The relationship between relative humidity and occupant health and comfort has not yet been fully established, but there is compelling evidence for a strong link between indoor relative humidity and occupant comfort. Derby et al. (2017) find (through a literature review [1985 to 2015]) that the impact of low humidity on comfort manifests as eye irritation, skin dryness, and static electricity.

Thermal Comfort, Eyes, and Skin. Low humidity does not affect thermal comfort as long as temperature is suitable (Derby et al. 2017). Higher temperature is generally considered necessary to offset decreased relative humidity (see ASHRAE Standard 55).

Figure 1. Optimum Humidity Range for Human Comfort and Health

(Adapted from Sterling et al. 1985)

Low relative humidity increases evaporation from the membranes of the nose and throat, drying the mucous membranes in the respiratory system; it also dries the skin and hair. Sunwoo et al. (2006) find higher eye evaporation rates or blink frequencies at lower humidities, but because tests were conducted on the time scale of minute to hours, long-term effects on comfort and visual acuity are unknown. In general, an ultra-low humidity environment (~2.5% rh) was found to be too dry and reduced worker comfort (Chou et al. 2005, 2007). In laboratory studies, low humidity increased transepidural water loss (TEWL) and skin discomfort (Wyon et al. 2006). However, in different studies within offices, humidification both yielded and did not yield a reduction in skin dryness. Very low humidity also increased the likelihood of static buildup. Nordström et al. (1994) document a significant decrease in electrostatic discharge complaints when relative humidity increases from 25 to 40%.

Indoor Environmental Quality (IEQ). Derby et al. (2017) determines that the effects of humidity on indoor environmental quality (IEQ) are modest. Results on the effects of relative humidity on volatile organic compound (VOC) emissions from building materials are inconclusive and depend on type of material and type of VOC. Emissions from materials may or may not vary with changes in temperature and humidity. In general, increases in temperature and humidity decrease the perceived quality or acceptability of the air. (Fang et al. 1998, 1999)

During one study, Fang et al. reported an increase of fatigue, headache, and difficulty thinking at lower levels of temperature and humidity. However, in another study, Norbäck et al. (1990) saw no correlation between room temperature and humidity and symptoms. Additionally, Fiedler et al. (2005) exposed women to brief (three-hour) mixtures of VOCs with and without ozone, and no significant subjective or objective health effects were reported as a result of the short VOC exposure.

Health. ASHRAE research project RP-1630 (Derby et al. 2017) findings indicate that low humidity (especially in cold air) triggers asthma (Bundgaard et al. 1982; Kaminsky et al. 1995), though the studies do not address specific lower humidity limits. Data included in house dust mite (HDM) literature by Arlian et al. support the conclusion that higher humidity levels encourage dust mite growth and subsequent HDM allergen production, and that controlling indoor relative humidity below approximately 50% can be an effective means of reducing house dust mite allergens. When considering the effects of low humidity on asthma, one must consider the interactions of allergies, ventilation, and viruses, as asthmatics are extremely susceptible to upper and lower respiratory tract infections.

The increased incidence of respiratory complaints during winter is often linked to low relative humidity. Epidemiological studies have found lower rates of respiratory illness reported among occupants of buildings with midrange relative humidity than among occupants of buildings with low humidity.

Extremes of humidity are the most detrimental to human comfort, productivity, and health. Figure 1 shows that the range between 30 and 60% rh (at normal room temperatures) provides the best conditions for human occupancy (adapted from Sterling et al. 1985). In this range, both the growth of bacteria and biological organisms and the speed at which chemical interactions occur are minimized. The trends of Figure 1 are, in general, correct, though the increasing widths of the bars designated “decrease in effect” are approximate at best. (Derby et al. 2017). Figure 1 has been adapted from its original published form. In the original work, an optimal range of 40 to 60% rh, was recommended. However, this has been expanded to include the 30 to 40% range, recognizing that higher humidity levels may not be feasible in cold climates.

Derby et al. (2017) make several observations across the comfort, IEQ, and health categories. Human subjects in this research were nearly always adults, and of these, a significant percentage of studies used university students, so it is unknown if the same conclusions apply to children, adolescents, and the elderly. Across the categories, ventilation was noted as a confounding factor, and this is a topic for future investigation. Time was noted as an important factor as well; most studies were conducted for a few hours, and long-term effects of low humidity are unknown.

The effect of relative humidity on occupant comfort has been considered a secondary concern compared to sensible temperatures. This oversight is partly because humans have no skin sensors for dryness. The role of relative humidity in occupant health, a more important consideration than comfort, has pushed management of indoor water vapor to the forefront. Relative humidity has a significant effect on the control of airborne infection.

One of the clearest examples of the importance of maintaining indoor rh between 40 and 60% for comes from a study of a hospital in the midwestern US. Over the course of one year, ten patient rooms were monitored for temperature, CO₂ levels, outdoor air fractions, lighting, visitor and clinician traffic, absolute and relative humidity, room pressurization, and RAC. These environmental parameters, measured every 5 to 30 minutes, were then statistically correlated with newly acquired healthcare-associated infections (HAIs) in approximately 300 patients residing in these rooms. Of all the room measurements, low relative humidity correlated the most significantly with increased HAIs (Taylor and Hugentobler 2016). In this study, as patient room relative humidity increased from 32 to 42%, the number of HAIs fell dramatically.

