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« Humidify = add water vapor to the air »


The need for dehumidification is well known to the general public because it is often necessary to dehumidify the walls of a house. The need for air humidification is less obvious but is nevertheless essential to ensure comfort within the building.

In a home, air humidification is mainly done through the use of the bathroom and the kitchen. In an office building, the need for additional humidification is greater because these humid rooms are not present.

We generally humidify during the same periods when the heating works; cold outside air, once warmed, is dry air. To ensure good thermal comfort, the air is humidified up to a relative humidity rate of around 40%.

Humidifying is energy-intensive and it is useful to correctly regulate the humidifier as well as the heating battery. The need for humidification is directly related to the air exchange rate and the desired set point.



A humidifier is a device designed to increase the humidity in the air. They are distinguished according to the origin of the heat of vaporization. Two techniques are possible and the resulting costs are very different.

The humidifier adds water vapor to the air:

  • either by allowing the evaporation of liquid water in contact with the air: adiabatic humidification

    • The heat of vaporization of liquid water is taken from the air which cools. If it is necessary to compensate for the adiabatic cooling, the vaporization energy must be provided by the heating system.

    • We talk about adiabatic, isenthalpic or constant enthalpy humidifiers. The vaporization energy is given by the fuel that powers the boiler (oil, gas, etc.).

  • either directly by injecting water vapor into the air flow: isothermal humidification

    • The heat of vaporization of the water is provided by the humidifier.

    • We talk about isothermal humidifiers because the addition of steam has almost no impact on the temperature of the air flow. The vaporization energy is given by the fuel that powers the humidifier (electricity, gas, etc.). The energy consumed by the humidifier has a higher cost (€/kWh) than that supplied by the heating battery.

Classification into 3 categories:

  • Adiabatic humidifiers without recycling

    • Microniser Rotary Atomiser

  • Adiabatic humidifiers with recycling

  • Steam isothermal humidifiers

    • Electrode steam generator

    • Resistance steam generator

    • Gas steam boiler



It is important to remember that the final responsibility is generally ensured by the user (or his maintenance service) because no system is 100% safe without respecting good practices for use and maintenance.

A steam humidifier does not generally require specific hygiene measures in addition to adequate maintenance. This is an important advantage because the risks are then very limited: the steam can condense in the duct, the lukewarm water tank (when not in use) can put bacteria in contact with the air, etc.

An adiabatic humidifier without recycling strongly limits the conditions for bacterial development. The quality of the water used is essential to guarantee an acceptable level of hygienic risk. Different water treatment techniques exist and will be recommended depending on the acceptable risk threshold and the desired redundancy.

An adiabatic humidifier with recycling presents a significant risk of bacterial development due to the stagnation of water in the retention tank. Antibacterial devices are therefore always necessary with this type of humidifier. This is the big weak point of recirculating humidifiers that excludes them from many projects.


The humidifier aims to reach a humidity set point. It is the BMS (Building Monitoring System) which "releases" the humidifier and sets its "setpoint". It is prudent to slave the operation of the humidifier to that of the fan, to avoid any risk of humidification in the absence of air flow. It is more efficient to regulate the humidification compared to a pulse setpoint. The BMS is able to consider a set of building parameters to choose the appropriate supply setpoint.

Steam humidifiers and adiabatic humidifiers without recirculation are directly capable of adding the desired amount of water to the airflow; they are self-regulating. Humidifiers with recirculation operate with dew point (saturation) regulation. Their regulation therefore requires the intervention of the pre-heating, post-heating and sometimes cold batteries.

  • Control of steam humidifier : the air is heated (or cooled) to the desired temperature in pulsation before being humidified by injection of hot steam (isothermal). A significant part of the energy is provided by the steam humidifier.

    • ​When the fresh air is hot and dry, the steam humidifiers run the risk of operating at the same time as the cold battery with a double high energy consumption.

  • Control of adiabatic humidifier without recycling: the air is heated (or cooled) to the desired enthalpy in pulsation before being humidified without heat input (isenthalpic). All the energy is supplied by the heating battery (which is generally very efficient and economical), the humidifier is only used for humidity control.

    • Non-recycling humidifiers can easily operate in proportional mode because it is very easy to precisely adjust the flow of water sprayed and absorbed by the air flow. With these systems, preference will therefore be given to continuous humidification at a value close to the desired humidity in the environment.

