Air separators and air vents remove free and dissolved air from closed heating and solar circuits. Fill water carries roughly 20 millilitres of dissolved air per litre, which leaves solution as the fluid heats and collects at high points — causing corrosion, noise, pump cavitation and stalled circulation. Vents release gathered air at high points; microbubble separators strip it continuously from the flow.
Why is there air in a sealed circuit at all?
Air arrives dissolved, trapped and by diffusion. Every litre of cold fill water carries roughly 20 ml of dissolved air at atmospheric pressure — more under system pressure — and Henry's law dictates that this air leaves solution exactly where you least want it: wherever the fluid is hottest and pressure lowest, meaning the top of the collector or the boiler heat exchanger. Filling always traps pockets in high points and fittings, oxygen diffuses slowly through non-barrier plastic pipes, and a circuit whose expansion vessel has lost pre-charge can pull air inward through threads and gaskets each time it cools. A sealed system is only as air-free as its weakest detail.
What damage does air actually do?
Four mechanisms, all expensive. Oxygen corrodes steel components and produces magnetite sludge that abrades pump bearings and clogs exchangers. Free bubbles make the familiar gurgling and ticking noises and cause circulator cavitation. Air pockets block circulation outright — an airlocked solar circuit stops flowing in full sunshine, the collector heads for stagnation temperature while the tank stays cold. And even before an airlock, entrained bubbles reduce heat transfer and falsify flow readings at the pump station. A circuit that needs re-venting every few weeks is not normal; it is a symptom of underpressure or a micro-leak.
Air vent, air separator or degasser: what is the difference?
| Device | Where it sits | What it removes |
|---|---|---|
| Manual bleed vent | Radiators, local high points | Collected air, on demand |
| Automatic float vent | System high points, collector field | Collected air, continuously |
| Microbubble separator | In the flow line at the hottest point | Bubbles as they leave solution |
| Vacuum degasser | Plant room, larger systems | Dissolved gases, actively |
The microbubble separator is the workhorse of modern practice: a mesh or lamella insert slows the flow, microbubbles coalesce and rise into a chamber, and a float valve expels them. Placed just downstream of the heat source — where solubility is lowest — it degasses the whole circulating volume pass by pass, typically within days of commissioning.
Why must automatic air vents in solar circuits be shut off?
Because of stagnation steam. When a solar circuit stagnates, vapour rises to the high points — exactly where automatic vents sit. An open float vent then blows off steam, which is lost propylene glycol fluid: after a few stagnation events the system is underfilled and underprotected. Standard practice is therefore a high-temperature-rated vent (metal float, 150 °C or more) with an isolation valve beneath it, opened for filling and commissioning and closed for normal operation. Ongoing degassing is delegated to a microbubble separator in the solar pump station at ground level, safely away from steam.
Where should air removal devices be installed?
Follow the solubility, then the geometry. The separator belongs in the flow close to the heat source — after the collector connection set or boiler — because hot, low-pressure fluid holds the least gas; it should also sit upstream of the circulator to protect the impeller from bubble erosion. Vents belong at every unavoidable high point of the pipework, with solar-rated hardware on the roof. Heating standards such as EN 12828 require venting provision at high points as part of normal system design — the difference between a compliant system and a quiet one is usually a single well-placed separator.
Frequently asked questions
Why does my solar system need venting again every spring?
It should not. Recurring air means the system is breathing: most often the expansion vessel pre-charge is too low, so the circuit dips into underpressure when cold and pulls air through fittings. Check pre-charge and system pressure before blaming the vents.
Should the automatic air vent on my collector stay open?
No. It is opened during filling and commissioning, then its isolation valve is closed. Left open, every stagnation event vents glycol vapour and slowly drains the system — one of the most common installation faults in solar circuits.
What causes gurgling noises in heating pipes?
Free air being dragged along by the flow. Vent the high points and the radiators, then check the pressure gauge — noise that returns within days indicates ongoing air ingress or a missing microbubble separator, not bad luck.
Does glycol fluid hold more air than water?
Glycol mixtures release dissolved gases more slowly and foam more readily during filling, which is why solar circuits are pressure-filled from the bottom and degassed with a separator rather than filled open and bled by hand.