Reimann et al. (2018) expand on the prior works by testing the hypothesis that raising the absolute humidity above seasonal values would have an impact on influenza virus survival and transmission. The researchers added humidifiers to several classrooms within a Minneapolis school and controlled the room humidity in the range of 40 to 60% (as recommended by Sterling et al. 1985). These rooms were compared with control classrooms where no humidity control was present and the air became considerably drier during the cold winter months. Air and object samples were tested to determine the presence of influenza and its ability to infect cells. Additionally, occupant surveys determined the number of influenza-like infections within the classroom. Results of this study showed a significant reduction in the number of influenza A virus samples in rooms with humidification. Furthermore, influenza-like infections occurred 2.3 times more regularly in the control (not humidified) rooms. While the authors recommend further research, they do note that the humidification could serve as a solution for reducing influenza or other viral outbreaks.

Another study in a longterm care facility monitored patient infections and indoor conditions over a four-year period. (Taylor and Tasi 2018). The most significant parameter associated with increased patient infections, especially respiratory and gastrointestinal (GI) infections, was low indoor relative humidity. As shown in Figure 2, patient health was most optimal at indoor rh 40 to 60%.

Derby et al. (2017) draw several general conclusions regarding humidification effects on bacteria, viruses, and fungi. Influenza virus survival exhibits a canonical dip between 40 and 80% rh across many studies, and for nearly all cases, virus survival declined the longer the length of exposure. It has been hypothesized that changes of pH within the aerosol, induced by evaporation, may trigger conformational changes of the surface glycoproteins of enveloped viruses, leading to a decrease in viral infectivity (Yang and Marr 2012).

(Patient respiratory and gastrointestinal infections from viral and bacterial pathogens were lowest at indoor rh of 40 to 60%)

Figure 2. Patient Infections at Indoor Relative Humidities

(Taylor and Tasi 2018)

Kudo et al. 2019 is the most recent publication on the effect of 40 to 60% rh for disease reduction and clearly describes the mechanisms involved. This landmark study provides insights for the seasonality of the influenza virus epidemics, whereby inhalation of dry air compromises the host’s ability to restrict influenza virus infection. The study shows that exposure of mice to low humidity conditions of 20% rh renders them more susceptible to influenza disease through impaired mucociliary clearance, decreased first line-of-defense against viral invasion of the respiratory tract, more lung damage from inflammation after viral disease, and defective tissue repair.

Conversely, 50% rh was protective to the infected mice and associated with almost twice the survival rate, as shown in Figure 3.

While previous studies have clearly shown that environmental conditions such as dry air affect transmission of the flu virus, this is the first to show the mechanism through which humidification decreases illness and mortality in an animal model that applies to humans.

Figure 3. Mice Survival Rates at 20 and 50% rh

(Mice maintained at 20 and 50% rh were infected with influenza A virus. Survival rates shown here.)

(Kudo et al. 2019)

These recent works build on previous work showing that at 50% rh, the mortality rate of certain organisms is highest, and the influenza virus loses much of its virulence. The mortality rate of these organisms decreases both above and below this value. High humidity can support the growth of pathogenic or allergenic organisms. As shown in Figure 4, humidity levels around 50% can be lethal to the Pneumococcus bacterium (Brundrett 1990). Similar effects can be seen in other microorganisms that cause serious health issues. Consequently, relative humidity in habitable spaces should be maintained between 30 and 60%.

Relative humidity also has a major role in the effects of different bacteria. Figure 5 shows the mortality of mice exposed to influenza under varying degrees of relative humidity (Brundrett 1990).

Figure 4. Mortality of Pneumococcus Bacterium

Maximum mortality for airborne Pneumococci comes when relative humidity is held at 55% rh. (Adapted from Brundrett [1990], Criteria for Moisture Control. Copyright Elsevier © 1990.)

Figure 5. Mortality in Mice Exposed to Aerosolized Influenza

Note that numbers of deaths and lung lesions were minimized when humidity was held between 40 and 60% rh.

(Adapted from Brundrett [1990], Criteria for Moisture Control. Copyright Elsevier © 1990.)

Electronic data processing equipment requires controlled relative humidity. High relative humidity may cause condensation in the equipment, whereas low relative humidity may promote static electricity. Also, rapid changes in relative humidity should be avoided because of their effect on bar code readers, magnetic tapes, disks, and data processing equipment. Generally, computer systems have a recommended design and operating range of 35 to 55% rh. However, the manufacturer’s recommendations should be adhered to for specific equipment operation.

The relative humidity required by a process is usually specific and related to one or more of several factors:

Typical conditions of temperature and relative humidity for storage of certain commodities and manufacturing and processing of others may be found in Chapter 15 of the 2019 ASHRAE Handbook—HVAC Applications.

Low humidity in winter may cause drying and shrinking of furniture, wood floors, and interior trim. Winter humidification should be considered to maintain relative humidity closer to that experienced during manufacture or installation.

For storing hygroscopic materials, maintaining constant humidity is often as important as, or even more important than, the humidity level itself. This is particularly true of artwork and museum artifacts, which are sensitive to rapid changes in humidity and temperature.The design of the structure should always be considered. Temperature control is important because of the danger of condensation on products through a transient lowering of temperature.

