Tap water is perfectly acceptable in most humidifiers, provided the tank is cleaned weekly.
The exception is an ultrasonic model in a hard-water area.
There the dissolved calcium leaves the tank with the mist and settles as a fine white film on the television, the skirting board and the top of the wardrobe.
The water you choose does not change how humid the room gets. It changes where the limescale ends up.
The minerals have to go somewhere, and the machine decides where
An ultrasonic humidifier vibrates a small disc at roughly two million times a second and flings microscopic droplets into the room.
Each droplet evaporates in mid-air. Whatever was dissolved in it does not.
That residue is the white dust, and it is the reason the debate about water exists at all.
An evaporative humidifier works the other way round: a fan draws air through a damp wick, and only water vapour leaves.
The minerals stay behind in the wick, which stiffens and eventually needs replacing.
A warm-mist unit boils the water, so the scale plates itself onto the heating element instead.
Same tap water, three entirely different places for the limescale to land.
Hard water starts around 200 mg/l, and much of England sits well above it
Water hardness is measured in milligrams of calcium carbonate per litre.
Below 100 mg/l is soft. Above roughly 200 mg/l is hard, and parts of the south-east reach 300 or more.
Typical mains hardness, in milligrams of calcium carbonate per litre. Your supplier publishes the figure for your postcode.
Most of Scotland, Wales, the north-west and the West Country are soft, and tap water there causes very little trouble.
London, the Home Counties, East Anglia and much of the Midlands are hard to very hard.
Suppliers publish a postcode look-up, sometimes in French degrees or Clark degrees rather than mg/l; one French degree is 10 mg/l.
| Water in the tank | White dust | Descaling | Cost |
|---|---|---|---|
| Tap, hard area | Heavy in ultrasonic units | Weekly | None |
| Tap, soft area | Barely noticeable | Fortnightly | None |
| Filter-jug water | Reduced, not removed | Weekly | Cartridges |
| Deionised or distilled | None | Rare | ยฃ2โยฃ4 per 5 litres |
Buy the water only if the machine sprays it. An evaporative humidifier can drink from the tap quite happily.
Boiling and filter jugs help rather less than people expect
A jug filter uses ion exchange resin, which takes out some of the calcium and magnesium but nowhere near all of it.
Boiling drops out temporary hardness as scale in the kettle, which helps only if you let the water cool and decant it carefully off the sediment.
Neither route gets you to zero, and zero is what an ultrasonic unit needs to stop dusting the room.
Deionised water from a motor factor costs a few pounds for five litres and is the cheapest genuinely mineral-free option.
That sounds trivial until you notice a large humidifier can get through five litres in a single day on its highest setting.
Demineralisation cartridges are the sensible middle
Most ultrasonic manufacturers sell a cartridge that sits in the tank, at ยฃ8 to ยฃ15, lasting a month or two.
They cut the dust substantially without committing you to hauling bottled water home.
They do not remove the descaling job, because scale still forms on the transducer under the water line.
Nor does distilled water make a dirty tank safe: standing water grows bacteria whatever its mineral content, which is why the weekly clean matters more than the water.
The practical rule is to keep mineral dust out of the mist rather than to worry about any particular dose of it.
With hard water in mind, it is the compact ultrasonic models that this affects.
Our Levoit Classic 300S review sets out the tank and cleaning numbers for one of them.
