Air is not a contaminant that occasionally gets into a water system. It is constantly being created inside it. Every litre of mains water entering a hydronic circuit arrives saturated with dissolved gas — typically 8 to 10 mg/L of dissolved oxygen at 10 °C — and as that water is heated, its ability to hold gas falls. The gas has to go somewhere, and if nobody has designed a path for it, it ends up exactly where it does the most damage: at the pump, in the heat exchanger, and at the top of risers where circulation simply stops.
Three mechanisms matter. First, heating releases gas, because solubility falls with temperature — a sealed system liberates air even though no new water enters it. Second, pressure drop releases gas: air comes out of solution wherever pressure is lowest. Third, every fresh fill and every automatic top-up adds newly oxygenated water, which is why an automatic fill valve left permanently open is one of the costliest mistakes in a plant room.
The consequence of that continuous supply of oxygen is corrosion. In the steel of radiators and boiler waterways, dissolved oxygen drives pitting corrosion, producing black iron oxide — magnetite — that settles as sludge. The sludge then blocks narrow passages, insulates heat-transfer surfaces and fouls the very air valves meant to remove the problem, which is how a system develops a self-perpetuating fault.
The effects are easy to recognise once you know the pattern. Air pockets at high points create air locks, and circulation in that branch simply ceases — radiators cold at the top and hot at the bottom are the classic signature. Air reaching a pump causes cavitation, eroding impellers and producing a gravel-like noise often misdiagnosed as bearing failure. Microbubbles clinging to heat-exchanger surfaces reduce heat transfer and force the boiler to work at higher temperature for the same output. Air also distorts the flow conditions that balancing valves and thermostatic controls depend on, so the system loses accuracy as well as capacity.
In domestic hot water installations air locks cause the same complaints: a pumped shower that runs, pulses, then runs dry; a secondary return that never reaches temperature; a tap that spits. The mechanism is usually an unvented high point trapping air brought in by the pump or a refill.
Air management is not about fitting more automatic air vents; it is about matching the device to the air.
Manual vents belong at every genuine high point and at the top of vertical risers; they are cheap, reliable and impossible to misread. Automatic air release valves — the float-operated brass vents — work well at local high points and on terminal units, but they are sensitive to sludge: a fouled float either weeps continuously or fails closed, and a leaking auto vent on a sealed system is a slow, invisible top-up that keeps adding oxygen. Where water quality is poor, protect them with a strainer upstream, or accept manual venting instead.
Air separators and microbubble coalescers are the high-value component in a hydronic system, and the design rule is specific: install them where the water is hottest and the pressure lowest, because that is where dissolved gas leaves solution most readily. In most systems that is the boiler or heat-pump flow connection, before the pump. The expansion vessel should connect at the same point, using a fitting designed to stop the vessel becoming a gas trap.
On the distribution side the analogous devices are specified to their own standards: air valves for water supply under EN 1074-4 and AWWA C512, while air admittance valves for drainage are a different product (EN 12380) and must never be substituted for a pressure air-release valve.
Commissioning standards exist precisely because air removal is a process, not a single action: BS EN 12828 for the design of water-based heating systems, EN 14336 for installation and commissioning, BS 7593 for cleaning, flushing and water treatment, and CIBSE's Guide B and Commissioning Code W for UK practice, with the ASHRAE Handbook covering hydronic air management internationally.
Fill the system slowly from the lowest point, venting at every high point as the water rises. Run the pump, then vent again — air clinging to surfaces will have been mobilised. Repeat until the vents stay dry. Record system pressure cold and at operating temperature; the rise should match the expansion vessel volume and pre-charge, and if it does not, suspect a failed vessel or trapped air rather than a faulty gauge.
Use a differential thermometer across the pump: a widening temperature difference with everything else unchanged points to falling flow, and air is a common cause. Check each emitter's flow and return temperatures to find branches that are not circulating, and listen at high points and the pump for gurgling or gravel noise. Finally, inspect every automatic vent for weeping, confirm the make-up loop is isolated, and record the sludge filter's condition as a baseline.
Air is a design input, not an accident. Separate it at the hottest, lowest-pressure point in the circuit, vent every high point, keep the system sealed after filling, protect float valves from sludge and remove the corrosion products feeding the problem. Ignore it and every fault in the building will trace back to the same few cubic centimetres of gas.
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**About the author**
*Zhejiang Xindong Sanitary Ware Co., Ltd. — Technical & Export Team*
Xindong has manufactured brass valves, fittings, thermostatic shower valves and mixer taps in Yuhuan, Zhejiang for more than 25 years, from a 30,000 m² plant with 380+ employees, in-house design and testing laboratories, and over 65 design and structure patents. Products are developed and tested in line with the approval regimes of the company's export markets, including WRAS, TMV2/TMV3, cUPC and NSF/ANSI requirements, and are supplied to importers, MEP contractors and water-treatment integrators across Europe, the Middle East and North America.
Contact: No.242 JinHai Road, Xuanmen Industrial Zone, Yuhuan, Zhejiang, China · +86-0576-87499008 · [email protected] · https://www.cn-xindong.com
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The company, founded in 1999 and located in Yuhuan,specializes in taps, thermostatic shower valves and shower kits. With over 30,000 sqm of space and 380+ employees, we hold numerous certifications and 65+ patents.
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