Electrostatic charges are generated when materials of high electrical resistance move against each other. The accumulation of such charges may have a variety of results: (1) unpleasant sparks caused by friction between two materials (e.g., stocking feet and carpet fibers); (2) difficulty in handling sheets of paper, fibers, and fabric; (3) objectionable dust clinging to oppositely charged objects (e.g., negatively charged metal nails or screws securing gypsum board to wooden studding in the exterior walls of a building that attract positively charged dust particles); (4) destruction of data stored on magnetic disks and tapes that require specifically controlled environments; and (5) hazardous situations if explosive gases are present, as in hospitals, research laboratories, or industrial clean rooms.

Increasing the relative humidity of the environment reduces the accumulation of electrostatic charges, but the optimum level of humidity depends to some extent on the materials involved. . Based on the study by Paasi et al. (2001), electrostatic discharge (ESD) for several materials is a concern below 30% rh.Figure 6illustrates the voltage that can be accumulated in the human body at different humidity levels. Relative humidity of 45% reduces or eliminates electrostatic effects in many materials, but wool and some synthetic materials may require a higher relative humidity.

Hospital operating rooms, where explosive mixtures of anesthetics are used, constitute a special and critical case. A relative humidity of at least 50% is usually required, with special grounding arrangements and restrictions on the types of clothing worn by occupants. Conditions of 22°C and 55% rh are usually recommended for comfort and safety.

Figure 6. Effect of Relative Humidity on Static Electricity from Carpets

Below 40% rh, perceptible shocks are more likely. (Adapted from Brundrett [1990], Criteria for Moisture Control. Copyright Elsevier © 1990.)

Air absorption of sound waves, which results in the loss of sound strength, is worst at 15 to 20% rh, and the loss increases as the frequency rises (Harris 1963). There is a marked reduction in sound absorption at 40% rh; above 50%, the effect of air absorption is negligible. Air absorption of sound does not significantly affect speech but may merit consideration in large halls or auditoriums where optimum acoustic conditions are required for musical performances.

Laboratories and test chambers, in which precise control of relative humidity over a wide range is desired, require special attention. Because of the interrelation between temperature and relative humidity, precise humidity control requires equally precise temperature control.

The maximum relative humidity level to which a building may be humidified in winter depends on the ability of its walls, roof, and other elements to prevent or tolerate condensation. Condensed moisture or frost on surfaces exposed to the building interior (visible condensation) can deteriorate the surface finish, cause mold growth and subsequent indirect moisture damage and nuisance, and reduce visibility through windows. If the walls and roof have not been specifically designed and properly protected with vapor retarders on the warm side to prevent the entry of moist air or vapor from the indoors, concealed condensation within these constructions is likely to occur, even at fairly low interior humidity, and cause serious deterioration.

Condensation forms on an interior surface when its temperature is below the dew-point temperature of the air in contact with it. The maximum relative humidity that may be maintained without condensation is thus influenced by the thermal properties of the enclosure and the interior and exterior environment.

Average surface temperatures may be calculated by the methods outlined in Chapter 25 of the 2017 ASHRAE Handbook—Fundamentals for most insulated constructions. However, local cold spots result from high-conductivity paths such as through-the-wall framing, projected floor slabs, and metal window frames that have no thermal breaks. The vertical temperature gradient in the air space and surface convection along windows and sections with a high thermal conductivity result in lower air and surface temperatures at the sill or floor. Drapes and blinds closed over windows lower surface temperature further, while heating units under windows raise the temperature significantly.

In most buildings, windows present the lowest surface temperature and the best guide to permissible humidity levels for no condensation. While calculations based on overall thermal coefficients provide reasonably accurate temperature predictions at mid-height, actual minimum surface temperatures are best determined by test. Wilson and Brown (1964) related the characteristics of windows with a temperature index, defined as (t – t_o)/(t_i – t_o), where t is the indoor window surface temperature, t_i is the indoor air temperature, and t_o is the outdoor air temperature.

The results of limited tests on actual windows indicate that the temperature index at the bottom of a double, residential-type window with a full thermal break is between 0.55 and 0.57, with natural convection on the warm side. Sealed, double-glazed units exhibit an index from 0.33 to 0.48 at the junction of glass and sash, depending on sash design. The index is likely to rise to 0.53 or greater only 25 mm above the junction.

With continuous under-window heating, the minimum index for a double window with a full break may be as high as 0.60 to 0.70. Under similar conditions, the index of a window with a poor thermal break may be increased by a similar increment.

Figure 7shows the relationship between temperature index and the relative humidity and temperature conditions at which condensation occurs. The limiting relative humidities for various outdoor temperatures intersect vertical lines representing particular temperature indexes. A temperature index of 0.55 was selected to represent an average for double-glazed, residential windows; 0.22 represents an average for single-glazed windows. Table 1 shows the limiting relative humidities for both types of windows at various outdoor air temperatures.

Figure 7. Limiting Relative Humidity for No Window Condensation

Vapor retarders are imperative in certain applications because the humidity level a building is able to maintain without serious concealed condensation may be much lower than that indicated by visible condensation. Migration of water vapor through the inner envelope by diffusion or air leakage brings the vapor into contact with surfaces at temperatures below its dew point. During building design, the desired interior humidity may be determined by the building enclosure’s ability to handle internal moisture. This is particularly important when planning for building humidification in colder climates.

