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While ASHRAE Heats Up, Could Hybrid Cooling Be the Future?

Gordon Johnson, senior CFD manager at Subzero Engineering, explores how ASHRAE's thermal guidelines are impacting data center design.

Sets of cooling towers in data center building
Wei Cao / Alamy

Thermal guidelines from the American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE) have been instrumental in setting global cooling standards for data centers and have evolved to promote energy efficiency.

According to ASHRAE, the recommended temperature near the inlet of servers should be as close to but not higher than 80.6°F (27°C).

This article explores how the guidelines have impacted data center design and how operational practices have had to adapt in order to hit the optimal temperature benchmark. We also look at how it challenges contractual service level agreements with data center service providers.

Hybrid Cooling for Data Centers

Many data centers are experiencing increasing power density per IT rack, rising to levels that just a few years ago seemed extreme and out of reach. Today, these levels are considered both common and typical while simultaneously deploying air cooling.

This increase has left data center designers and managers wondering if air-cooled IT equipment (ITE), along with containment used to separate the cold supply air from the hot exhaust air, has finally reached its limits and if liquid cooling is the long-term solution.

Moving forward it’s expected that data centers will transition from 100% air cooling to a hybrid of air and liquid cooling, with all new and existing air-cooled data centers requiring containment.

Related:Power and Cooling: Design for today must consider long-term

With air cooling currently being the primary source used to cool data centers, ASHRAE has worked towards making this technology as efficient and sustainable as possible. ASHRAE's thermal guidelines, focusing on cooling the ITE in the data center, have had a significant impact on data center design and operational practices for years leading to improvements in energy efficiency and overall performance.

Data Center Cooling Criteria

A common set of criteria for cooling IT servers with the participation of ITE and cooling system manufacturers, entitled 'TC9.9 Thermal Guidelines for Data Processing Environments', has been published by ASHRAE since 2004.

Focusing on efficiency and reliability, several revisions have been published, with the latest (Revision 5) being released in 2021 (PDF). This latest generation TC9.9 highlights a new class of high-density air-cooled ITE (H1 class). This new class of ITE will need to be cooled to more restrictive supply temperatures than the previous class of servers.

The Revision 5 guidelines also focus more on cooling high-density servers and racks with a trade-off in terms of energy efficiency due to lower cooling supply air temperatures recommended to cool the ITE.

Related:Is Chip Cooling the Answer to Data Center Sustainability?

How Can Data Centers Comply With ASHRAE's Guidelines?

There are several considerations a data center operator must be aware of in adherence to the ASHRAE guidelines:

Cooling system design

Raising the temperature set points within the ASHRAE guidelines has prompted some data centers to deploy alternative cooling techniques that consume less energy, such as economizers or evaporative cooling.

Airflow management

The subject of improving airflow management should also receive attention. To achieve efficient cooling within the recommended temperature range, many data centers have focused on hot and cold aisle containment systems, with raised floors and optimized rack layouts used to minimize hot and cold air mixing.

It's by preventing hot spots and improving overall airflow that data centers can maintain the recommended temperatures without relying on excessive cooling.

HVAC equipment

Optimization of heating, ventilation, and air conditioning (HVAC) systems should also be reviewed under the latest ASHRAE guidelines. Advanced options such as variable speed drives, energy-efficient chillers, and direct-to-chip liquid cooling can efficiently cool high-density racks while enabling precise temperature control and reducing energy consumption.

Monitoring and automation

The modern data center utilizes real-time sensors to monitor temperature and humidity ranges throughout the operation. Automated controls are often used to adjust and maintain cooling parameters, airflow and equipment operations to preserve optimal conditions and improve energy efficiency. In addition, a real-time monitoring system that can remove thermal risk, optimize cooling capacity, and minimize energy waste should be deployed whenever possible.

Equipment improvements

With the exception of the new H1 Class of high-density servers, most new servers, storage systems, and networking devices now operate within wider temperature parameters without compromising performance or reliability. This can allow data centers to raise ambient temperatures while ensuring equipment longevity.

Education and training

Training programs and certifications have also been developed to ensure operators are knowledgeable about ASHRAE guidelines to implement them effectively.

Service level agreements (SLAs) typically outline the terms and conditions between a service provider and a customer regarding the quality and performance of the services being provided. However, while SLAs do not normally explicitly adhere to the ASHRAE thermal guidelines, they can incorporate relevant aspects of these guidelines to ensure appropriate environmental conditions for the services covered in the agreement:

  1. Temperature and humidity range: Recommended temperature and humidity ranges to help ensure optimal operating conditions for sensitive equipment. The SLA can specify the acceptable temperature and humidity ranges, which should align with the relevant recommendations.

  2. Environmental monitoring: The guidelines emphasize the importance of monitoring and documenting environmental conditions. The SLA can outline requirements for regular monitoring and reporting, especially temperature levels. It can also specify the frequency of monitoring, acceptable deviations from the recommended ranges, and the actions to be taken in case of disparities.

  3. Redundancy and resilience: The need for redundancy and backup systems to maintain stable environmental and operating conditions cannot be stressed enough. The SLA can include provisions related to redundant cooling systems, backup power supplies and failover mechanisms to ensure uninterrupted service delivery.

  4. Maintenance: The SLA can address the regular maintenance and preventive measures necessary to comply with the guidelines. This can include scheduled inspections, filter replacements, cleaning procedures and calibration of environmental monitoring and automated equipment.

  5. Compliance and audit: ASHRAE guidelines serve as industry standards for maintaining thermal conditions. The SLA can specify that the service provider must comply with these guidelines and allow the customer or authorized third party to perform periodic audits or inspections to ensure adherence.

But what to do when ASHRAE updates its guidelines? 2021 was the last time the TC9.9 document was revised, and as new technologies are constantly being considered and improved, it's important to note that the guidelines are subject to periodic updates.

If your SLA has it written that it refers to the most recent ASHRAE publications, the agreement should be revised in consultation with experts in the field to ensure the SLA aligns with the latest recommendations.


ITE is evolving and will continue to do so. ITE power and cooling has and will continue to push the limits due to packaging density and HPC. Both air cooling and liquid cooling experts agree that both technologies will coexist for many years to come, with future data centers transitioning from 100% air cooling to a hybrid of air and liquid cooling, driven by energy consumption concerns and sustainability goals.

It's important to recognize that by adopting ASHRAE's thermal guidelines for recommended temperature and humidity ranges, data centers have already improved energy efficiency, optimized cooling systems and embraced innovative technologies.

However, it should be highlighted that data center designers and managers need to take a holistic approach to cooling. While the data center can contribute to reducing operational costs, environmental impact, and overall resource consumption by sticking to the guidelines, we always need to consider what makes the most sense for each individual business case.

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