    • Their regulation is the simplest because the energy supply and humidification are separated, which also makes it possible to avoid any competition between the systems with the associated parasitic energy consumption. Non-recirculating humidifiers are the most precise and responsive but are more suitable for centralized humidification.

  • Control of adiabatic humidifier with recirculation: the air is preheated (or cooled) to the dew point enthalpy of the desired humidity in pulsation before being humidified without heat input to the dew point ( isenthalpic) and finally to be postheated to the desired temperature in pulsation. All the energy is supplied by the pre- and post-heating batteries, the humidifier is only used for humidity control.

    • A special feature of humidifiers with recycling is that a post-heating battery is required for their regulation. The regulation of humidifiers with recycling is therefore complex and presents risks of excessive humidification (even in winter) and simultaneous operation of the heating and cooling batteries.



​With a steam humidifier, the heating battery must be able to provide the energy needed to reach the desired dry temperature. Humidification energy is consumed by the humidifier itself.

With an adiabatic humidifier, the heating battery must be able to provide all the energy necessary to reach the humidity and temperature set point. All the energy is supplied by the heating battery; the humidification is isenthalpic.

A post-heating battery is only necessary for adiabatic humidifiers with recycling. As with steam humidifiers, humidifiers without recycling such as the Microniser can operate without a post-heating battery. They can therefore be placed either at the end of the humidification box or in a watertight section of the supply duct.


Steam humidifiers have the advantage of being able to inject steam directly into the duct, without necessarily a humidification box. These humidifiers will be preferred in small spaces. Gas steam boilers must be placed in a separate room (boiler room).

Adiabatic humidifiers must be placed in a sealed humidification box.

The casing of adiabatic humidifiers without recycling can possibly be inserted directly into the supply duct even if it is always preferred to place this casing in the air handling unit.

For adiabatic humidifiers with recycling, this box must be located between the pre-heating battery and the post-heating battery.


For humidification, a distinction is made between energy consumption and the cost of this consumption. The energy efficiently supplied to the air flow is the same in all cases to reach the humidity and temperature set point. The energy losses can be different but above all the energy cost will be greatly different depending on the energy source used.

Two sources of consumption are present:

  • the mechanical energy of the pumps (for spraying, for example)

  • the energy needed to vaporize the water

The first is negligible, at most a few % against the second! On the other hand, the energy required to change water from liquid to vapor state is high. Depending on the type of humidifier, the cost of this energy will be very different:

  • the adiabatic humidifier works from liquid water: it is the air that will provide the vaporization energy. It is therefore the heating battery which, indirectly, will provide the heat. The cost price of the heat of vaporization will be fixed by the price of fuel in the boiler room.

  • the steam humidifier works either with steam from a steam boiler and we find ourselves in a situation similar to the previous point, or with steam of electrical origin whose operating cost is multiplied by a factor 3 or 5.

To turn 1 kg of water at 10°C into a vapor state, it is necessary to supply a heat of:

4,19 kJ/kg/K x 90 K + 2 257 kJ/kg = 2 634 kJ/kg = 0,73 kWh/kg


Depending on the cost of energy, we can estimate the cost of vaporization:

  • Electricity: 0,2 €/kWh => 0,145 €/kgwater

  • Gaz/fuel : 0,035 €/kWh => 0,025 €/kgwater

Steam humidifiers consume a large amount of energy to evaporate water. The main energy losses are linked to hot water deconcentration, maintaining the tank at high temperature and control auxiliaries. The energy cost will be 3 to 5 times higher for an electric steam humidifier than for a gas steam humidifier due to the difference in €/kWh.

Adiabatic humidifiers consume very little energy because the energy supply to the air flow is made by the heating battery (or the pre- and post-heating batteries for adiabatic humidifiers with recycling). Energy losses are linked to the parasitic consumption of the humidifier (atomiser, pump, etc.) but these losses are very limited. The energy cost is always very low because these humidifiers make it possible to supply energy via the most efficient source (heating battery). The adiabatic cooling effect may also make it possible to limit the use of the cold battery in the inter-seasons.

In certain circumstances (hot and dry air), the steam humidifiers and the adiabatic humidifiers with recycling will have to operate at the same time as the cooling battery to reach the set point. This leads to significant energy consumption. This is never the case with adiabatic humidifiers without recycling for which the energy supply is totally independent of the regulation.



In practice, the cost related to water consumption is negligible in the financial comparison between different types of humidifiers. On the other hand, users are increasingly aware of their water consumption and are asking for water-saving solutions.