The humidification load depends primarily on the rate of natural infiltration of the space to be humidified or the amount of outdoor air introduced by mechanical means. Other sources of moisture gain or loss should also be considered. The humidification load H can be calculated by the following equations:

For ventilation systems having natural infiltration,

(1)

For mechanical ventilation systems having a fixed quantity of outdoor air,

(2)

For mechanical systems having a variable quantity of outdoor air,

(3)

where

Interior design conditions are dictated by the occupancy or the process, as discussed in the preceding sections on Enclosure Characteristics and on Environmental Conditions. Outdoor relative humidity can be assumed to be 70 to 80% at temperatures below 0°C or 50% at temperatures above 0°C for winter conditions in most areas. Additional data on outdoor design data may be obtained from Chapter 14 of the 2017 ASHRAE Handbook—Fundamentals. Absolute humidity values can be obtained either from Chapter 1 of the 2017 ASHRAE Handbook—Fundamentals or from an ASHRAE psychrometric chart.

For systems handling fixed outdoor air quantities, load calculations are based on outdoor design conditions. Equation (1) should be used for natural infiltration, and Equation (2) for mechanical ventilation.

For economizers that achieve a fixed mixed air temperature by varying outdoor air, special considerations are needed to determine the maximum humidification load. This load occurs at an outdoor air temperature other than the lowest design temperature because it is a function of the amount of outdoor air introduced and the existing moisture content of the air. Equation (3) should be solved for various outdoor air temperatures to determine the maximum humidification load. It is also important to analyze the energy use of the humidifier (especially for electric humidifiers) when calculating the economizer setting in order to ensure that the energy saved by “free cooling” is greater than the energy consumed by the humidifier.

In residential load calculations, the actual outdoor design conditions of the locale are usually taken as –6.7°C and 70% rh, while indoor conditions are taken as 21.1°C and 35% rh. These values yield an absolute humidity difference (W_i – W_o) of 0.0040 kg per kilogram of dry air for use in Equation (1). However, the relative humidity may need to be less than 35% to avoid condensation at low outdoor temperatures (see Table 1).

Ventilation of the humidified space may be caused by natural infiltration alone or natural infiltration in combination with intentional mechanical ventilation. Natural infiltration varies according to the indoor-outdoor temperature difference, wind velocity, and tightness of construction, as discussed in Chapter 16 of the 2017 ASHRAE Handbook—Fundamentals. The rate of mechanical ventilation may be determined from building design specifications or estimated from fan performance data (see ASHRAE Standard 62.1).

In load calculations, water vapor removed from the air during cooling by air-conditioning or refrigeration equipment must be considered. This moisture may have to be replaced by humidification equipment to maintain the desired relative humidity in some industrial projects where the moisture generated by the process may be greater than that required for ventilation and heating.

Estimates of infiltration rate are made in calculating heating and cooling loads for buildings; these values also apply to humidification load calculations. For residences where such data are not available, it may be assumed that a tight house has an infiltration rate of 0.5 air changes per hour (ach); an average house, 1 ach; and a loose house, as many as 1.5 ach. A tight house is assumed to be well insulated and to have vapor retarders, tight storm doors, windows with weather stripping, and a dampered fireplace. An average house is insulated and has vapor retarders, loose storm doors and windows, and a dampered fireplace. A loose house is generally one constructed before 1930 with little or no insulation, no storm doors, no insulated windows, no weather stripping, no vapor retarders, and often a fireplace without an effective damper. For building construction, refer to local codes and building specifications.

Hygroscopic materials, which have a lower moisture content than materials in the humidified space, absorb moisture and place an additional load on the humidification system. An estimate of this load depends on the absorption rate of the particular material selected. Table 2 in Chapter 15 of the 2019 ASHRAE Handbook—HVAC Applications lists the equilibrium moisture content of hygroscopic materials at various relative humidities.

In cases where a certain humidity must be maintained regardless of condensation on exterior windows and walls, the dehumidifying effect of these surfaces constitutes a load that may need to be considered, if only on a transient basis. The loss of water vapor by diffusion through enclosing walls to the outdoors or to areas at a lower vapor pressure may also be involved in some applications. The properties of materials and flow equations given in Chapter 26 of the 2017 ASHRAE Handbook—Fundamentals can be applied in such cases. Normally, this diffusion constitutes a small load, unless openings exist between the humidified space and adjacent rooms at lower humidities.

The introduction of a hygroscopic material can cause moisture gains to the space if the moisture content of the material is above that of the space. Similarly, moisture may diffuse through walls separating the space from areas of higher vapor pressure or move by convection through openings in these walls (Brown et al. 1963).

Moisture contributed by human occupancy depends on the number of occupants and their degree of physical activity. As a guide for residential applications, the average rate of moisture production for a family of four has been taken as 320 g/h. Unvented heating devices produce about 1 kg of vapor for each kilogram of fuel burned. These values may no longer apply because of changes in equipment as well as in living habits.

Industrial processes constitute additional moisture sources. Single-color offset printing presses, for example, give off 200 g of water per hour. Information on process contributions can best be obtained from the manufacturer of the specific equipment.

There are three major categories of supply water: potable (untreated) water, softened potable water, and demineralized (deionized [DI] or reverse osmosis [RO]) water. Either the application or the humidifier may require a certain water type; consult the humidifier manufacturer’s literature.

In areas with water having a high mineral content, precipitated solids may be a problem: they can clog nozzles, tubes, evaporative elements, and controls. In addition, solids allowed to enter the airstream via mist leave a fine layer of white dust over furniture, floors, and carpets.

