Controlling humidity in hospitals

Summary: In this information sheet we discuss the importance of controlling humidity in air-conditioned, heated and/or ventilated premises, according to their specific needs of use.  In particular, we show that humidity is important not only for the comfort of occupants but also for the consequences for their health if the control of the levels required by law is deficient. We look at the current regulations on this matter, both for general use and for hospitals, to which specific requirements apply. A very important characteristic of the system is the efficient design and fitting of used vapour ducts, to prevent spills of condensate that could present the risk of creating sources of infections. Finally, we analyze the energy and economic cost of the systems used to supply humidity according to the energy source, and the equipment used. It is not uncommon for the cost of the energy associated with humidifying to result in the system being left unused.


Key words: Humidity, vapour, isothermic, dispersion, cost.




There are various parameters involved in ambient comfort:

  • Ambient temperature
  • Ambient humidity
  • Air movement through the premises
  • Air quality (cleanliness, harmful gases etc.).

The effect of temperature which is well-known, and the conditions can vary from 20ºC to 28ºC, according to the circumstances.

The influence of humidity content in air is less well-known, which is why its importance is rarely acknowledged, both when it is too high and too low.

  • It affects the health of the respiratory tracts.
  • If high, it can cause colonies of bacteria to grow.
  • It increases the effect of static electricity if it is too low, causing headaches, etc.
Figura 1. Mortalidad de ratones infectados por un virus en función de la humedad relativa del laboratorio
Figura 1. Mortalidad de ratones infectados por un virus en función de la humedad relativa del laboratorio
Increase in the humidity content in textiles in relation to RH | Fisair
Figure 2.
Control de humedad en hospitales | Fisair
Figura 3.
Figure 4 - humidity intervals
Figure 4 - humidity intervals


The range of required values is indicated in the Regulations on Design Conditions in air-conditioned spaces.

Interior designs conditions | Fisair
  • Limitation in swimming pools: 65%HR, max. 30ºC.
  • Return adiabatic cooler when there is heat recovery.
  • Sanitation quality humidified steam.
  • Interior hospital air quality requirement to UNE standard 100713.

Spaces co-exist in hospitals for many different uses, with diverse needs and different design conditions.

In cases such as consulting rooms, waiting rooms, foyers, administration areas, and also in hospitalization areas, the needs are not very different from what would be required in buildings in the services sector (offices, entertainment venues, hotels, etc.) although there may be special requirements in some cases, because there is long-term occupancy.

There is a regulation that covers the requirements for hospitals, which are generally based on the standard UNE-100.713.

Regulation UNE 100713-2005 | Fisair

It can be seen that, in general, somewhat higher temperatures apply (24-26 ºC) that those specified by the RITE regulation for winter, which can be justified in spaces for patients, but could be excessive in other spaces.

For operating theatres, delivery rooms and similar spaces, the range is greater: between 22 and 26ºC.

HVAC for operating theatres has several particular features, which are essential due to the use and conditions of the space.

–     Internal space, not influenced by outside conditions.

–     Internal and ventilation load only.

–     Very high Air Renewal Load.

–     Very strict requirements in terms of temperature and humidity.

In any case, the conditions are set by the surgeons.

It is essential to bear in mind that the system called for is an “All Outside Air” one and a Large Flow, so the increase in temperature and specific humidity in the air passing through the space is small, hence one needs to ensure that the injection conditions established are met exactly, since the inertia of the inside air is minimal.

To clarify these concepts, let us analyse a standard operating theatre:

-Surface                            30 m2

-Volume                             80 m3

-Air flow                             2,400 m3/h


Refresh rate                    30 refreshments/hr

Age of the air at the outlet                       2 minutes

The internal heat load of the space is determined by:

Sensitive            Lights,  Electrical-medical equipment, Persons.

Latent                  Sterilization Equipment (as applicable), Persons.

In general, we can say that the increase in temperature is usually less than 2ºC and the humidity supplied is no higher than 0.5 g/kg of inserted air.

