Control choices impact system choice and performance

A wide range of relative humidity control — from ±1% to ± 5% RH — is achievable with current humidification system control technology; however, note that humidity control rests not only with the control system, but with the complete building system. Individual building dynamics, such as temperature or air changes, can affect the accuracy and control capability of any humidification system. 

The factors to consider when specifying humidity control are:

Desired relative humidity percentage (set point)
Typical RH ranges are:
     •  Comfort, static control: 35% to 40% RH.
     •  Paper storage, printing: 40% to 50% RH.
     •  Clean room: 35% to 55% RH.
     •  Medical facilities: 35% to 50% RH for general areas; 60% to 80% for critical care areas.

Control accuracy required
How critical is control accuracy and what is an acceptable variance from the desired RH set point? Some manufacturing processes tolerate RH fluctuations of only ±1% from set point; humidification provided to improve human comfort can fluctuate up to ±5% from set point.

Space temperature
The amount of moisture air can hold correlates directly to air temperature — this is why it's called "relative humidity." If air temperatures fluctuate, RH levels fluctuate. Accurate RH control requires stable temperature control.
Component quality
The control system is only as good as the least accurate component in the system. Selecting humidity controls that match the application can eliminate many system difficulties.

Component location
Sensor and transmitter location has a significant impact on humidifier performance.
Learn more: Location, location, location: Sensor and transmitter locations are critical

Component types
Two control components that help ensure drip-free dispersion are a duct high limit humidistat and an airflow proving switch connected to the humidifier controller.

A duct high limit humidistat ensures that RH does not exceed a typical set point of 85% to 90% at the humidistat location, downstream of the dispersion assembly. To ensure proper operation, locate the duct high limit humidistat far enough downstream of the dispersion assembly to allow steam to become fully absorbed and equally distributed in the airstream. Otherwise, short cycling can occur, resulting in an unsatisfied humidification demand.

Systems with constant air volume can use on/off-type high limit humidistats. Systems with VAV require a higher level of control than a constant air volume system and must use modulating high limit humidistats to track airflow changes. Changing airflow quantities require the use of both space- and duct-mounted humidity controls used in conjunction with a programmable logic controller to modulate humidifier output.

An airflow proving switch, mounted in the duct downstream of the dispersion assembly, switches open a safety circuit if airflow in the duct stops or decreases below an acceptable level, disabling the humidifier. Constant volume systems can use pressure- or sail-type airflow proving switches. VAV systems must use sail-type airflow proving switches because pressure-type switches are not accurate at low airflows.

A common control component used in cold climates is a temperature offset transmitter mounted on an inside pane of an exterior window. This device transmits window temperature to the controller, which lowers the RH set point when the outdoor temperature drops, preventing window condensation.