Many humidifiers may bleed off or actively replace some or all of the water passing through the equipment to reduce the concentration of salts in the recirculating water. This allows for less frequent maintenance at the cost of increased water usage.

Dust, scaling, biological organisms, and corrosion are all potential problems associated with water in humidifiers. Stagnant water can provide a fertile breeding ground for algae and bacteria, which have been linked to odor and respiratory ailments. Bacterial slime reacts with sulfates in the water to produce hydrogen sulfide and its characteristic bad odor. Regular maintenance and periodic disinfecting with approved microbicides may be required (Puckorius et al. 1995). This has not been a problem with residential equipment; however, regular maintenance is good practice because biocides are generally used only with atomizing humidifiers.

Industrial pan humidifiers, when supplied with water that is naturally low in hardness, require little maintenance, provided a surface skimmer bleedoff is used.

Water softening is an effective means of eliminating mineral precipitation in a pan-type humidifier. However, the concentration of sodium left in a pan as a result of water evaporation must be held below the point of precipitation by flushing and diluting the tank with new softened water. The frequency and duration of dilution depend on the water hardness and the rate of evaporation. Dilution is usually accomplished automatically by a timer-operated drain valve and a water makeup valve.

Demineralized or reverse osmosis (RO) water may also be used. The construction materials of the humidifier and the piping must withstand the corrosive effects of this water. Commercial demineralizers or RO equipment removes hardness and other total dissolved solids completely from the humidifier makeup water. They are more expensive than water softeners, but no humidifier purging is required. Sizing is based on the maximum required water flow to the humidifier and the amount of total dissolved solids in the makeup water.

Certain microorganisms are occasionally present in poorly maintained humidifiers. To deter the propagation and spread of these detrimental microorganisms, periodic cleaning of the humidifier and draining of the reservoir (particularly at the end of the humidification season) are required. Research by Unz et al. (1993) on several types of plenum-mounted (evaporative and steam) residential humidifiers showed no evidence of organism transmission originating from the humidifier. Ruud et al. (1993) also determined that humidifiers (evaporative and steam) did not add bacteria or particles to the heated airstream. Misting type systems, such as ultrasonic, fogging nozzles, and other systems that atomize water present a higher risk for distributing bacteria into the air if their reservoirs become contaminated. When using these types of products, care must be taken to heed the manufacturers cleaning and hygiene instructions to ensure safe operation of the system.

Residential humidifiers designed for central air systems depend on airflow in the heating system for evaporation and distribution. General principles and description of equipment are as follows:

Pan Humidifiers. Capacity varies with temperature, humidity, and airflow. Vapor is introduced into the air by evaporation.

device.

Wetted-Element Humidifiers. Capacity varies with air temperature, water temperature, humidity, and airflow volume. Vapor is introduced into the air by evaporation. Air circulates over or through an open-textured, wetted medium. The evaporating surface may be a fixed pad wetted by either sprays or water flowing by gravity, or a paddle-wheel, drum, or belt rotating through a water reservoir. The various types are differentiated by the way air flows through them:

Atomizing Humidifiers. The capacity of an atomizing humidifier does not depend on the air conditions. However, it is important not to oversaturate the air and allow liquid water to form in the duct. The air’s ability to absorb moisture depends on the temperature, flow rate, and moisture content of the air moving through the system. Small particles of water are formed and introduced into the airstream in one of the following ways:

Figure 9. Residential Humidifiers

Self-Contained Electrode Steam Humidifiers (Figure 9G). These units operate by passing an electric current directly into ordinary tap water, thereby creating heat energy to boil the water, and produce steam vapor. The humidifier usually contains a plastic bottle (see Figure 10E) that is supplied with water through a solenoid valve. Periodic and partial drains maintain a desirable solids concentration and the correct electrical flow. Units are available for steam distribution into the duct with a steam nozzle or wand, or into a room with a booster fan.

Many portable or room humidifiers are used in residences heated by nonducted hydronic or electric systems, or where the occupant is prevented from making a permanent installation. These humidifiers may be equipped with humidity controllers.

Portable units evaporate water by any of the previously described means, such as heated pan, fixed or moving wetted element, or atomizing spinning disk. They may be tabletop-sized or a larger, furniture-style appliance (Figure 9F). A multispeed motor on the fan or blower may be used to adjust output. Portable humidifiers usually require periodic filling from a bucket or filling hose.

Some portable units are offered with an auxiliary package for semipermanent water supply. This package includes a manual shutoff valve, a float valve, copper or other tubing with fittings, and so forth. Lack of drainage provision for water overflow may result in water damage.

Some units may be recessed into the wall between studs, mounted on wall surfaces, or installed below floor level. These units are permanently installed in the structure and use forced-air circulation. They may have an electric element for reheat when desired. Other types for use with hydronic systems involve a simple pan or pan plate, either installed within a hot-water convector or using the steam from a steam radiator.

Figure 10. Industrial Isothermal (Steam) Humidifiers

Humidifiers must be installed where the air can absorb the vapor; the temperature of the air being humidified must exceed the dew point of the space being humidified. When fresh or mixed air is humidified, the air may need to be preheated to allow absorption to take place.

Heated Pan Humidifiers. These units offer a broad range of capacities and may be heated by a heat exchanger supplied with either steam or hot water (Figure 10A). They may be installed directly under the duct, or they may be installed remotely and feed vapor through a hose. In either case, a distribution manifold should be used.