This can become a financial element when it is necessary to treat the water. Indeed, the sizing of the softener or the osmosis unit is different for a system with or without recycling, with or without deconcentration.

With a steam humidifier, water losses mainly come from the necessary deconcentration of the tank. During evaporation, the concentration of salts and minerals increase in the tank. A loss of energy is also linked to this deconcentration because the water evacuated is generally around 80°C.

In the case of adiabatic humidifiers without recirculation, the water loss is related to the evaporation efficiency of the water in the air flow. This efficiency mainly depends on the set point to be reached and the operating conditions (dimensions of the humidification box, air flow speed, turbulence, droplet distribution, etc.), as well as the type of droplet separator.

In the case of humidifiers with recycling, the water consumption essentially depends on the deconcentration of the retention tank. This consumption can be 3 to 10 times higher than that of the other two types of humidifiers.


“Humidify = risk of injecting bacteria and dust into the air”

​The quality of the water must be defined taking into account two constraints:

  • Ensure hygienic operation because water-air contact favors the spread of bacteria

  • Control operating and maintenance costs

Spray humidifiers without recycling consume a limited flow of water: the water flow is then totally vaporized. During evaporation, the salts and minerals contained in the water are then found in the humidification box and on the drop separator. Maintenance is then necessary to remove the deposits. Reverse osmosis water is generally recommended to limit the maintenance of the humidification chamber and the droplet separator.

Adiabatic humidifiers with recycling consume a higher water flow due to the need for deconcentration. They usually work with softened water. Frequent maintenance is then necessary to keep the humidifier and the humidification unit in good condition.

Steam humidifiers ensure sterility and the absence of dust in the humidified air. However, the salts present in the feed water settle in the steam generator, causing a lot of maintenance problems. The operating cost is therefore high.

Resistance steam humidifiers can operate with reverse osmosis water to limit maintenance costs. This is impossible with electrode steam humidifiers, which need minimal water conductivity to operate.



The vast majority of users base their choice on the investment. They also all agree that it is the total cost (investment + operation) that interests them. It is important to take these two costs into consideration, over a period relevant to the user (3, 5, 10 or 20 years).

The optimal humidifier will depend on the expected performance but also on the conditions of use which will be all the more predominant the longer the decision horizon. The investment quickly becomes negligible compared to the operating and maintenance costs.

In addition, the Microniser humidifier is offered in Full Service (0€ investment) with payment in monthly instalments.

The illustration shown is only valid in certain operating contexts. Do not hesitate to contact us for a comparison specific to your application.



Proper maintenance is essential for the proper functioning of all types of humidifiers.

The maintenance of steam humidifiers mainly consists of cleaning the steam tank and/or replacing the electrodes and resistors. Electrode humidifiers are much more sensitive than resistance humidifiers. This maintenance is generally expensive.

The maintenance of adiabatic humidifiers without recycling is generally quite simple (replacement of bearings, etc.). Maintenance of the humidification chamber and the drop separator is also necessary if the feed water is not osmosis.

Maintenance of adiabatic humidifiers with recirculation must be carried out frequently to guarantee good hygiene. Periodic disinfection is necessary. This maintenance is quite costly in terms of working time.


For steam humidifiers, the recommended power is 750W per kg/h of steam. The steam ramp will be sized according to the dimensions of the supply ducts.

A 280W Microniser atomiser can effectively spray up to 70L/h, or 4W per kg/h of steam. The number of atomisers will be determined according to the necessary steam flow, the surface of the humidification box, the air flow etc. There is generally 1 atomiser for 10,000 m³/h.


​Steam humidifiers provide guarantees at the hygienic level, but their operating cost is high due to the cost of the electric kWh: if they are chosen, a dimensioning limited to minimum needs is required. It is a very flexible solution for a small office building.

Adiabatic humidifiers with recycling are economical since the energy input is provided by the building's boiler, but they are sensitive to bacterial proliferation: if they are chosen, serious maintenance and air quality control measures will be needed.

Adiabatic humidifiers without recycling (Microniser) combine both hygienic and economical qualities, but the size is sometimes incompatible with the space available in an air treatment unit. These techniques are essential for ambient humidification of large halls or for “centralized” humidification in large air handling units (>20,000 m³/h).

For more information on the design of your humidifier, we also invite you to consult the energie+ website.

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