Although industry debate about the research on acceptable humidity levels may continue, there is a growing realization that commercial buildings need some form of humidity control to provide optimum occupant comfort and productivity.

System designers have a variety of options to both dehumidify and humidify the air within a commercial building. Central systems may be best in some instances, and elsewhere localized systems may be more appropriate and economical to install, operate, and maintain.

The design challenge for today’s professional engineer responsible for the HVAC design is balancing building construction and energy codes and standards such as ASHRAE 55, 62, and 90.  The basic fundamental theoretical analysis of peak cooling, heating, humidification, and ventilation requirements of a building and the spaces within the building are simply an extension of engineering fundamentals learned in a collegiate course like heat transfer, fluids, and thermodynamics. The application of products and the controls of those products in a dynamic environment occupied by humans with differing metabolisms, comfort desires, and expectations becomes the challenge in controlling the part-load performance of the HVAC systems.


Liability of Moisture

A concise way to come to grips with the seriousness of moisture within a building is to recall what the Whole Building Design Guide (a program of the National Institute of Building Sciences) states on the Design Guidance page at 


Moisture control has become a significant liability issue for the MER (Mechanical Engineer of Record). A very negative trend has been developing in the industry recently whereby buildings are making occupants sick due to growth of mold. There is no one reason to explain why such a proliferation of mold contamination cases has blossomed, but there are some fundamental factors that the MER must keep in perspective while designing a facility, such as:


All water generation sources inside the facility need to be understood and minimized.

Construction of the building envelope must be properly applied to the climate zone in question. The relationship of vapor retarders and air barriers needs to be correctly understood.

All possible relative building air pressure relationships (internal and external) need to be understood to avoid bringing undesirable, untreated moisture into the facility.

Moisture in the air and in the building structure must be understood and controlled. Whether the moisture is from the outside, from processes inside the building, or added to the HVAC air stream through a humidification process, there must be an integrated approach that takes into account not only the controls but the building envelope construction details.


Psychrometric Analysis

Seven properties make up the psychrometric chart that the industry uses to design the spaces of commercial buildings to provide comfort to the occupants. These properties are: dry bulb and wet bulb temperatures, relative humidity, specific humidity (grains of water vapor per pound of air), vapor pressure, dew point, and enthalpy.

Humidity is generally talked about in terms of “relative humidity” in casual conversation.  Relative humidity is the amount of moisture in the air expressed as a percentage of the maximum amount of moisture that the air is capable of holding at a specific temperature. Many people don’t fully understand what relative humidity is, but they know when they feel that their skin is dry or there is “static electricity” caused by the air being dry. On the high end, they know that they have a harder time breathing when the “humidity” is high. Weather reporters generally talk about the “humidity” for the day and they are really referring to the “relative humidity,” not the specific humidity of the atmospheric conditions.

When engineers are designing the HVAC systems for buildings, modern codes and standards require that they must take humidity into account now more than ever. Beyond occupant comfort concerns, some products, such as wood, that are used in the architectural finishes and furnishings in a building also have humidification tolerances that must be maintained in order to maintain the products performance and the warranty on the product. Coordination between the architect and the mechanical engineer is essential to ensure that the humidification system has the capacity and controls to maintain these required product humidity limits.


On the market

Professional engineers will generally use theoretical calculations combined with some rule-of-thumb  experience factors to evaluate and document their HVAC loads for heating and cooling of a building.  These calculations also take into account the humidity and air velocities within the occupied spaces of commercial buildings. Once the theoretical calculations are completed, there is a need to see what products on the market are available to maintain target conditions.

For humidification, there are essentially three main methods to add humidification to an airstream:  steam, water vapor, and water droplets. Heat sources for adding additional grains of water to an airstream include electric steam generators, gas fired steam generators, and steam-to-steam or water-to-steam heat exchangers.

In the commercial facility market, the products and controls become more sophisticated than in residential humidifiers that many home systems have, but the psychrometric principles are the same.

Commercial humidifiers can be localized units located in the space, or compact units near the space that have short distribution piping to a distribution manifold within the supply air duct serving a specific space. These types of units will have canisters that need to be replaced with new full canisters of water as part of the maintenance of the system.

