What Is the Difference Between ASHRAE's Recommended and Allowable Data Center Environmental Limits? - Part 1

Overview

This 2019 post by Ian Seaton was published on the Upsite Technologies blog. Seaton tabulates ASHRAE's recommended and allowable server inlet envelopes for classes A1-A4 and explains what actually distinguishes them: allowable limits are tied to OEM warranties, while the recommended envelope is not.

The recommended inlet is 18-27 C (64.4-80.6 F) for all A classes. The allowable ranges are 15-32 C for A1, 10-35 C for A2, 5-40 C for A3 and 5-45 C for A4. Allowable limits matter contractually because they align with OEM warranty specifications; neither envelope is a code. Exceeding the limits is therefore not a legal violation but a warranty and reliability risk.

A server's class is usually inferred from the widest allowable range that fits the vendor's published minimum and maximum temperatures. ASHRAE's X-factor method relates hours in each band to failure rate; reliability is referenced to a constant inlet of 20 C (68 F), and annual hours by band determine relative failure rate. Seaton also discusses the performance, noise and cost implications of operating in the allowable range.

The post is the first part of a series and concentrates on the limits and class determination; as a blog post it does not replace the official standard. The values in the record come from Table 1 of the post. The most important takeaway for a designer is that the difference between the recommended and allowable limits is not just a technical preference but a separation of warranty and operating risk.

Key points

1. Recommended inlet is 18-27 C (64.4-80.6 F) for all A classes.
2. Allowable: A1 15-32 C; A2 10-35 C; A3 5-40 C; A4 5-45 C.
3. Allowable limits matter contractually because they align with OEM warranty specifications; neither envelope is a code.
4. Class is usually inferred from the widest allowable range that fits the vendor's published min/max temperatures.
5. X-factor reliability is referenced to a constant 20 C (68 F) inlet; annual hours by band determine relative failure rate.
6. The post discusses the performance, noise and cost implications of operating in the allowable range.
7. The post is a 2019 blog entry and the first part of a series.
8. Related reference: ASHRAE Thermal Guidelines for Data Processing Environments (4th ed.).

Using it in design

• Server catalogue entries should store the manufacturer's min/max inlet temperature and derive the ASHRAE class from it.
• Design checks should compare hall set points and economizer excursions against both envelopes.
• Warn at the recommended limit and raise an error at the allowable (warranty) limit.
• Link annual hours distribution to the X-factor to show the effect on relative failure rate.

Who should read it

Mechanical designers and operators setting data centre set points, economizer operation and server temperature limits.

Related standards

• ASHRAE Thermal Guidelines for Data Processing Environments (4th ed.)

Publication details

Authors

Ian Seaton

Publisher

Upsite Technologies blog

Year

2019

Document language

English

Source type

Manufacturer

Access

Free to read

Licence

Copyright retained; citation only — the publisher’s terms apply to the original document

Citation