Air trapped in a closed loop system causes gurgling noises, uneven heating or cooling, and can accelerate corrosion. Removing it requires a combination of proper bleeding, correct system pressure, and in many cases, dedicated air separation devices. The specific approach depends on whether you’re dealing with a hydronic heating system, a chilled water loop, or a PC liquid cooling setup, but the underlying physics are the same: air wants to collect at high points, and dissolved gases come out of solution when water is hot and pressure is low.
Why Air Gets Trapped in Closed Loops
Even a properly sealed system contains air. Fresh fill water carries dissolved oxygen and nitrogen. Every time you add makeup water, you introduce more. Air also sneaks in through threaded fittings, valve packings, and any point where system pressure drops below atmospheric pressure. In hydronic systems, maintaining a minimum of 4 to 5 psi above atmospheric pressure at the highest point in the system prevents this air ingress and allows venting devices to work properly.
Temperature plays a direct role in how much gas water can hold. Cold water dissolves more air than hot water. As water heats up inside a boiler or heat source, dissolved gases come out of solution and form bubbles. This is why you’ll often hear the most gurgling shortly after a system fires up.
Manual Bleeding: The First Step
The simplest way to purge trapped air is through bleed valves, the small valves located at high points in the system such as the tops of radiators, fan coil units, or air vents at system peaks. You’ll need a bleed key or a flathead screwdriver depending on the valve type, plus a small container to catch water.
Start with the system circulator pump turned off. Open each bleed valve slowly, beginning at the lowest radiator or terminal unit and working your way up to the highest. You’ll hear hissing as air escapes. Once a steady, bubble-free stream of water flows out, close the valve. After bleeding all points, check system pressure at the fill gauge and top off if it has dropped. Then turn the pump back on and let the system circulate for 15 to 20 minutes before repeating the process. It often takes two or three rounds to clear stubborn pockets.
Manual bleed valves are inexpensive and reliable, but they require you to physically check each one. In larger buildings or systems with many zones, this becomes impractical, which is where automatic venting comes in.
Automatic Air Vents
Automatic air vents use an internal float mechanism. When air accumulates inside the vent body, the float drops and opens a small orifice, releasing air until water rises and lifts the float shut again. These are installed at every high point in the piping and at locations where air naturally collects, such as the tops of risers and above heat exchangers.
Automatic vents eliminate the need for manual intervention and reduce the risk of human error, making them the standard choice for larger systems or those with frequent pressure fluctuations. They do require occasional inspection. Mineral deposits or debris can foul the float mechanism, causing either constant weeping or failure to vent. A small isolation valve beneath each automatic vent lets you service or replace it without draining the system.
Air Separators and Microbubble Removal
Bleed valves and automatic vents handle free air, the visible bubbles that collect at high points. But a significant amount of air exists as microbubbles suspended throughout the water. These tiny bubbles are too small to rise on their own and too dispersed to collect at a vent. Over time, they cause corrosion, pump cavitation, and reduced heat transfer.
Air separators solve this problem. The best location for an air separator is where the water temperature is highest and the pressure is lowest, because that’s where dissolved gases are least soluble and most eager to come out of solution. In a heating system, this means installing the separator at the outlet of the boiler or heat source. The expansion tank connection and system fill valve should be located nearby at what’s called the “point of no pressure change,” which keeps the separator working at peak efficiency.
Older style air scoops use internal ramps or mesh to slow water flow and coalesce bubbles, which then rise to a vent at the top. They work well when flow velocity is within their design range, but they have limitations with microbubbles. Microbubble resorbers use a different internal geometry, often a coalescing media, to capture and merge even the smallest bubbles. They reach a fully deaerated state faster than conventional air scoops, though both types eventually achieve the same level of air removal given enough operating time.
Getting System Pressure Right
Proper pressurization is one of the most overlooked factors in air removal. If any point in the system drops below atmospheric pressure, air gets sucked in through vents, seals, and fittings, and no amount of bleeding will keep up.
For systems where the pressure gauge is at the bottom (which is typical), calculate the required cold fill pressure by taking the height in feet from the gauge to the highest point in the system, dividing by 2.3 to convert to psi, and then adding 4 to 5 psi. So a system with 20 feet of height between the gauge and the top needs roughly 13 to 14 psi cold fill pressure (20 รท 2.3 = 8.7, plus 5). This ensures every point in the loop stays above atmospheric pressure during all operating conditions.
An undersized or waterlogged expansion tank can also cause pressure swings that pull air in during the cooling cycle. If you’re constantly rebleeding a system, check the expansion tank’s pre-charge pressure before assuming you have a leak.
Chemical Oxygen Scavengers
In commercial and industrial closed loops, chemical treatment adds another layer of protection. Oxygen scavengers are additives that react with dissolved oxygen in the water, converting it into harmless compounds before it can cause corrosion. Common formulations use sulfite or nitrite-based chemistry, sometimes combined with polymer dispersants and metal passivators that coat internal pipe surfaces.
Different formulations target different system types. Hot water loops typically use nitrite-based inhibitors to prevent oxygen pitting. Chilled water systems use inhibitors designed for lower temperatures where corrosion patterns differ. Systems containing aluminum components need specialized azole-based treatments to protect that specific metal. Chemical treatment doesn’t replace mechanical air removal, but it neutralizes the dissolved oxygen that separators and vents can’t fully eliminate.
Removing Air From PC Water Cooling Loops
Custom and all-in-one (AIO) PC liquid cooling loops follow the same physics but on a smaller scale. The key principle: make sure the pump is never the highest point in the loop. Air should collect in the radiator or reservoir, not in the pump head, where it causes rattling, reduced flow, and premature wear.
For a custom loop with a reservoir, fill the reservoir, power on the pump briefly, then top off as the water level drops. Repeat until the reservoir holds a stable level. Then tilt the entire PC forward and backward at about 45 degrees while the pump runs to work stubborn bubbles out of bends and fittings. Rotating the case in several directions helps dislodge air pockets that cling to internal surfaces. If you have traditional spinning hard drives, power them down before tilting.
For sealed AIO coolers where you can’t add fluid, set the pump to its highest performance mode and let it run for an hour. The aggressive flow rate pushes bubbles out of the pump chamber and into the radiator where they cause less noise and interference. Orienting the radiator so its tubes connect at the bottom (with air rising to the top of the radiator, away from the hoses) keeps air from cycling back through the pump.
New PC loops typically need a few days of operation before all microbubbles work their way out. Occasional gurgling during the first week is normal and resolves as air migrates to the reservoir or radiator and escapes through the fill port or is absorbed.
Ongoing Maintenance
Air removal isn’t a one-time event. Every time you drain and refill part of the system, add makeup water, or service a component, you reintroduce air. Seasonal startup in heating systems almost always requires a fresh bleed cycle. Automatic vents and air separators reduce the manual workload significantly, but periodic checks ensure they’re still functioning.
Monitor system pressure monthly. A gradual pressure drop usually means air is accumulating (or water is leaking). Persistent air problems after multiple bleedings often point to an undersized expansion tank, a failed tank bladder, or a fitting somewhere that’s pulling in air under vacuum conditions. Fixing the root cause saves you from chasing symptoms indefinitely.