliminatedUnchangedBought by the gallon
Demineralized / deionizedEliminatedEliminatedUnchangedBought by the gallon
Soft tap waterReducedReducedUnchangedFree
Hard tap waterFullHeavyUnchangedFree
Carbon-filtered tap (jug)Barely reducedBarely reducedUnchangedFilter cartridges
Bottled spring / mineralCan be worse than tapCan be worse than tapUnchangedExpensive

What it costs

This is the part of the recommendation nobody quantifies, so here it is in gallons rather than dollars. We do not print a price for distilled water: prices vary by region and by retailer and we do not have a live price feed for supermarket goods, so quoting one would be exactly the kind of remembered number this site refuses to publish. The volumes, though, are pure arithmetic.

For a bedroom unit that is about 115 gallons across a winter, which is a couple of gallon jugs a week - manageable, if annoying to carry. For a 4 gallon-a-day large-room unit it is 728 gallons a season, which is roughly four gallon jugs a day, every day, from October to March. At that scale the recommendation stops being practical advice and starts being an argument for buying a different type of machine.

So the honest recommendation splits by machine type

What we would actually do
Your situationWhat to do
Ultrasonic unit, hard waterDistilled, or switch machine type. This is the combination the EPA guidance is aimed squarely at, and it is where white dust is worst.
Ultrasonic unit, soft waterTap is workable if you accept fortnightly descaling. Distilled is still better.
Evaporative unit, any waterTap. The minerals stay in the wick by design, and the wick is a consumable you were replacing anyway. Hard water shortens wick life - see filter care.
Warm mist unitTap is fine for the air. The minerals precipitate in the boiling chamber, so expect heavier descaling.
Large-room unit over 2 gal/dayBuying distilled is impractical at that volume. Choose an evaporative machine and use tap.

The options people try that do not work

  • Bottled spring or mineral water. Contains dissolved mineral by definition. Some brands are harder than tap water, so this can make white dust worse while costing more than distilled.
  • A carbon filter jug. Built to improve taste and remove chlorine, not to remove total dissolved solids. It makes almost no difference to white dust or scale.
  • Boiled and cooled water. Precipitates some temporary hardness and leaves the rest. Also not sterile once cooled and standing.
  • Reverse-osmosis water, if you have it plumbed in. This one does work - RO removes most dissolved solids, and if you already have a system it is effectively free distilled water. Worth knowing.

The upstream question

If you find yourself weighing 728 gallons of bought water against a machine you already own, the framing is wrong. The white dust and scale problem is a property of ultrasonic humidifiers specifically, and it is one of the two genuine differences between the machine types - the other being the filter bill, which runs the other way. That trade is laid out in full on evaporative vs ultrasonic and costed on filter-free vs filtered.

Questions people actually ask

Do you have to use distilled water in a humidifier?

The EPA advises using water with low mineral content such as distilled, because ultrasonic and impeller units are very efficient at dispersing tap-water minerals into the air. For an ultrasonic unit on hard water it is the right call. For an evaporative unit the minerals stay in the wick rather than in the air, so tap water is workable.

How much distilled water does a humidifier use?

The same as its daily output. Over a 182-day heating season that is about 115 gallons for a 0.63 gal/day bedroom unit, 364 gallons for a 2 gal/day large-room unit and 728 gallons for a 4 gal/day one. At the top of that range, buying water stops being practical and becomes an argument for a different machine type.

Does distilled water prevent humidifier mold?

No. Distilled water is mineral-free, not sterile, and it stops being sterile the moment it sits in an open tank in a warm room. It eliminates white dust and scale. It does nothing for bacterial growth, and the EPA's every-third-day cleaning instruction applies regardless of which water you use.

Can you use tap water in an ultrasonic humidifier?

You can, and you will get white dust in proportion to your water hardness plus scale inside the tank. The EPA specifically advises against it for this machine type. If you want to keep using tap water, an evaporative unit is the type that tolerates it.

Is bottled water better than tap for a humidifier?

Usually not. Spring and mineral waters contain dissolved mineral by definition and some are harder than tap water, so they can make white dust worse while costing considerably more. Distilled or demineralized water is a different product and is the one to look for.

Sources

Every figure on this page came from one of the documents below, read on the date shown. We do not cite other review sites, forum posts or video reviews as sources of fact.

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