Quick answer: For 2,000 m³/h of winter air at 0 °C / 80 % RH heated to 21 °C and humidified to 40 % RH, a humidifier needs about 7.6 kg of water per hour. A humidifier control system adds moisture when space or supply-air RH falls below target; sizing also includes process losses.
Choose the humidifier type by energy, water quality and hygiene needs (steam, ultrasonic, evaporative or high-pressure spray), control it from a room or return-air sensor with a high-limit sensor in the duct to prevent condensation, and treat the water to avoid mineral dust and microbial growth.
Quick answer
- Load (kg/h) = air mass flow × (target − incoming moisture content).
- Ultrasonic: very low electrical power, needs treated (demineralised) water.
- Always add a duct high-limit sensor (for example 80–90 % RH) downstream.
Why humidify
| Application | Typical target |
|---|---|
| Offices and public buildings in cold climates | About 30–50 % RH for comfort and to limit static |
| Printing, paper, textiles, woodworking | Product-specific, often 45–60 % RH, to stop dimensional changes and static |
| Electronics manufacturing | Controlled RH to limit electrostatic discharge |
| Greenhouses and propagation | High RH for cuttings and seedlings |
| Cold stores for fruit and vegetables | High RH to reduce weight loss |
| Hatcheries | Stage-specific RH; see incubator settings |
Related: incubator settings.
The US EPA notes that humidifiers must be kept clean and used with care to avoid microbial growth and mineral dust (US EPA, Use and care of home humidifiers).
Sizing: worked example
Load (kg/h) = air volume (m³/h) × 1.2 × (x_target − x_incoming) ÷ 1,000
- Outdoor air: 0 °C / 80 % RH → about 3.0 g/kg.
- Target: 21 °C / 40 % RH → about 6.2 g/kg.
- Ventilation air: 2,000 m³/h → 2,000 × 1.2 × (6.2 − 3.0) ÷ 1,000 ≈ 7.6 kg/h.
Add moisture absorbed by products (paper, wood, textiles) and any leakage, then a margin.
Humidifier types
| Type | How it works | Energy | Water quality | Notes |
|---|---|---|---|---|
| Steam (electrode or resistive) | Boils water to steam | High: about 0.75 kWh per kg of steam | Tolerant (electrode needs some conductivity) | Hygienic; no cooling effect |
| Ultrasonic | Vibrating transducers make a fine mist that evaporates | Very low electrical power | Needs demineralised (RO/DI) water to avoid white dust | Cools the air as mist evaporates |
| Evaporative (wetted media) | Air passes over wet media | Low | Needs water treatment and bleed-off | Cools the air; capacity limited by air conditions |
| High-pressure spray/fog | Nozzles atomise water | Low | Needs treated water | Large capacities; greenhouses, industry |
Adiabatic types (ultrasonic, evaporative, spray) need heat to evaporate the water: about 0.68 kWh per kg, which shows up as a cooling effect. That is welcome in warm rooms and must be supplied by heating in winter.
Our ultrasonic humidifiers (RYJS series, 3 to 102 kg/h) are used for industrial rooms, greenhouses, cold stores and supply-air humidification.
Control
| Element | Role |
|---|---|
| Room or return-air sensor | Main control signal |
| Duct high-limit sensor | Stops humidification if duct air approaches saturation (typically 80–90 % RH) to prevent wet ducts |
| Airflow switch | Stops the humidifier if the fan stops |
| Modulating control | Matches output to load (better than on/off for stable RH) |
| Dew point limit | Prevents condensation on cold windows and surfaces |
In cold weather, lower the indoor RH target as outdoor temperature falls if windows or walls condense.
Water quality and hygiene
- Use demineralised water (reverse osmosis or deionised) for ultrasonic and spray systems.
- Drain and flush reservoirs; avoid standing water.
- Clean and disinfect on a schedule.
- Check the water system for microbial risk as part of building water management.
Commissioning and maintenance
- Calibrate the room and high-limit sensors.
- Check that the absorption distance in ducts is long enough so mist evaporates before reaching bends or filters.
- Inspect nozzles, transducers and media regularly.
FAQ
Frequently asked questions
How do I size a commercial humidifier?
Air mass flow × (target moisture content − incoming moisture content), plus product and leakage loads.
What is the most energy-efficient humidifier?
Ultrasonic and other adiabatic humidifiers use far less electricity than steam, but the evaporation heat must come from the air or heating system.
Why does my ultrasonic humidifier leave white dust?
Minerals in the water. Use demineralised water.
What RH should a humidifier maintain in winter?
Often 30–50 % for comfort; lower when windows condense in very cold weather.
Sources
- US EPA, Use and care of home humidifiers.
- ASHRAE Handbook—HVAC Systems and Equipment, Humidifiers.
- NIST, Hygrometers.
