What do ASHRAE W-classes mean?

They classify liquid-cooled IT by the maximum facility water supply temperature it accepts: W17 up to about 17 °C, then W27, W32, W40 and W45. The higher the class, the warmer the water the equipment tolerates — unlocking chiller-free cooling with dry coolers — at the cost of tighter chip-level thermal margins. The classes complement, and are separate from, the A1-A4 room-air classes.

Which W-class do I need for free cooling?

It depends on climate: W32 suits dry coolers for much of the year in temperate regions, W40 makes chiller-less operation feasible across most of Europe and North America, and W45 extends free cooling further — but tropical sites with design wet-bulbs near 28-30 °C lose margin even at W45. Match the class to your site’s wet-bulb statistics, not to vendor marketing.

Reference + lookup · 2 min read

ASHRAE W-classes (W17–W45)

ASHRAE sorts liquid-cooled equipment into coolant supply temperature classes: W17, W27, W32, W40, W45 — the number is the maximum facility water supply temperature in °C. Higher classes reject heat into warmer water, enabling chiller-free operation — if the IT hardware supports it.

The table

Class	Max facility supply	What it implies for the site
W17	≈ 17 °C	Chilled water required — compressors run year-round; historic baseline
W27	≈ 27 °C	Cooler climate or partial free cooling; chiller mostly shoulder-season
W32	≈ 32 °C	Most temperate sites' free-cooling target; dry coolers viable much of the year
W40	≈ 40 °C	Chiller-less operation in most of Europe/North America via dry coolers
W45	≈ 45 °C	Warm-water direct-to-chip; the class behind "free cooling almost everywhere" claims — and the rating point of typical L2A cabinets

FIG. 1 — The class number ≈ maximum facility water supply temperature in °C. Quality, chemistry and wet-bulb limits apply on top — see notes.

Look up your site

Your facility water supply (°C)

Needs class ≥ 32

W32: the common free-cooling target — dry coolers cover much of the year in temperate sites

Reading it honestly

W-class is about the facility loop, not the room. Room air classes (A1-A4) are a separate ladder for air-cooled IT; do not mix them in a quote.
Warmer water trades chiller hours for chip margin. A W45 loop lets the site skip refrigeration, but every degree of warmer coolant raises cold-plate approach and chip temperatures — capacity and silicon limits pay for the efficiency.
Temperature is only half the spec. Classes assume water quality targets (corrosion, biology, particulates); PG25 with inhibitors is the usual answer on the technology loop (fluids).
Tropics cap the ladder. With design wet-bulbs near 28-30 °C, even W45 loops lose dry-cooler margin — the physics behind wet-bulb limits and SS 726.

Wet-bulb & free cooling · Standards overview · Approach temperature · Glossary

Sources

ASHRAE liquid cooling guidance ("Water-Cooled Servers" framework, 2019) and the W-class family as cited in equipment datasheets. Class value ≈ max supply temperature; confirm exact envelope in the current ASHRAE edition and your vendor's spec.

ASHRAE W-classes (W17–W45)

The table

Look up your site

Reading it honestly

Related

Sources