Precision air conditioner humidity control and alarm management

In a server room, temperature is the value everyone tracks; humidity is often not thought of until an alarm comes. Yet air that is too dry invites static electricity, and air that is too humid invites condensation and corrosion. One of the most important points where a precision air conditioner differs from a comfort air conditioner is its ability to control this humidity.

In this article, drawing on our field experience, we explain how humidity control works, what the most frequent alarms tell you and how alarm management should be set up.

Why does humidity matter in a server room?

Humidity is a two-way risk for electronic equipment: low humidity creates a risk of static discharge, and high humidity a risk of condensation and corrosion. In winter the outdoor air is low in humidity, and in a heated indoor space relative humidity can easily fall below 20%. In summer, door openings and uncontrolled fresh air intakes quickly raise the room's humidity.

ASHRAE guidelines define humidity by dew point and an upper relative humidity limit; within the recommended range, relative humidity is generally not to exceed 60%. In the field, many facilities still operate with a target of 40-60% relative humidity. Which target to choose should be determined according to the equipment manufacturers' values and the facility's operating policy.

How does a precision air conditioner control humidity?

A precision air conditioner switches to either humidification or dehumidification mode based on the reading of the humidity sensor in the return air. Both operations take place inside the same unit, managed by the control board.

01Humidification

Most precision air conditioners have an electrode-type steam humidifier. The water in the cylinder is boiled by current passing between the electrodes and the steam is released into the airflow. Some models use infrared lamp or ultrasonic humidifiers. The cylinder scales up depending on the hardness of the feed water and needs to be replaced at certain intervals.

02Dehumidification

To dehumidify, the unit takes part of the evaporator out of service or reduces the fan speed; the coil surface then drops below the dew point and moisture in the air condenses. Because the air would be over-cooled during dehumidification, some units reheat the air with an electric heater or hot gas.

If one of two precision air conditioners in the same room is humidifying while the other dehumidifies, energy is wasted. This is usually caused by the sensors reading different values; the units should be linked together so that they work jointly.

What does a high humidity alarm tell you?

A high humidity alarm comes when the return air humidity exceeds the upper limit of the set point, and its most frequent cause is uncontrolled outside air entering the room. Doors left open, unsealed wall penetrations and incorrectly adjusted fresh air systems are the first places to look.

• The unit cannot switch to dehumidification mode: Compressor capacity may be insufficient or control settings may be restricting dehumidification.
• Sensor error: A dirty or drifted humidity sensor may read higher than the true value.
• Drain problem: Water in a blocked drain pan evaporates again and raises the room's humidity; it also creates a risk of water leakage.

What does a low humidity alarm tell you?

A low humidity alarm mostly indicates that the humidifier is not working. The most frequent cause is a scaled or worn-out cylinder; when the cylinder replacement warning is ignored for a long time, the unit stops humidifying altogether.

Other causes include a water supply valve left closed, a blocked supply solenoid, low water pressure and a humidifier contactor failure. Feed water that is overly softened or pure water also reduces conductivity in electrode-type humidifiers and prevents steam production. For this reason, water quality should match the values specified by the manufacturer.

It is normal for low humidity alarms to become more frequent in winter, because when cold air from outside warms up, its relative humidity drops quickly. In this period the humidifier runs much longer and the cylinder scales up faster. Having the cylinder's condition checked during maintenance before winter begins prevents a significant share of the alarms that would come in January.

Checks that a home user or facility staff can safely do include noting the alarm history, confirming the water supply valve is open, and making sure the room doors are kept closed. Cylinder replacement, electrode connections, and control board settings must be done by a qualified technician, because the humidifier runs on high current.

How should alarm management be set up?

Alarm management means classifying every alarm by severity and getting it to the right person in the right amount of time. If all alarms arrive through the same channel with the same urgency, the team quickly becomes desensitized to them; this is called alarm fatigue.

Critical alarms should be connected to the building management system, to monitoring software via SNMP or Modbus, or at the very least to a GSM alarm device through a dry contact. A written list should state who to call for each alarm and who the second contact is.

The alarm infrastructure itself should also be tested. One of the problems we see most often in the field is a notification system installed years ago that no longer reaches anyone because the phone number changed or the SIM card expired. Deliberately triggering at least one alarm during maintenance visits and confirming that the notification actually arrives is a check that takes only a few minutes but is very valuable.

What should you do first when an alarm goes off?

The first step is not to reset the alarm but to take a record. Note the time of the alarm, the temperature and humidity values shown on the display, and any changes made in the room in recent days. This information speeds up diagnosis.

Alarms that are reset repeatedly without finding the cause can turn a small fault into a big one. If the same alarm keeps recurring, the unit cannot solve the problem on its own. We described a similar approach for chillers in our article on compressor fault symptoms.

Preventing humidity problems from the start

Most humidity alarms are prevented by periodic maintenance: sensor calibration, timely replacement of the humidifier cylinder, cleaning of the drain line, and checking the room's airtightness. For the room's overall temperature targets, see our server room temperature guide.

At YOİ Mühendislik, during precision air conditioning maintenance we compare humidity sensors against a reference instrument and review the alarm list together with the facility. The maintenance interval and scope are determined during the site survey.