Some typical conditions requested by surgeons for humidity injection are: 22ºC and 45% HR.

This means a specific humidity of x= 7.5 g/kg.

To meet these conditions, we require:

– In winter, supplying humidity to the intake air.

– In winter, extracting humidity from the air.

Air dehumidification is usually achieved by passing air through a coil fed with cooled water, which comes from the hospital’s general air-conditioning system; this often presents difficulties because frequently the temperature at which the cold water reaches the cold coil of the HVAC unit is not sufficiently low to dehumidify all of the air required, when the outside ambient conditions include high humidity.

There are two procedures for humidification, (supplying humidity to air):

Isothermic humidification

Adiabatic humidification.

Adiabatic Humidification

Air for humidification is passed through a moist panel, on which water evaporates as it joins the air current; or alternatively, liquid water is pulverized and likewise evaporates.

The process is adiabatic as the latent heat of evaporation is taken from the air itself, which cools; the enthalpy of the assembly remains constant, but the sensitive heat has been converted to latent heat.

Humidificadores Isotérmicos y Adiabáticos
HEF 2-3 Contact Panel Units Picture | Fisair's Photo
Figure 7.
Idustrial air humidifier | Fisair's Photos
Figure 8.

Due to the risks of spreading Legionella and other bacteria, adiabatic systems are not normally used in hospitals.


Isothermal humidification

Having ruled out adiabatic systems as explained above, the remaining option is to feed steam.

Saturated steam production units are used, at atmospheric pressure (100 ºC). There are systems with heat supplied by electricity, or by an external thermal fluid, generally steam or super-heated water.


Electrical approaches

By Immersed Electrodes

– These require water with sufficient electrical conductivity, between 125 and 1,250 micro Siemens/cm..

The water must be of “sanitary quality”. Drinking water from the public water supply generally meets the requirements.

Isothermal humidification with electrode based units | Fisair
Figure 9.

By Electrically resistant elements

  • Will work with de-mineralized or osmotized water.
  • Achieve a very accurate regulating of production.

The units have built-in systems to dispose of the deposited salts automatically.

Isothermal humidification with resistance based units | Fisair
Figure 10.

Both systems have a high operating cost as they consume electrical energy.


Conventional Steam Boiler

This is expressly prohibited by the Regulations, as the steam produced does not have the “sanitary quality/grade” required.


By External Fluid

They are hygienic steam production units, which draw heat from an external fluid, in general, steam or super-heated water.

The production, distribution and dispersion system is illustrated in the diagram.

Controlling humidity in hospital | Fisair
Figure 12.
Steam production 5-726kg/h graphic | Fisair

Steam Dispersion

Good design and implementation of the dispersion system is fundamental for the unit to work smoothly as a whole. It is not uncommon for defects in the conduction route or the poor positioning of the dispersion unit, the “steam lance”, to ruin the performance of the whole and lead to it falling into disuse.

In the following figures one can see the various units:

Controlling humidity in hospitals | Fisair
Figure 14.
Steam distribution | Fisair
Figure 15.
Imagen de evacuación de condensado | Fisair
Figura 16.

The importance must be stressed of positioning the lance and evacuation of the condensate produced correctly in order to prevent drips inside the HVAC unit or duct.

Absorption distance

The steam that is injected into the air current is saturated (100ºC); on coming into contact with the cooler air, it condenses, forming a mist (microscopic particles of liquid water) at air temperature, since the mass of the steam is very small in comparison with that of the air and the temperature remains without any noticeable change. This mist progressively dissipates, evaporating into the air, until it is completely absorbed.

The above process takes some time for the mixture and evaporation, so that from the injection point until full absorption, there is a section of the air current, called the “absorption distance” within which there must be no obstacles or protrusions in the conduction, to prevent the risk of liquid water being deposited on the surface in contact with the air.

To prevent this, there are various shapes of lances and graphics to select the suitable dispersion units according to the specific conditions of the site selected.