Steam heat exchangers are commonly used in heated-pan humidifiers, with steam pressures ranging from 35 to 105 kPa (gage). Hot-water heat exchangers are also used in pan humidifiers; a water temperature below 115°C is not practical.

All pan-type humidifiers should have water regulation and some form of drain or flush system. When raw water is used, periodic cleaning is required to remove the buildup of minerals. (Using softened or demineralized water can greatly extend time between cleanings.) Care should also be taken to ensure that all water is drained off when the system is not in use to avoid the possibility of bacterial growth in the stagnant water.

Direct Steam Injection Humidifiers. These units cover a wide range of designs and capacities. Steam is water vapor under pressure and at high temperature, so the process of humidification can be simplified by adding steam directly into the air. This method is an isothermal process because the temperature of the air remains almost constant as the moisture is added. For this type of humidification system, the steam source is usually a central steam boiler at low pressure. When steam is supplied from a source at a constant supply pressure, humidification responds quickly to system demand. A control valve may be modulating or two-position in response to a humidity sensor/controller. Steam can be introduced into the airstream through one of the following devices:

Units must be installed where the air can absorb the discharged vapor before it comes into contact with components in the airstream, such as coils, dampers, or turning vanes. Otherwise, condensation can occur in the duct. Absorption distance varies according to the design of the humidifier distribution device and the air conditions within the duct. For proper psychrometric calculations, refer to Chapter 1 of the 2017 ASHRAE Handbook—Fundamentals. Because these humidifiers inject steam from a central boiler source directly into the space or distribution duct, boiler treatment chemicals discharged into the air system may compromise indoor air quality. Check chemicals for safety, and carefully avoid contamination from the water or steam supplies.

Self-Contained Steam Humidifiers. These units convert ordinary city tap water to steam by electrical or gas energy using either electrodes, resistance heater elements, infrared lamps, or gas combustion. Steam is generated at atmospheric pressure and discharged into the duct system through dispersion manifolds; if the humidifier is a freestanding unit, the steam is discharged directly into the air space or mixed in the airstream. Some units allow use of softened or demineralized water, which greatly extends the time between cleanings.

Steam Distributors. Humidifiers that produce steam require a steam distributor to introduce the steam into an airstream or conditioned space. Correct selection and installation of both the steam distributors and the steam lines from the humidifier are essential for proper performance of the humidification system.

There are three common steam distributor types:

Figure 11. Room Fan Distributor

Individual-Tube and Short Absorption Manifold Considerations. Both individual-tube and short absorption manifold distributors require tubes placed directly in an airstream. Increasing the number of tubes tends to shorten steam absorption distance, because the steam is more evenly distributed across the cross section of the airstream; however, it also increases the exposed area for heat transfer and thus also increases airstream heat gain and condensate losses in the distributor. It is best practice to minimize the number of tubes used while still maintaining absorption within the available distance. This approach reduces condensate losses and airstream heat gain while also reducing installation cost. To further reduce losses and improve efficiency, many manufacturers offer insulated distributors.

Tubes and short absorption manifolds should both be located in a straight section of ductwork with laminar airflow. Avoid placing these distributors immediately after bends or in areas where turbulent airflow can be expected. Eddies and recirculation currents in these locations can cause the steam to be drawn into duct walls and condense.

Although not strictly required, it is good practice to include a drain pan in the distributor section to protect against possible leaks or condensation from the distributor.

Duct-mounted sensors and controls should be placed far enough away from the distributor for the moisture to mix adequately with the airstream. As a rule of thumb, high-limit controls and humidity sensors should be installed downstream at a minimum distance of five times the expected absorption distance.

Atmospheric Steam Lines. The steam lines that connect the humidifier to the steam distributors are another important part of the humidification system. Steam lines must be correctly sized, properly routed, made of correct material, have adequate drainage, and be insulated.

Sizing. Follow manufacturer guidelines when selecting an overall diameter. Lines that are too narrow create restriction that is difficult for the humidifier to overcome and can reduce system efficiencies. Lines that are too large have very low steam flow velocities and can cause high condensate losses. As a starting point, consider matching the diameter of the steam line to the outlet of the humidifier and maintaining this diameter through to the distributor.

Routing. Most stand-alone isothermal humidifiers generate atmospheric steam [i.e., steam at a very low pressure (1.5 to 4.5 kPa)]. As such, steam flow in these lines cannot travel long distances. Consider the following when routing steam lines:

Materials. Material selection is an important aspect of steam line design. Common materials for atmospheric steam lines include copper tubing, stainless steel tubing, and hose. Compared to tubing, piping is costlier to install, has thicker walls, and has higher containment ratings that are not necessary for atmospheric steam. Steam hose is typically made of a flexible rubber or polymer material. Restrict steam hoses to short runs of 3 m or less, because the materials can soften and sag over time. Ensuring adequate support of steam hose is important to avoid low points where condensate can collect and block steam flow.

Plastic tubing and black iron piping are generally not recommended as materials for atmospheric steam lines. Certain types of plastic tubing can emit odors or become brittle from repeated heating cycles. Similarly, black iron pipe can emit odors from oils used during the manufacturing process and is prone to corrosion.

Draining. Condensate forms inside steam lines from heat losses and from cool lines on start-up. To minimize the risks of choking steam flow or discharging condensate from the distributor, this condensate should be removed through drains along the steam lines. It is best practice to place a drain and trap immediately before the distributor to collect and drain condensate before it can enter the distributor. Additionally, drains and traps should be placed every 4.5 m on steam line runs, and at any low points in the line. Low points often occur as the steam line is routed under beams, ductwork, or other piping.

