Humidity control and static electricity in data centers

Older data centers were often designed to hold relative humidity in a narrow band, commonly around 45 to 50 percent. Low humidity was feared because of static electricity, and high humidity because of condensation and corrosion. Holding that band, especially in a Canadian winter, takes a lot of energy: humidifiers run for months, and cooling units sometimes dehumidify and reheat at the same time.

Industry guidance has changed. ASHRAE TC 9.9 now recommends a much wider humidity range for IT equipment, based on research and field experience. This guide explains the current guidance, the real risks, and how to control humidity efficiently.

Current ASHRAE guidance

ASHRAE TC 9.9's thermal guidelines define a recommended envelope and several allowable classes for IT equipment. For most equipment (class A1 to A4), the recommended envelope is:

• Dry bulb temperature from 18 to 27 °C (64 to 81 °F)
• Dew point from -9 °C (16 °F) up to 15 °C (59 °F)
• Relative humidity no higher than 70 percent in the current (fifth) edition, provided gaseous contamination is low; earlier editions used 60 percent, and ASHRAE advises a lower limit, around 50 percent, where corrosive gases are present

The guidance specifies humidity in terms of dew point at the low end because absolute moisture content is what matters for static and condensation, and because dew point does not change as air is heated or cooled along its path through the room. The allowable classes extend further, and the latest edition adds guidance on gaseous contamination and a high-density class with a narrower range. See ASHRAE TC 9.9 thermal guidelines.

Always check the specific requirements of your IT vendors and any equipment such as tape libraries, which may need tighter limits.

Static electricity

Electrostatic discharge (ESD) can damage sensitive components when a charged person or object touches equipment. Low humidity makes it easier for charge to build up on people and materials.

ASHRAE-sponsored research into ESD in data centers found that the risk at low humidity is controlled mainly by grounding practices, not by humidity alone. Conductive or static-dissipative flooring, grounded racks and personnel grounding (such as wrist straps when handling components) provide most of the protection. This research supported lowering the recommended minimum to a -9 °C (16 °F) dew point.

Practical ESD measures:

1. Ground racks and equipment frames according to manufacturer and electrical code requirements.
2. Use static-dissipative flooring or floor tiles, and verify their resistance periodically.
3. Use wrist straps or grounded work surfaces when working inside equipment.
4. Avoid carpets and materials that generate charge in the white space.
5. Train staff on ESD handling.

High humidity and corrosion

The bigger risk at the upper end is corrosion, particularly when gaseous contaminants such as sulphur compounds or chlorine are present. High humidity accelerates corrosion of copper and silver on circuit boards. This is why the current ASHRAE guidance ties its 70 percent upper limit to low measured corrosion rates, and recommends keeping relative humidity lower, around 50 percent, where gaseous contamination is a concern.

Gaseous contamination can be assessed with reactive monitoring coupons or sensors that measure copper and silver corrosion rates, classified using ISA-71.04 severity levels. Facilities near industrial sources, wastewater treatment, pulp and paper mills or heavy traffic, and those using direct outdoor air economizers, should consider monitoring.

Condensation is a separate concern. It occurs when surfaces are colder than the dew point of the surrounding air. In data centers, this mainly matters for chilled water coils and pipes, liquid cooling components and equipment brought in from cold vehicles in winter.

Humidity challenges in Canada

Winter

Cold outdoor air holds very little moisture. When it is brought in for ventilation or economizing and heated, relative humidity falls sharply. Data centers with direct airside economizers can see dew points well below the recommended minimum on very cold days. Options include:

• Mixing return air to raise moisture content
• Adding humidification only when dew point drops below the chosen minimum
• Using indirect economizers that do not bring outdoor air into the white space
• Accepting the lower allowable limits where all IT equipment is rated for them

See humidification in cold climates for humidifier types and their energy impact.

Summer

Humid summer conditions in Ontario, Quebec and Atlantic Canada can push dew points above 15 °C (59 °F). Direct airside economizers must switch modes before outdoor dew point exceeds the limit. In closed systems, dehumidification is usually handled by a dedicated outdoor air unit, not by every cooling unit.

Efficient control strategies

Centralizing humidity control and widening deadbands is one of the simplest ways to cut energy in older facilities. It is a common finding in retro-commissioning.

Sensors and monitoring

Humidity sensors drift over time and need regular calibration. Mismatched sensors on multiple units are a common cause of units fighting. Place sensors where they represent the conditions IT equipment sees, typically at rack inlets in the cold aisle, and use dew point for control. Trend dew point alongside temperature to see seasonal patterns.

Frequently asked questions

Do I need humidifiers at all?

Many facilities with indirect cooling and no outdoor air economizer rarely need humidification, because the only moisture loss is through ventilation and infiltration. Facilities with direct airside economizers in cold climates may need some. Check against the dew point limit you have adopted.

What about tape libraries and older equipment?

Some media and legacy equipment have tighter environmental requirements. If they are critical, place them in a zone with dedicated control rather than imposing tight limits on the whole room.

Key takeaways

• ASHRAE now recommends a wide humidity range, from a -9 °C (16 °F) dew point up to a 15 °C (59 °F) dew point and 70 percent RH where gaseous contamination is low.
• Grounding and ESD practices, not tight humidity control, are the main defence against static damage.
• Corrosion risk rises with high humidity and gaseous contaminants; monitor where contamination is possible.
• Centralize humidity control, control on dew point and widen deadbands to save energy.
• Canadian winters make humidity management important for direct airside economizers.