Venting & Deaeration
Air in Data Center Cooling Loops: Why Venting and Deaeration Matter at Fill
Trapped and dissolved air causes corrosion, pump cavitation and weak heat transfer. How to vent and deaerate data center cooling loops at fill.
By Denham McCollum, COO, Prime Equipment Solutions | The Readiness Report | Mechanical Readiness 101
Key takeaways
Air shows up in three forms: pockets at high points, entrained bubbles and gas dissolved in the water.
It causes oxygen corrosion, pump cavitation, noise and poor heat transfer, and large pockets can block flow.
ASHRAE's data center cooling committee calls air management "the sleeper" problem in direct-to-chip cooling.
Filling from the lowest point and venting every high point removes most free air. Separators and degassers handle the rest.
Vacuum filling removes trapped air, but glycol blends should be sampled afterward to confirm they stayed uniform.
Air in a cooling loop is any gas that stays in the system after fill, and getting it out is part of commissioning, not an afterthought. New loops start full of air. Some vents out during fill, but pockets, bubbles and dissolved gas often remain. Left in, they corrode metal, damage pumps and quietly cut cooling performance.
What forms does air take in a cooling loop?
Caleffi's hydronics engineering guide lists three:[1]
"Stationary air pockets at high points"
"Entrained air bubbles"
"Gases dissolved within water"
Dissolved air is the one people miss. Caleffi notes that at 65°F and 15 psig, "up to 3.6% of the molecules in a container of water can be dissolved gases." Heat the water to 170°F and its capacity drops to 1.8% of its volume. Drop the pressure too and it falls to about 0.6%.[1] As the water warms or pressure falls, that gas comes out of solution as bubbles.
What damage does air cause?
Corrosion. "Air is about 23% oxygen, and oxygen in contact with ferrous metals such as steel and cast iron causes corrosion."[1]
Pump cavitation. Gaseous cavitation "often occurs at the orifice of valves or the inlet of circulators."[1]
Poor heat transfer. "A given volume of water can absorb almost 3500 times more heat than the same volume of air."[1]
Blocked flow. A large enough air pocket in a tall system can keep the pump from pushing water over the top.[1]
Why does air matter more in liquid-cooled data centers?
Because the margins are thinner. StorageReview's summary of ASHRAE Technical Committee 9.9's May 2026 coolant bulletin calls air management "the sleeper" issue, noting that "entrained air cuts heat-transfer effectiveness, cavitates pumps, accelerates corrosion" and can stretch commissioning schedules.[2]
The bulletin also notes that air problems can show up weeks after startup, because degassing works best at high temperatures a loop may only reach under heavy IT load.[2]
How should a cooling loop be filled to keep air out?
Vertiv's guidance for single-phase technology cooling systems gives a practical baseline:[3]
"Fill from the lowest point in the system to mitigate air entrapment."
Run fill pumps "at speeds to prevent over pressuring the system or introducing air bubbles."
"Fill until all air bleeders are no longer expelling air."
Install "enough air vents... across long piping runs and at local high points."
Don't place automatic air vents above IT equipment, "as they can leak during operation."
ASHRAE's bulletin adds design points: vent every high point, avoid trap shapes and unvented flexible hoses, and place air separators on the warm, low-pressure return near the CDU inlet. Once the loop is confirmed air-free, "cap the automatic vents or isolate them with a valve."[2]
What tools remove the air that venting misses?
High-point vents, manual or automatic, release pocketed air where it collects.[1]
Microbubble air separators capture entrained bubbles, which "have extremely low rise velocities and are easily entrained by moving fluids."[1]
Vacuum degassers pull dissolved gas out of the water. One maker says its unit removes "dissolved and free gases" using Henry's law.[4]
Vacuum filling removes trapped air during fill. ASHRAE's bulletin cautions that "glycol mixtures can partially separate under vacuum," so the blend should be sampled and lab-tested afterward.[2]
Why does air matter during hydrostatic testing?
Safety. Test procedures call for crews to "systematically release air from all high points of the system" before pressurizing. Water is "virtually incompressible," but compressed gas stores far more energy, about "200 times more energy" than water at the same pressure and volume, and "the energy can be released with deadly force."[5]
Where Prime Equipment Solutions fits
Prime Equipment Solutions fills, vents and circulates systems as part of mechanical flushing, glycol injection and hydrostatic testing, with one team responsible for getting the loop clean, full and air-free before turnover.
Need a loop that's full, clean and air-free at turnover? Request a Quote
Frequently asked questions
What are the types of air in a hydronic system?
Stationary air pockets at high points, entrained air bubbles moving with the water, and gases dissolved in the water.
What problems does air cause in a cooling loop?
Oxygen corrosion, pump cavitation, noise, reduced heat transfer and, with large pockets, blocked flow.
How do you remove air from a cooling loop?
Fill from the lowest point, vent every high point, and use microbubble air separators or vacuum degassers for entrained and dissolved air.
Is vacuum filling safe for glycol coolant?
It removes trapped air well, but ASHRAE TC 9.9 cautions that glycol mixtures can partially separate under vacuum, so sample and lab-test the blend afterward.
Why does water release air when it heats up?
Warmer water holds less dissolved gas. Caleffi's data shows water's dissolved-gas capacity falls from 3.6% at 65°F to 1.8% at 170°F at 15 psig.
Sources
Caleffi North America, "Separation in Hydronic Systems," idronics #15.
StorageReview, "ASHRAE TC 9.9 Flags Four Ways Direct-to-Chip Cooling Loops Go Wrong," Aug 30, 2026.
Vertiv, "Guidelines for Deployment of Single Phase Technology Cooling System," SL-80056 Rev A, Feb 2025.
MEPcontent, "The new SpiroVent Superior S250," Jul 27, 2021.
Wermac, "Pressure Testing Practice."