A condensate drain should consist of a full-sized tee placed in the steam line, and a pressure trap. At a minimum, the trap must be sized to resist the maximum expected duct static pressure.

Condensate drained from steam lines can be recovered back to the humidifier, returned to a water treatment system, stored for irrigation and toilet usage, or directed to drain as directed by local codes. Steam line condensate is hot, often near 100°C, and should be cooled before draining. Commonly, a high-temperature condensate pump or a condensate cooling tank is used for this purpose.

Insulating. Heat transfer from steam lines can cause steam to condense back to liquid water. Losses in steam lines can reduce the overall system efficiency and reduce the amount of steam being distributed to the space. In the worst cases, significant line losses can prevent the humidifier from maintaining the desired humidity in the space. Therefore, atmospheric steam lines should be insulated with a suitable insulation for the chosen material.

Atomizing Humidifiers. Water treatment should be considered if mineral fallout from hard water is a problem. Optional filters may be required to remove mineral dust from humidified air (Figure12A). Depending on the application and the water condition, atomizing humidifiers may require a reverse osmosis (RO) or a deionized (DI) water treatment system to remove the minerals. It is also important to note that wetted parts should be able to resist the corrosive effects of DI and RO water. Atomizing humidifiers introduce fine droplets or a fog, directly into the airstream. A mist elimination system is suggested for all atomizing-type humidifiers.

There are four main categories of atomizing humidifiers:

Figure 12. Industrial Adiabatic (Atomizing and Evaporative) Humidifiers

Wetted-Media Humidifiers. Rigid-media humidifiers (Figure 12H) use a porous core and the process of evaporation. Water is circulated over the media while air is blown through the openings. These humidifiers are adiabatic, cooling the air as it is humidified. Rigid-media cores are often used for the dual purpose of winter humidification and summer cooling. They depend on airflow for evaporation: the rate of evaporation varies with air temperature, humidity, and velocity.

The rigid media should be located downstream of any heating or cooling coils. For close humidity control, the element can be broken down into several (usually two to four) banks having separate water supplies. Individual pumps or solenoids controlling water flow to each bank are activated as humidification is required.

Rigid-media humidifiers have inherent filtration and scrubbing properties because of the water-washing effect in the filter-like channels. Only pure water is evaporated; therefore, contaminants collected from the air and water must be flushed from the system. A continuous bleed or regular pan flushing is recommended to minimize accumulation of contaminants in the pan and on the media. A cycles-of-concentration method can be used to minimize scale build-up and water discharge.

Hybrid Humidifiers. Hybrids (Figure 12I) combine a nozzle-type humidifier and a rigid-media humidifier. They are used in ducted or air handler applications. Nozzles, which typically operate at low pressure, are placed upstream of a rigid-media core and spray water into the airstream toward media. Droplets that do not evaporate are captured on the media and evaporated or drained off. The media functions as both a mist eliminator and an evaporator. Hybrid humidifiers normally use either RO or DI water to prevent mineral precipitation from fouling the media.

Evaporative Cooling. Atomizing and wetted media humidifiers discharge water at ambient temperature. The water absorbs heat from the surrounding air to evaporate the fog, mist, or spray at a rate of 2500 kJ per kilogram of water. This evaporative cooling effect (see Chapter 41) should be considered in the design of the system and if reheat is required to achieve the final air temperature. The ability of the surrounding air to efficiently absorb the fog, mist, or spray depends on its temperature, velocity, and moisture content.

Each type of humidifier has its own strengths and best applications (Table 3). 

Mechanical sensors depend on a change in the length or size of the sensor as a function of relative humidity. Many humidity-sensitive materials are available, such as nylon, human hair, wood, and animal membranes that change length with humidity changes. The most commonly used sensors are synthetic polymers or human hair. They can be attached to a mechanical linkage to control the mechanical, electrical, or pneumatic switching element of a valve or motor. This design is suitable for most human comfort applications, but it may lack the necessary accuracy for industrial applications.

A humidity controller is normally designed to control at a set point selected by the user. Some controllers have a setback feature that lowers the relative humidity set point as outdoor temperature drops to reduce condensation within the structure.

Electrical sensors change electrical resistance as the humidity changes. They typically consist of two conductive materials separated by a humidity-sensitive, hygroscopic insulating material (polyvinyl acetate, polyvinyl alcohol, or a solution of certain salts). Small changes are detected as air passes over the sensing surface. Capacitive sensors use a dielectric material that changes its dielectric constant with relative humidity. The dielectric material is sandwiched between special conducting materials that allow a fast response to changes in relative humidity.

Electronic control is common in laboratory or process applications requiring precise humidity control. It is also used to vary fan speed on portable humidifiers to regulate humidity in the space more closely and to reduce noise and draft to a minimum.

Electronic controls are now widely used for residential applications because of low-cost, accurate, and stable sensors that can be used with inexpensive microprocessors. They may incorporate methods of determining outdoor temperature so that relative humidity can be automatically reset to some predetermined algorithm intended to maximize human comfort and minimize any condensation problems (Pasch et al. 1996).

Along with a main humidity controller, the system may require other sensing devices:

Further information on humidity sensors can be found in Chapter 37 of the 2017 ASHRAE Handbook—Fundamentals.

