Chapter 7: Humidification

7.1. Theory and Applications

7.2. Commissioning the Humidification System

7.2.1. Functional Testing Field Tips

Key Commissioning Test Requirements

Key Preparations and Cautions

Time Required to Test

7.2.2. Design Issues Overview


7.1. Theory and Applications

Active humidification systems are complex, expensive to operate, and maintenance intensive, so these systems are seldom employed unless they are essential. Eliminating unnecessary humidification systems can yield substantial benefits. Active humidification is an energy intensive process that can also create moisture problems if not properly designed, installed and implemented. Thus, the commissioning of these systems can be critical to their success.

Humidification systems are typically applied to health care facilities and process environments. A health care application in an arid environment may actually require humidification for some areas even during the summer months when HVAC processes in most environments would be dehumidifying. In the past, computer facilities also were prime candidates for humidification to ensure that equipment was free from static electricity, thereby eliminating problems associated with electrostatic attraction. Current computer technology has largely eliminated this need, but it is a good idea to review the computer equipment manufacturer’s requirements.

Once the decision to actively humidify is made, the method of providing it will need to be evaluated. Methods for humidification include:

·       Direct or indirect steam injection

·       Evaporative approaches

·       Compressed air driven

·       Ultrasonic

·       Air washers

·       Sprayed coils

Ultrasonic humidification is a new technology that has the potential to provide humidification for a fraction of the energy cost associated with some of the more traditional systems. Regardless of the method of providing the humidification, it is critical that the humidified area be properly designed for the environment that will be produced. In most instances, these considerations will need to be applied to all portions of the building that can directly interact with the humidified area. Successful operation of most humidification systems is highly dependent on the control system serving it. Key considerations include the sensing technology employed and the safety and operational interlocks provided.

Another important consideration for any humidification application is that adequate absorption distance be provided downstream of the humidifier. This ensures that all of the water added to the air by the humidification process is fully vaporized before it encounters any surfaces upon which moisture droplets could impinge.

When considering humidification, it is important to remember that the measurements are frequently made in terms of relative humidity, the vapor content of the air relative to what it could hold at the current dry bulb temperature. For example, air that is saturated at 55ºF (100% relative humidity) will have a relative humidity of 50% when heated to 75ºF. The specific humidity and dew point of the air are unchanged. Thus, if an air handling system was serving an area by supplying 55º air and the area was to be maintained at a relative humidity of 50%+/-5%, it would be necessary to deliver air at or near saturation at the discharge of the unit. While easy and nearly impossible to avoid the saturated discharge condition when in a cooling cycle where moisture is being extracted from the air by passing it over a condensing cooling coil, it can be difficult to achieve it in a humidification cycle where moisture is added to the air stream via active humidification.

7.2. Commissioning the Humidification System

The following tables outline the benefits and background information associated with testing humidification systems. Refer to Functional Testing Basics for guidance related to all functional testing activities, regardless of the component or system being tested.

The third table, Section 7.2.3 Design Issues Overview, presents issues that can be addressed during the design phase to improve system performance, safety, and energy efficiency. These design issues are essential for commissioning providers to understand even if design phase commissioning is not a part of their scope, since these issues are often the root cause of problems identified during testing. 

7.2.1. Functional Testing Field Tips

Key Commissioning Test Requirements

Humidification processes tend to be energy intensive. The obvious costs are in the direct energy and water consumed to vaporize liquid water into the air stream. The parasitic burdens associated with the jacket heating systems used by direct and indirect steam injection systems may also lead to significant energy costs. The 2 to 4ºF temperature rise in the HVAC air stream serves no useful purpose when humidification is not required. In addition to representing unnecessary heating energy consumption, this temperature rise often represents a load on the chiller plant. Unnecessary heat transfer losses in the steam distribution system can also be significant, especially when there are no other requirements for steam. Thorough commissioning of the humidification system will help ensure that humidification is only being provided to the extent necessary and that the parasitic loads are minimized when humidification is not required.

Humidification system functional testing typically has the following objectives:

1   Verification of system performance and capacity.

2  Verify that capacity control systems match system performance to the current load condition, at low load as well as at design.

3  Verify that safety interlocks shut down the humidification process when they are tripped.

4  Verify that operational interlocks shut down the humidification process when conditions are not appropriate for humidification.

5  Verify that the capacity control system also remains in control of the process during sudden load changes associated with start-up and shut down, without tripping limit controls or experiencing operational problems like “rain” from the ducts.

Acceptance criteria for the various humidification system tests will vary with the test performed. In general, the equipment and systems need to meet the requirements of the design intent as documented on the construction documents.

Key Preparations and Cautions


The excess moisture injected into an HVAC system by an out of control humidification process can quickly lead to significant water damage. Caution should be exercised when subjecting the system to test conditions that could result in loss of control of the humidification process. Examples of these test conditions include step changes like start-ups, major set point changes or shut downs, as well as functional tests of the limit control systems in which the capacity control system is overridden to create the actual limit situation. It is a good idea to station personnel in the vicinity of the humidifier who are trained and prepared to manually shut down the humidification process.