Imagen de ubicación del sistema de dispersión en climatizadores | Fisair
Figure 17.
Maintenance and regulation location of probes | Fisair


Cost of Steam

  • Example: Hospital
  • Area: Cantabrian Region
  • of HVAC units: 35
  • Total Air Flow: 340,900 m3/h
  • Winter design cond.: 0º-89%-(x= 3.4 gr/kg as)
  • Winter Inside spaces:
    • Operating theatres and similar: 22ºC-45%  (x=7.5 gr/kg as)
    • Other Uses: 23ºC-50%   (x=81 gr/kg as)



The needs for humidifying vary according to the outside conditions.

The data on humidifying needs have been taken from the Project Design.

The internal design conditions are:

  • Operating theatres and similar: 22ºC-45% (x=7.5 gr/kg as)
  • Other uses:                23ºC-50%  (x=8.1 gr/kg as)

-Areas:   24h/day, 365 days/year

12h/day, 5 days/week

Weather Data: ATECYR [Spanish HVAC Association]  “Bilbao Three-hourly Data”

Design Conditions:

  • Total Humidification Capacity: 1,828 kg/h
  • Total power required:
    • Water enthalpy 10ºC …………………..42 kJ/kg
    • water enthalpy 100ºC………… 2,675 kJ/kg
    • Increase in enthalpy……………2,633 kJ/kg
    • Equivalent to…………………………0.731 kWh /kg v (useful net power)
    • Assuming………………………0.8 kWh/kg (gross power)
    • Total power required:
    • 8 kWh/kg x 1,828 kg/h=  1,462 kW
  • Electrical Equipment:
    • Loss Distrib. (3%)          1,507kW

Fluid Output:

– Loss Gener. and Distr. (18%)                      1,783 kW

Direct Gas

Loss  Generation (10%)                            1,662 kW

Design Conditions | Fisair
Design Conditions | Fisair
Design Conditions | Fisair
Design Conditions | Fisair
Design Conditions | Fisair
Design Conditions | Fisair
Design Conditions | Fisair

Energy costs

Energy that has to be supplied:

4,244,651 kg v/year x 0.8 kWh/kh v=  3,395,721 kWh/year

Electricity consumption: 3,395,721 kWh/0.97=     3,500,743 kWh

Gas consumption:

With fluid:          3,395,721 kWh/0.80=     4,244,651 kWh

Direct Gas:       3,395,721 kWh/0.90=     3,773,023 kWh

Cost of Electricity:     0.12 €/kWh

Cost of Natural Gas:     0.05 €/kWh


  • Electrical Equipment
  • Potential Additional Contract: 1,507kW x 0.8(Cs)= 1,200 kW
  • Additional fixed cost:
  • 1,200 kW x 14€/kWmonth x 12month/year= 201,600 €/year
  • Gas unit (direct and indirect):
    • Not considered


  • Electrical Equipment
  • Energy Consumption:
  • 3,395,721 kWh/year x 0.12 €/kWh=    407,86 €/year
  • Thermal Fluid
  • Energy Consumption:
  • 3,395,721/0.8  kWh x 0.05 €/kWh=      212,232 €/year
  • Direct Gas
  • Energy Consumption:
  • 3,395,721/0.9 kWh/year x 0.05 €/kWh=     188,651 €/year


  • Electricity:
  • Units
  • Electrical Install. (transformers, lines, panels, etc.)
  • Estimated Total Investment €782,000
  • Thermal Fluid
  • Unit
  • Installation (boilers, pipes, insulation, etc.)
  • Estimated Total Investment €630,000
  • Direct Gas:
  • Units
  • Electrical Install. Gas
  • Estimated Total Investment €365,000
Comparative table | Fisair


–     Controlling humidity in hospitals calls for a considerable consumption of energy which not uncommonly means that the humidity control units are unused.

–     More often than not, this is achieved using independent units powered by electricity.

–     The technology and equipment exists to drastically reduce the running cost in comparison with electrically-powered units.





Ashrae Handbook 2008 Systems and Equipment

Lew Harriman et al. 2006  Ashrae.

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