Warm mist sterilises the water; cool mist saves the electricity
Both types raise the humidity in a room. The difference is what happens to the water on its way out.
A warm mist unit boils it. A cool mist unit does not.
Running cost, noise, safety and the white film on your furniture all follow from that single decision.
Cool mist is two different machines wearing one name
"Cool mist" covers two mechanisms that behave nothing alike.
Ultrasonic models vibrate a metal disc thousands of times a second and throw water off it as a visible fog.
Evaporative models blow air through a damp wick. Their output is invisible, because it is genuinely vapour.
The distinction matters most if you live somewhere with hard water, which is most of southern and eastern England.
An ultrasonic unit atomises whatever is dissolved in the tank, minerals included.
Those minerals settle as a fine pale film on nearby surfaces โ the white dust owners complain about.
An evaporative wick leaves the minerals behind in the wick instead. No dust, but you buy replacement wicks.
Evaporative models also self-limit. Saturated air stops accepting vapour, so over-humidifying with one is difficult.
An ultrasonic unit will cheerfully push a small bedroom past 70% and hand you condensation on the windows.
Warm mist buys a sterile output for roughly ten times the power
Boiling water kills what is living in it.
That is the honest advantage of warm mist, and in a tank nobody cleans it is not a small one.
A neglected cool mist tank does not merely fail to help โ it aerosolises whatever has grown inside it.
The steam also warms the room slightly, which some people prefer in January.
The cost is electricity, because a heating element is a heating element.
At roughly 25p a unit, eight hours of warm mist a night comes to about 60p, against roughly 6p for ultrasonic.
Across a winter that is the difference between a rounding error and a visible line on the bill.
There is also the obvious point: a warm mist unit holds near-boiling water.
Keep it off the floor and out of reach of anyone who might pull the tank over.
Hybrid models default to cool and switch to warm on demand, which is the sensible compromise if you want both.
Our Levoit LV600S review goes through how that switching behaves in practice.
In a British house the real question is whether you need one at all
Most UK homes have the opposite problem.
Condensation on the bedroom windows in October. Black spotting in the corner of a north-facing wall. A musty smell in the wardrobe.
That is excess moisture, and a humidifier will make every bit of it worse.
Genuinely dry indoor air here is almost entirely a heating effect. Cold outside air warmed indoors holds a much lower relative humidity.
So it turns up in a heated room during a cold snap, and disappears again by April.
The range worth aiming for is 40-60% relative humidity.
Below 40%, throats and sinuses dry out and static starts building. Above 60%, dust mites and mould take over.
A ยฃ10 hygrometer answers that question before a ยฃ90 humidifier answers the wrong one.
Buy the meter first.
