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Commissioning Humidification & Dehumidification Systems

If you commission nothing else in a new or renovated building, be sure to consider commissioning the humidification and dehumidification systems. The fact that a project includes active humidification and/or dehumidification controls means that the proper operation of those systems is critical to the facility owner. These systems cost more to install and will cost more to operate throughout the life of the building. An owner will not choose to include them without a good reason to justify the expense.

The benefits of commissioning humidification and dehumidification systems is verification that they function as intended (i.e., the owner gets what they are paying extra for) and that they do so as efficiently and inexpensively as possible (i.e., the owner will not have to pay more than necessary to achieve their desired environmental control in the future).

Another way of looking at benefits is to consider the risks associated with not commissioning the systems. These risks may include:

  • Potential moisture-related problems such as mold and bacterial growth due to excessive humidification or uncontrolled condensation from humidification equipment.

  • If humidification is needed for health reasons, the inability to achieve design intent levels of relative humidity may result in colds, eye and nasal irritation, and fatigue.

  • If humidity control is intended to protect or preserve building contents, whether they be museum collections, manufactured products, or electronic equipment, the cost of damage to those contents can be extremely high compared to the cost of commissioning. In some cases, the contents may be considered priceless and impossible to recover from environmentally-imposed damage.

  • An insidious and often long-to-manifest-itself problem with improper humidity control systems is damage to a building’s structure. This can be due to poor or non-existent vapor barrier application or leaking humidifier components, Both of these can result in moisture collection inside of building elements where no one can see them without destructive investigation. If this occurs in a cold climate, the moisture can freeze and thaw with the weather and result in invisible and potentially catastrophic structural failure.

Because humidification/dehumidification control is not “standard” for most building types, many engineers don’t have a lot of experience with these systems and their proper design and control. More importantly, facility managers don’t necessarily have experience with their maintenance and intended operation. Training is a key element to the success of commissioning, and that is especially true for buildings with active humidity control.

Examples of actual experiences with humidity control systems are as follows:

  • A museum facility with individual zone “booster” humidifiers in the supply air ductwork had severe problems due to out-of-control condensation and “steaming” from the ceiling supply diffusers. The “final straw” was the collapsing of the conservation laboratory ceilings due to excessive moisture absorption due to out-of-control humidifiers. The commissioning process discovered a systemic mechanical problem with the modulating humidifier valves and un-tuned humidification control loops.

  • Another museum was found to be supplying humidified air as a “warm air wash” to exterior windows in a climate where winter outdoor temperatures drop to below 0oF on a regular basis. The resulting condensation and frosting on the window panes and frames was not only unsightly but also contributed to the rapid deterioration of the wooden window frames and the inability to open a required egress door.

  • Many traditional dehumidification control strategies depend on maintenance of low discharge air temperatures from the central air handler cooling coil in order to achieve the intended dewpoint levels throughout the building. Reheat is almost always required to reach the desired space temperature and relative humidity levels. Untrained building operators regularly want to raise the discharge air temperature setpoint to “save energy” (both cooling and reheat) without understanding the importance of maintaining the design setpoint.

  • An extreme example of lack of meaningful training was an elementary school where the building engineer refused to operate the boilers (for reheat) in the summer, regardless of being told it was imperative. That school was cool and damp for months at a time, because the air handlers continued to operate at a constant low discharge air temperature setpoint without any way of raising the space temperature and lowering the relative humidity.

No one should treat water in buildings lightly. Recent literature is full of examples of moisture-related building issues, most notably poor indoor air quality, that are costing building owners, operators, and occupants millions each year. Humidification and dehumidification systems are all about managing water and deserve the level of rigor and attention afforded them by the commissioning process.



Engineered Systems, December, 2003

Rebecca Ellis, PE, LEED AP, CCP, CxA
Questions & Solutions Engineering
1079 Falls Curve
Chaska, MN  55318