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Humidifiers in HVAC: steam versus adiabatic

Updated 23 October 2026 · SEO Dons Editorial

How does each type of humidifier add moisture?

A steam humidifier boils water and releases the steam into the air. An adiabatic humidifier evaporates liquid water into it. CIBSE Journal describes the steam process as almost pure latent heating, a vertical line at constant dry-bulb temperature on the psychrometric chart, with a small sensible heating effect from the tube that carries the steam. It describes adiabatic humidification, which characterises atomising spray and ultrasonic humidifiers and the rarely used recirculating spray and wetted pad types, as a constant-energy process shown along the wet-bulb line.

TypeHow it makes moistureWhat the maker Condair says
Electrode steamPasses current through water to boil itControl around +/-5%RH. Needs water minerals, so cannot operate on RO water
Resistive steamHeater elements transfer heat to the waterControl around +/-2%RH on mains water. Can operate on RO water
Gas-fired steamSteam raised by gasOutputs start from around 40 kg/h, so suited to larger duties. Any water type
SprayNozzles produce fine droplets that evaporate in the airControl around +/-4%RH
Wetted mediaAir passes through a continually moistened matrixNo aerosol, so it can sit in a short section of an air handling unit
UltrasonicAdiabatic, per CIBSE JournalNo figure quoted here

These figures are a manufacturer's statements about its own products. The humidifier sits in the air handling unit or in the room, and the choice affects the rest of the unit.

How do steam and adiabatic compare on energy?

The difference is where the latent heat of evaporation comes from. A steam unit supplies it from electricity or gas. In an adiabatic process, CIBSE Journal says, the energy is drawn from the sensible heat of the air, reducing its dry-bulb temperature while increasing its moisture content. Two consequences follow from that description.

  • The unit itself uses little energy, but the air leaves cooler. If the supply temperature matters, the heating coil upstream has to warm the air first, so heat is still spent, though it can come from heat recovery or a cheaper source.
  • In summer the cooling can be a benefit, which is the basis of evaporative cooling.
  • Wetted media add resistance. CIBSE Journal says evaporative media inherently obstruct airflow, and the extra fan power is an energy penalty that has to be outweighed by the savings.

Compare on the whole air path: humidifier input, heating coil duty and fan power. No percentage saving is given here because no official or recognised body quoted on this page publishes one for UK buildings. Makers publish savings for their own products; check them against your own load.

Where is humidity control needed, and to what range?

SettingPublished guidanceSource
OfficesGood-practice relative humidity is 40% to 60%. Where there is humidity control, set it above 40%. Mould grows above 60%CIBSE Journal (2021, 2020, 2011)
Archives35% to 60% relative humidity and 13 to 20 degrees CGOV.UK valuation guidance
HealthcareSet room by room in HTM 03-01, not reproduced here. See healthcare ventilationNHS England
Labs, print, museums, server roomsNone of the sources checked gives a UK range. Use the process or equipment maker's stated limitsNone

Tolerance matters as much as the target. A band of +/-2%RH is a different job from +/-5%RH, and it narrows the choice of unit. Dehumidification is the opposite problem, covered in commercial dehumidification.

What legionella duties apply to humidifiers?

HSE lists humidifiers and air washers among the other systems that could be a source of legionella growth. Its test is whether a water system has any of these factors:

  • legionella present in the system water, where the water temperature may be between 20 and 45 degrees C
  • a means of creating or spreading breathable droplets (aerosols)
  • stored or re-circulating water
  • a likely source of food, such as sludge, rust, scale, organic matter or biofilms

The actions are in Part 3 of HSG274. HSE says you must carry out a risk assessment, that these systems should be kept clean and monitored regularly where appropriate, and that there is a duty to keep records of maintenance and monitoring results. For an employer or someone in control of premises, the duty is to understand and manage legionella risks, and not every system needs elaborate controls. HSE does not rank humidifier types, so each unit is assessed against the factors.

Question to put to each unitWhy it matters
Does it store or recirculate water?One of HSE's factors
Does it spray or atomise?Spraying creates droplets, which is HSE's aerosol factor. CIBSE Journal notes evaporation in its exhaust-air example produces vapour rather than droplets
How warm does the water get, and for how long does it sit?The 20 to 45 degree band
Where can scale or biofilm form?A food source
Who is the competent person, and where are the records?Duty to monitor and keep records

CIBSE Journal says modern exhaust-air evaporative cooling systems, which use adiabatic humidifiers, incorporate automated flushing, draining and drying cycles to prevent stagnation, with water quality monitoring and sometimes ultraviolet sterilisation. It also notes that sprays lost favour in comfort air conditioning because of maintenance concerns. A manufacturer, Condair, describes steam as intrinsically hygienic, but that is a supplier's view and the assessment still applies. The wider duty is in the legionella and air conditioning guide.

What does maintenance involve?

CIBSE's ventilation guidance says a visual check of equipment, including condensate drain pans and humidifier reservoirs, should be made at least once a week. Beyond that, the routine depends on the type. Condair says an electrode unit deposits minerals as limescale in a replaceable plastic cylinder that periodically needs replacing, while an evaporative unit has minerals removed by water treatment or flushed to drain. Water quality therefore feeds straight into the service plan, along with the cleaning and records HSE expects. The humidifier tasks sit within the AHU maintenance checklist.

How should you choose?

  1. Set the humidity target and tolerance from the use of the space.
  2. Decide whether cooling the air is wanted, and what heat is available to warm it.
  3. Check the water supply, drain and whether RO or softened water is needed.
  4. Decide who owns the legionella risk assessment, cleaning and records.
  5. Check space and pressure drop in the air handling unit.

Readers weighing humidification in an existing or new air handling unit can send an enquiry through the site's form.

Sources

Common questions

Does a steam humidifier avoid legionella risk?

HSE's page does not exempt any type. It lists humidifiers as systems that could be a source and tests each against its factors, such as stored or re-circulating water and a food source. Condair describes steam as intrinsically hygienic, but that is a manufacturer's view, so every unit still belongs in the risk assessment.

Do adiabatic humidifiers cool the air?

Yes. CIBSE Journal says the energy to evaporate the water is drawn from the sensible heat of the air, lowering its dry-bulb temperature while it gains moisture. That can be useful in summer and a cost in winter, when the air may need heating before or after.

What relative humidity should an office be kept at?

CIBSE Journal gives good-practice relative humidity as 40% to 60%, and reports CIBSE guidance that, where humidity is controlled, it should be set above 40%. A separate CIBSE Journal module says mould grows where relative humidity is greater than 60%.

Can a steam humidifier run on reverse osmosis water?

It depends on the type. Condair says its electrode units need water minerals and cannot operate on RO water, while its resistive units can. Gas-fired units, it says, can be used with any water type.

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