Commissioning To Fight Humidity Problems
The benefits of commissioning humidification and dehumidification systems are 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 rh may result in colds, eye and nasal irritation, and fatigue.
- If humidity control is intended to protect or preserve building contents, whether they are museum collections, elements of long-term experimental research, 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 nonexistent vapor barrier application or a leaking humidifier component. 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 collapse 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 untuned 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 0 degrees F 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.
- This same museum was a perfect example of another almost unfathomable but nonetheless frequent design error: There were separate, but open-to-each-other, environmental control "zones" throughout the facility, each with different temperature and rh criteria. In short, different rooms were to be controlled to different space dewpoints, but there were no barriers (doors, vapor barrier, etc.) between the zones. Moisture moves extremely easily through the air of unrestricted openings and construction "cracks" and somewhat easily through permeable solid building materials. Under this scenario, the humidification and dehumidification loads on the spaces with the more rigorous dewpoint criteria are much larger than they need to be.
In the case where the design engineers understand this issue and actually size the humidification and dehumidification systems to make up for the moisture loss/gain through these "openings," the critical spaces themselves can be maintained as desired, but the other spaces which don't require such rigorous control will have similar dewpoint levels to the adjacent critical spaces. This is unnecessarily expensive both from a first cost (i.e., much larger capacity equipment and associated central utilities) and from an operating cost (much higher energy consumption) standpoint throughout the life of the facility. In the case where the design engineers and architects don't communicate well enough to realize that these "holes" exist and/or that they may be a problem, the engineers will likely undersize the humidification and dehumidification systems. The installed equipment will attempt to humidify/dehumidify as best it can, but it will end up providing a relatively even dewpoint throughout the facility which is potentially slightly higher than the requirement for the noncritical spaces and significantly lower than the requirement for the critical spaces. No one will be happy.
- In a printing plant environment, vapor would intermittently issue out of overhead circular diffusers off one branch of a multibranch duct system. The vapor would then condense on the diffuser surfaces and start dripping onto the printing equipment and occupants in the space below. This would go on for a week or so, stop for a week or two, and then reappear again. This caused a real nuisance for the printing operation and began to produce water stains on the diffusers. The humidification system had a high-limit duct humidistat installed that seemed to be working according to local humidity readings just downstream of this sensor.
Further investigation uncovered the fact that the offending duct branch takeoff was within six feet of the humidifier manifold and the discharge air temperature off of this particular air-handling system was resetting down to 52 degrees to 53 degrees during a call for airside economizer operation to maintain internal space setpoint. The high-limit humidistat was located about eight to ten feet downstream of the humidifier manifold. Consequently, under these conditions, the vapor trail off of the manifold got longer and more concentrated, and was then diverted down the first duct branch due to the ductwork configuration and the system balancing. Very little of the excess vapor ever made it to the high limit sensor even though it was installed in a typically recommended location, and indoor rain soon followed.
- A wise feature to include in a humidification system is an interlock with the air handler for that system, so that humidification is turned off if there is no airflow. This prevents an over-accumulation of moisture and condensation within the portion of the air-handler system containing the humidifier manifold. One frequent deficiency discovered during verification testing is the lack of this interlock being made for all shutdown conditions of the air handler. While the interlock is present for a direct shutdown, it is not present for shutdown due to duct smoke detector activation or some other air handler interlock with a separate mechanical unit (energy recovery, return air fan, etc.). This problem seems to be most prevalent when the interlock is through the control software rather than a hardwire interlock. In a retro-commissioning effort where the manifold was just upstream of the supply fan inlet, the centrifugal fan wheel had experienced significant rusting and deterioration because the humidifier did not shut off when the fan did.
- As an extension of the preceding example, airflow proving devices, whether a sail switch or a differential pressure sensor, have also been found to be defective during verification testing. These devices are usually linked to causing an automatic humidification shutdown if no airflow is sensed in an air distribution system that provides humidification control. Sometimes it has been just a matter of adjusting the operational settings to restore operation, and at other times the unit has actually been defective.
- Other supporting devices that have been found defective during verification testing include steam valves that leak when in their supposedly "closed" positions. Therefore, even when the valve responds correctly to a valid control signal, the humidification system fails to meet its functional requirement to stop introducing moisture because of the internal leakage. This obviously also causes wasted steam and increased operational costs, besides poor humidification control.
- Another culprit can be the makeup water fill valve installation on steam-to-steam generator type humidifiers. One AHU penthouse installation that was tested had its solenoid fill valve operate correctly, but could not replenish its reservoir within the required time interval to allow continuous humidification to occur. The makeup valve actually demonstrated correct functioning for both open and closed operations.
The problem ended up being that the water system upstream of the valve could not deliver the minimum required pressure to create the intended fill flow rate. The installation in this case was above an auditorium, remote from the normal O&M personnel areas, and at a high enough elevation that some type of pressure booster assist was required but not provided. As a further aside, the reduced pressure backflow preventer in the makeup line leaked from a weep hole onto the penthouse floor with no direct capture of the leakage to be funneled into a drain system. The ultimate "drain system" was the auditorium ceiling.
- A similar steam-to-steam generator installation served a 100% outside air system which had very tight (50% rh plus or minus 3% rh) rh tolerances for a space with high air changes. Whenever the level in the humidifier tank dropped to the point of requiring makeup water, the cold makeup water flow cooled the volume of water in the tank to a point that steaming stopped for the duration of the fill cycle and then for a few minutes afterwards while it warmed up again. With 100% dry outside air flowing past the humidifier injection point with no steam added, the space rh dropped precipitously during the makeup cycle and, below a certain outside air temperature, failed to recover the space humidity level before the next fill cycle started. This resulted in a downward spiral and inability of the equipment to maintain space conditions for much of the winter.
A solution to this problem, which is apt to be more prevalent as we see more local steam-to-steam generators at air-handling equipment in laboratory and health care facilities, could be to provide hot makeup water to minimize the "downtime" of the steam generation. This would certainly help, but not eliminate the cyclic nature of the impact of the makeup water process. Another more costly solution, but one that could avoid any dip in humidification production capacity, would be redundant parallel steam generators set up to allow one to be in "fill" mode while the other is at full steam generation capacity and vice versa.
- A computer room had two air-handling systems supporting it. One was with a downflow air conditioner supplying an underfloor distribution system. The other was an overhead system for augmenting the underfloor cooling and providing humidification. At one point in the operating history of this particular computer room, the overhead system provided excessive moisture beyond its capability to effectively contain the resulting condensation. This caused rusting of the perforated supply air diffusers that eventually started to rain red dust and other rust particles onto the top of the computer equipment. This particle distribution continued long after the offending humidification system had been shut off as the "fix" employed by the maintenance personnel. An oversized single steam generator may be another solution but is likely to cause other problems during low load operation with respect to turndown capacity.
- 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 rh 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 rh.
No one should treat water in buildings lightly. Recent literature is full of examples of moisture-related building issues, most notably poor IAQ, that are costing building owners, operators, and occupants millions of dollars 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. ES