In centrally humidified structures, the humidity controller is most commonly mounted in a controlled space. Another method is to mount the controller in the return air duct of an air-handling system to sense average relative humidity. Figure 13 shows general recommended locations for the humidistat for a centrally air-conditioned room.

Figure 13. Recommended Humidity Controller Location

The manufacturer’s instructions regarding the use of the controller on counterflow furnaces should be followed because reverse airflow when the fan is off can substantially shift the humidity control point in a home. The sensor should be located where it will not be affected by (1) air that exits the bypass duct of a bypass humidifier or (2) drafts or local heat or moisture sources.

In many applications, humidifiers can be integrated with the building management system (BMS). These types of systems can be set up with simple analog interaction with the humidifier, or by various communication protocols.

For analog-type systems, the management system can enable/disable the unit, send proportional signals for direct humidity control, or redirect humidity sensor signals. These types of systems use various analog and digital outputs and inputs to command the humidifiers to operate, and at what level, as well as receive alarm states.

Communication protocols can be used as well to interact with and control humidification equipment. Common protocols include BACnet^®, LonWorks^®, Modbus^®, TCP/IP, and SNMP. In some cases, these protocols can fully control the humidifier, replacing the need for any analog wiring, or can monitor the status and operation of the unit, or a combination of both. Some equipment providers can also supply a user interface system that offers online access to the status and operation modes of the units. See ASHRAE Standard 135 for more information.

Often, humidifiers are installed directly in the space they are humidifying, using humidity controllers (humidistats) for control, either installed directly within the space or mounted in the duct system of the ventilation unit serving the area. It is critical that space humidistats be located where they can sense the actual temperature and humidity of the controlled space. Normally, this requires that they are located at the working level of the facility on an interior wall or column. Humidistats should not be installed in the following areas:

In these areas, local changes in temperature or moisture cause the sensed relative humidity level to differ from the space as a whole.

It is good practice to install a high-limit humidistat to protect the space from possible excessive humidity in case the controlling humidistat fails or is damaged. Normally, this high-limit humidistat is an on/off type with a set point of 5 to 10% above the desired space conditions. It is then wired to open the control circuit of the humidifier and turn the humidifier off in case the control humidistat fails to properly shut the system off.

Humidifiers are commonly installed in ductwork or an air-handling unit (AHU) to control the temperature and humidity in the space or building as a whole. For these systems, best practice is to include the following controls:

The space humidity controller can mount either directly within the conditioned space or in the return duct leaving the conditioned space. If using the return duct method, ensure that the humidistat senses conditions representative of the space as opposed to elevated temperatures or humidity levels resulting from machinery, stratification at the ceiling, or other process in the space.

When installing humidifiers in the AHU or supply duct, consider the psychrometrics of the airstream where the humidifiers are located. Warm air can absorb more water vapor than cold air, and the temperature and volume of air flowing through the ventilation system must be able to absorb the moisture being added to the airstream without becoming saturated. If the supply air is significantly cooler than the space design, the supply air may not be able to transport the required amount of moisture to satisfy the humidification load. For example, space design conditions of 24°C and 50% rh require 0.00944 kg of moisture per kilogram of air. Supply air at 13°C can only hold 0.00937 kg of moisture per kilogram of air and will be fully saturated under these conditions. The excess moisture that cannot be absorbed will likely become deposited in the duct or AHU, where it can cause bacterial growth, odors, or corrosion.

When using steam for humidification, the characteristics of the steam distributor should be considered. Adequate distance must be allotted for the steam to fully absorb to prevent unintended condensation on coils and other surfaces in the ventilation system. It is important to keep in mind that hot steam distributor tubes often contribute a small amount of heat to the airstream, which can increase the supply air temperature.

When using adiabatic humidification, consider the energy required to evaporate the water being discharge into the airstream. Evaporating a kilogram of water extracts 2500 kJ/kg from the airstream. As a result, the airstream leaving the adiabatic humidifier will be significantly cooler than the airstream entering the humidifier. Air preheating may be required to offset this cooling effect.

Control in variable-air-volume (VAV) systems is much more demanding than in constant-volume systems. VAV systems, common in large, central-station applications, control space temperature by varying the volume rather than the temperature of the supply air. Continual airflow variations to follow load changes in the building can create wide and rapid swings in space humidity. Because of the fast-changing nature and cooler supply air temperatures (13°C or lower) of most VAV systems, special modulating humidity controls should be applied.

Best results are obtained by using both space and duct modulating humidity sensors with an integrating device, which in turn modulates the output of the humidifier. This allows the duct sensor to respond quickly to a rapid rise in duct humidity caused when airflow to the space decreases as temperature conditions are satisfied. The duct sensor at times overrides the space humidistat by reducing the humidifier output to correspond to decreasing air volumes. This type of system, commonly referred to as anticipating control, allows the humidifier to track the dynamics of the system and provide uniform control. Because of the operating duct static pressures of a VAV system, using an airflow proving device is recommended to detect air movement.

Each individual humidifier should have a dedicated space control, high-limit humidistat, and air proving switch. All should be calibrated and checked for accurate control capabilities when commissioning the system and periodically as recommended by the control manufacturer. Properly commissioning the humidification system according to the manufacturer’s instructions is essential for long-term reliability, sustainability, performance, and ultimately owner satisfaction with the system.