If the humidifier equipped HVAC system is started up and commissioned during a period of time when the humidification system is not required, then it may be best to lock-out/valve out the humidifier until suitable conditions exist to allow it to be commissioned under load. An active but untested and untuned humidifier can quickly cause problems when it begins operating for the first time without any formal commissioning.

Test Conditions

In most instances, the humidification element will need to be in place to verify the functionality of the control, safety and interlock systems although it may be possible to verify some of the operational and safety interlocks without actually operating the humidifier.

Verification of the capacity control and performance of the system will require operating under a variety of load conditions. Humidification loads can be very difficult to simulate, thus testing of the system will need to occur under conditions where humidification is actually required.

Test the system under low load as well as peak load conditions. It is not uncommon for a system that has been tuned to perform satisfactorily at design or near design conditions to have problems operating at low loads. As a result, multiple site visits spread out over the course of the first operating year should be anticipated when commissioning humidification systems.

Instrumentation Required

Instrumentation requirements will vary from test to test but typically will typically be available in the standard tool kit listed in Functional Testing Basics. For applications where humidity levels must be precisely controlled, a precision hygrometer may be necessary to verify performance.

Time Required to Test

Operational and safety interlock tests can often be accomplished in an hour or less. Tests targeted at verification of the capacity control system and overall system performance often require observation over the course of a day or more with hour long periods of relatively intense activity interspersed with periods of low activity where monitoring of the system performance is the primary function. Often, this type of testing can be coordinated with other tests.

Multiple site visits to test under actual load conditions (both design and low load) can be costly, especially if there are no other tests to be performed concurrently and/or the site location requires travel time. For example, if a project is started up and commissioned during the summer months, it is likely one or two additional trips will be required to the site during the fall and winter months to properly commission the humidification system. If there are no other tests or observations to be made while at the site, then the commissioning provider's productivity may be low during portions of the test cycle where the response and performance of the system is simply observed.


7.2.2. Design Issues Overview

The Design Issues Overview presents issues that can be addressed during the design phase to improve system performance, safety, and energy efficiency. These design issues are essential for commissioning providers to understand, even if design phase commissioning is not a part of their scope, since these issues are often the root cause of problems identified during testing.

Is the building designed to support a humidified environment?

Surface temperatures of walls, windows, ducts, piping, and equipment in the humidified environment all need to be considered in light of the ambient dew point when the humidification system is active to avoid condensation problems. The location of vapor barriers also becomes an important consideration. Failure to take these issues into account can result in significant performance problems and IAQ concerns.

Are areas adjacent to the zone that is actively humidified designed as if they were humidified environments?

The difference in vapor pressure between the humidified zone and the non-humidified zone will cause vapor to migrate from the area that is actively humidified to the other areas in the building regardless of their need for humidification. Thus, the humidity level in all areas of a building with humidified zones will tend to be elevated even if humidity is not actively controlled at all locations.

Do the design conditions for the system reflect the true, worst case situation for the location?

Since maintaining the required humidity level is critical for many projects with active humidification systems, it is wise to look at the performance of the systems on the worst-case days, not just the design days.

Is the sensing technology employed appropriate for the requirements of the load served?

The sensing technology used by humidification control systems is not as robust or accurate as the more familiar temperature and pressure sensing technology encountered in HVAC systems. The newer technologies have made significant improvements over what was available a decade ago, but high levels of accuracy (+/-3% or better) are expensive and maintenance intensive.

Have appropriate safety interlocks been provided?

Operating a humidifier without air flow can cause significant damage to the air handling equipment and area served in the form of condensed moisture and “rain” out of the duct system. Operating without an adequate water level for electrically motivated evaporative systems can ruin the humidifier. The lack of robustness in some control technologies makes high limit controls an important consideration in many applications.

Have appropriate operational interlocks been provided?

Steam injection technologies require a warm-up cycle to ensure that steam, not large water droplets, is injected into the air stream. This technology also requires control to shut down the jacket heating system when humidification is not required to eliminate an unnecessary temperature rise and related energy waste. Evaporative technologies require reliable, well-maintained make up and blow down and may also require some sort of treatment for the make up water to prevent excessive scaling.

Is the physical configuration of the system in the area of the humidifier appropriate?

To successfully add moisture to the air stream of an HVAC process without causing water damage to the system or area served, the physical configuration of the system at the humidifier location must be properly configured. The airflow over the humidification element should be uniform and at a relatively low velocity. Adequate distance should be provided downstream of the humidification element for the moisture that has been added to become completely vaporized. As a general rule, the lower the velocity of the air stream, the lower the absorption distance requirement becomes for complete vaporization of the moisture injected by the humidifier.

Most systems providing a high relative humidity level in the occupied zone will require generating nearly saturated air at the discharge of an air handling system that is providing air at or below the space sensible temperature, an air handling system providing a cooling process. In extreme cases, it may be necessary to reheat the air prior to the humidification process to enable the desired relative humidity level to be generated in the area served.