In larger HVAC systems, the steam humidification part of the system becomes more involved and usually includes a boiler with treated water, steam distribution piping, and a steam manifold within the main supply air duct near the air handler discharge. In addition, a condensate piping system is needed to remove the condensate within the steam piping manifold. The condensate piping requires maintenance to ensure the trap is operational.

The basic types of commercial humidifiers include: heated-pan humidifiers, wetted-element humidifiers, atomizing humidifiers, direct-steam-injection humidifiers, jacketed steam humidifiers, self-contained steam humidifiers, and portable humidifiers. There are many manufacturers of commercial humidifiers. Most manufacturers make multiple types of commercial humidifiers and offer technical guidance as to the pros and cons of the various types based on the application in which the humidifier is being installed.

The location of a steam distribution manifold is an engineering decision that must take into account the distance within a duct to absorb the moisture so that it is fully absorbed into the airstream.  Without full absorption, moisture will collect in the duct system and can either leak out of the duct or form a pool of water that can then run through the duct system and cause other damage, including saturating any duct liner that may be in the supply air duct system. 

When a supply air duct does have duct liner, care must be taken to not install the liner in the duct at a point too close to the humidifier manifold; otherwise, the liner will get wet. Darkness, dampness, and dirt within a supply air duct can then lead to a mold-friendly environment.

Most manufacturers as well as ASHRAE Handbook chapters and other engineering books have system schematics to show how various humidification system and product types are to be designed and for proper operation. It is prudent to review multiple sources of information to gain a more comprehensive understanding of the reasons why various humidifier types may be best for specific commercial applications. Product and installation costs as well as operation and maintenance costs are part of the design consideration. However, what is most important is the required performance of the humidification system, and this may require that the selection looks beyond the least expensive options.



The current ARCOM MasterSpec section for Humidifiers is section number 28 84 13 in the CSI 2014 MasterFormat Numbers & Titles index. These optional commercial building humidifier types are further delineated as follows:


23 84 13.13 Heated-Pan Humidifiers

23 84 13.16 Wetted-Element Humidifiers

23 84 13.19 Atomizing Humidifiers

23 84 13.23 Direct-Steam-Injection Humidifiers

23 84 13.26 Jacketed, Steam Humidifiers

23 84 13.29 Self-Contained Steam Humidifiers

23 84 13.33 Portable Humidifiers


The specification of humidification products and systems can be in one of the four forms of specifications that the Construction Specification Institute delineates in their MOP: proprietary (open or closed), reference standard, descriptive, or performance specifications. There may be a need in some cases to use a combination of these methods within Contract Documents in order to fully specify the products and system.

Professional engineers should have a clear understanding of these CSI specification formats. Specifications become part of the legal document in the contract between the owner and contractor. It is essential to have the humidification products specified clearly, concisely, completely, and correctly in order to get the proper products to meet the design intent.



Humidification system products and controls must be installed and commissioned in order to ensure proper operation. When contractors don’t follow the manufacturer’s installation instructions, they have an increased risk and liability if there is a resulting issue with the humidification system.

In the same regard, the engineer or construction administrator who reviews the installation should take care to check to see that the installation does indeed meet the specific manufacturer’s installation instructions. The authority having jurisdiction (AHJ) who inspects the installation should also have a clear understanding of the code requirements and manufacturer’s installation requirements including the piping to and from the humidifier.

Most humidifier specifications in the CSI — MasterFormat have a “Part 3 — Execution” section with statements similar to these:


A. Installation shall meet or exceed all applicable federal, state, and local requirements, referenced standards and conform to codes and ordinances of authorities having jurisdiction.

B. All installation shall be in accordance with manufacturer’s published recommendations.


The contractor will of course know what product they are buying and will have access to obtaining the appropriate manufacturer’s published recommendation. However, these specified Contract Document requirements also put a duty on the installing contractor to understand and comply with “all applicable federal, state, and local requirements, referenced standards, and conform to codes and ordinances of authorities having jurisdiction.” 

