What Water to Put in a Humidifier

The answer depends on the type of humidifier. An ultrasonic unit disperses everything that is dissolved in the water. Steam and evaporative units leave the salts inside themselves. The difference in design determines both the water requirements and the nature of the problems.

Three Types and Three Different Answers

Ultrasonic humidifier. A piezoelectric element vibrates at high frequency and tears the finest droplets off the water surface. An aerosol goes into the air. The water in the droplet evaporates, while everything dissolved remains as solid microparticles and spreads around the room.

Steam humidifier. The water boils, and pure water vapor goes into the air. The salts remain in the boiling chamber and form scale on the electrodes or the heating element.

Evaporative humidifier, also known as a traditional humidifier or air washer. A fan drives air through a wetted wick or rotating discs. Here too, only pure water evaporates. The salts remain on the wick and in the tray.

Hence the rule. With an ultrasonic unit, minerals end up in the lungs and on the furniture. With steam and evaporative units, they end up in the appliance itself.

What White Dust Is

The white film on furniture, electronics and the windowsill near a humidifier is dried salts from tap water. The US Environmental Protection Agency describes the mechanism directly: minerals from the water are dispersed by ultrasonic and impeller humidifiers and deposited on surfaces in the room.

The amount of dust is directly proportional to the total dissolved solids in the water. A liter of water with a dry residue of 500 mg/L will leave half a gram of solids in the room. Over a week of operation that is already several grams, and this entire quantity passes through the air you breathe.

EPA recommends not using tap water in ultrasonic and impeller humidifiers. Instead, use bottled water labeled "distilled" or a demineralization cartridge. Water labeled "spring," "artesian" or "mineral" is not suitable, because the minerals have not been removed from it.

What Manufacturers Say

The instructions for household humidifiers say the same thing in softer terms. For its HUL585 series of ultrasonic humidifiers, Honeywell states that a high mineral content in the water causes a white mineral deposit on surfaces near the unit. Two solutions are offered: distilled water or a demineralization cartridge.

The same documents contain limitations that are often forgotten.

• Fill with cold water only. Warm and hot water must not be used.
• Do not add essential oils, medications or inhalation liquids to the tank.
• Clean the unit weekly; remove scale by soaking in white vinegar for about 20 minutes, followed by rinsing.
• Target relative humidity 40–60%.

EPA gives a stricter upper limit. It is better to keep indoor humidity no higher than 50%, so as not to encourage the growth of biological organisms.

What Must Not Be Used

Mineral bottled water. Its total dissolved solids are higher than those of tap water. There will be more dust, not less.

Softened water from an ion exchange filter. Ion exchange does not reduce total dissolved solids. It exchanges calcium and magnesium for sodium in equivalent amounts. The total mass of dissolved substances stays the same, and the white dust does not go away.

Boiled water. Boiling precipitates part of the carbonate hardness, but does not remove sulfates, chlorides and sodium. Part of the water also evaporates, and the remaining solution becomes more concentrated.

Water with additives. Oils, fragrances and "disinfecting" liquids destroy the plastic of the tank and the piezoelectric element. The instructions explicitly prohibit this.

How to Obtain Suitable Water

Distilled water from a pharmacy or auto parts store. A simple option for one small unit.

Demineralization cartridge. It is installed in the humidifier tank and works on the ion exchange principle with a mixed resin bed. Its service life is limited and depends on the hardness of the raw water. On hard water, the cartridge is used up faster than the stated life.

Reverse osmosis. Justified if a drinking water treatment system is already installed in the home. Osmosis permeate has low total dissolved solids and is suitable for an ultrasonic humidifier.

For a steam or evaporative humidifier, ordinary tap water is acceptable. But the harder it is, the more often you will have to remove scale and replace the wick. Softened water is appropriate here, because sodium salts do not form dense scale.

Hygiene Matters More Than the Choice of Water

Standing water in the tank is a medium for bacteria and mold. Ultrasonic and impeller humidifiers disperse microorganisms into the air along with the aerosol more effectively than other types. Inhaling such an aerosol can cause an inflammatory reaction in the lungs.

The minimum routine according to EPA recommendations is as follows.

• Every day, empty the remaining water, wipe all surfaces dry and fill with fresh water.
• Every three days, clean the unit, remove deposits with a brush and dry it completely.
• If the manufacturer has not specified a cleaning agent, use a 3% hydrogen peroxide solution with thorough rinsing.
• During long periods of non-use, drain the water and store the unit dry.

Watch for condensation on windows and walls. It means the humidity is too high and needs to be reduced.

What this means for a project in Tashkent

• Water in Tashkent is hard in approximately 85% of the city, and the picture is the same in the other regions of the republic. An ultrasonic humidifier on tap water will here inevitably produce white dust.
• Old central mains add iron and suspended solids. The suspended solids settle on the piezoelectric element's membrane and reduce the dispersion output.
• If a reverse osmosis system is already installed in the apartment, use its water for the humidifier. Buying distilled water separately is then unnecessary.
• In winter, while the heating is running, keep humidity in the 40–50% range and monitor it with a hygrometer rather than by feel.