A humidifier doesn't boil water — it sings through one metal disc under the surface, vibrating water straight into mist. Scale covering that throat turns the song into a hum. And the white dust isn't the machine failing — it's your tap water's calling card.

Where the mist goes (no heater, no fan — all on one metal disc)

An ultrasonic humidifier doesn't "steam" water — a metal diaphragm in the base vibrates at ultrasonic frequency, shattering the water touching it straight into mist. Vibration is its only talent, and scale is its only nemesis: calcium and magnesium in tap water precipitate into a white crust on the disc, muffling the vibration — the machine hums, but the mist is gone. So no-mist has exactly three accounts: the disc is filmed over (#1), the float is stuck (the machine thinks it's empty), or the water is too hard (weak mist + white dust).

Match the evidence

• Hums / lights on, but no mist → Scale on the diaphragm: unplug, empty the base, pour white vinegar in to cover the disc and soak for 45 minutes (not a minute longer — long soaks eat the rubber seals), scrub gently in circles with a soft toothbrush, rinse three times. Never scrape with anything metal — one scratch is the beginning of that disc's retirement.
• Shuts off while the tank still has water → Stuck float or unseated tank: first reseat the tank — you should see bubbles rising in the base when it's seated right (bottom-fill models twist clockwise until locked); then check the float — lift it with a fingertip five times, it should move like a piston; if it's sluggish, soak in vinegar for 20 minutes.
• White dust on furniture, mist weaker than before → Hard water: the white dust is minerals riding the mist onto your furniture — not a fault, your tap water's calling card. Dump stale water, wipe the film off the nozzle and intake, switch to distilled water or fit a demineralization cartridge.

Two easy misjudgments: in a cold room, cool mist condenses and vanishes right at the nozzle — hold your hand a hand-span above the outlet; damp coolness means it's working. And evaporative (no-mist) models are invisible by design — don't judge them by an ultrasonic machine's standard. One honest limit: do the dancing-droplets test — tank off, power on, look straight at the metal disc: tiny droplets dancing on it within 10 seconds = the disc lives, the blockage is in the mist path; not a single droplet, still silent after cleaning = the diaphragm is cracked or it's the board. For these small appliances the repair bill usually beats a new machine — even the tech will tell you to replace it.