A contractor should not take for granted or ignore codes and standards or assume that the spec language is just incidental verbiage in the Contract Documents. This part of the contract is signed by the contractor with the client. This is not a duty of the Professional Engineer to explain to the contractor. This is not a duty of the manufacturer or the manufacturer’s rep to explain to the contractor. This is not a duty of the facility owner to explain to the contractor with details on the drawings. This is a duty clearly delineated in the Contract Document specifications with which the installing contractor must research and comply.

Two other commonly specified installation criteria for commissioning agent and contractor to keep in mind:


C. Examine duct layout at location of humidifier, and conditions for compliance with requirements or installation tolerances and other conditions affecting performance.

D. Install humidifiers with required clearance for service and maintenance.


The contractor must understand the operation of the humidifier at peak and part-load conditions to ensure the installation will not affect performance. If the installation location show on the Contract Document drawings does not allow proper installation, then the contractor would be wise to submit an RFI (Request For Information) early in the process (rather than, say, let the issue become a concern raised during commissioning or even later) to the owner for review and clarification by the design professionals.

Some specifications may require the factory to participate in the process as noted in the following two paragraphs that are sometimes included in the Contract Documents:


E. Manufacturer’s Field Service: Engage a factory-authorized service representative to inspect, test/adjust components, assemblies, and equipment installations, including connections. Report results in writing.

F. Engage a factory-authorized service representative to train owner’s maintenance personnel to adjust, operate, and maintain humidifiers.



The best product selection, specification, and installation can be rendered useless if the system is not operated and maintained properly by the building owner’s staff or facility maintenance contracting company, so adequate pre-occupancy training is essential. Many of the components within a commercial humidification system require maintenance and occasional calibration in order to operate correctly through the life of the product.



A lawsuit several years ago involved an IAQ issue that ultimately was tracked back partially to improper boiler water chemical treatment by the facility manager and maintenance staff. The monitoring of the chemicals within the water or steam systems that are used to add moisture to the air that the occupants of a commercial facility inhale is a maintenance responsibility. The maintenance of steam traps associated with humidification steam distribution headers is likewise important.

It is wise to have a very structured maintenance program in place for every humidification system in a commercial building. The program should incorporate all of the maintenance procedures and recommendations in the specific product IO&M manuals for the products in the system. In addition, the facility manager should ensure that all industry standards are understood and incorporated into the maintenance program. These include the humidity sensor ranges of operation being within the limits of the recommended minimum and maximum space humidity levels as recommended by industry organizations like ASHRAE.

Engineers need to coordinate with the architects to ensure proper product maintenance access space is provided. If a product is specified that will not fit in the space provided, including the accessibility space for maintenance, then the architect and engineer are taking greater risks.



In commercial applications, the humidity sensor (humidistat) can be located in a variety of places, depending on the system requirements and the control strategy. Location could be in the air handler itself, in the return air duct, in the supply air duct, or in the occupied space. If located in the occupied space, care needs to be taken so that the space is representative of all of the spaces served. If in the air handler or duct system, care must be taken to understand that this average sensor reading is not necessarily representative of any specific space, so appropriate tolerances must be allowed to not over- or under-humidify a specific space.

A humidistat is a device that measures the humidity. Humidity measurement instruments usually rely on measurements of some other quantity such as temperature, pressure, mass, or a mechanical or electrical change in a substance as moisture is absorbed. One instrument used for measuring the content of moisture in the atmosphere is a hygrometer. Newer style electronic devices use the temperature of condensation or changes in electrical capacitance or resistance to measure humidity differences. Accuracy, calibration, and commissioning are important for the life of a humidity sensor.



When it comes to providing the desired and required space conditions within a commercial building there can be a point where the KISS principle can become too focused on keeping it cheap and not providing the appropriate products and controls to meet the required codes and standards dictating today’s commercial building occupied spaces.

Facility owners, engineers, contractors, and the code authorities having jurisdiction must continue to go back to the engineering principle basics. They need to challenge each other (and themselves) to ensure that humidification of these commercial buildings are designed, installed, operated, and maintained properly to optimize the comfort and productivity of the occupants as well as provide the required conditions for the equipment and materials of construction within the commercial building.