What causes air in a hydronic heating system?

A hydronic heating system is a sealed loop of water circulating from your boiler through radiators or in-floor pipes. When it’s working right, there shouldn’t be much air in it at all. So when you hear gurgling, get cold spots at the tops of radiators, or find yourself bleeding the system often, air has found a way in – and it’s worth understanding how.

Some air is normal when a system is first filled, but recurring air points to a fault. In Metro Vancouver and the Fraser Valley, where many homes run boilers hard through long, wet winters, air problems are a frequent service call.

HeatLand has diagnosed and fixed hydronic air issues since 1999. Here’s what causes air in these systems and how to stop it coming back.

Fresh Water and Refilling

Every time water is added to a hydronic system – during initial filling, after maintenance, or when topping up lost pressure – that fresh water carries dissolved oxygen and air. As the system heats up, those dissolved gases come out of solution and form bubbles, exactly the way a pot of water releases bubbles before it boils. Those bubbles rise and collect at high points and in radiators. In a healthy, sealed system that’s filled once and left alone, air separators and vents remove this initial air over the first few weeks and the problem settles. But if you’re constantly topping up pressure because the system keeps losing it, you’re continuously introducing fresh, oxygen-rich water – and that means continuous air. This is why recurring air is often really a symptom of a leak or pressure-loss problem elsewhere. It also matters for corrosion: repeatedly adding oxygenated water speeds up internal rust. The fix isn’t just bleeding the air out; it’s identifying why the system needs refilling in the first place and, where appropriate, ensuring a proper corrosion inhibitor is in place to protect components.

Leaks and Low Pressure

Leaks are one of the biggest culprits behind persistent air. It seems counterintuitive – a leak lets water out, so how does air get in? The answer is pressure. Wherever water can seep out, air can be drawn in at points that sit above the pressure line or when the circulator pump creates low-pressure zones. Small weeps at pipe joints, radiator valves, pump seals, or fittings may be too minor to leave puddles but still let the system lose pressure and pull air in over time. As pressure drops, you top up, adding oxygenated water, and the cycle repeats. Common leak points in Metro Vancouver homes include aging valve packings, pump gaskets, and joints stressed by years of heating and cooling. Air drawn in at the pump’s suction side is especially common. Safety note: a slow leak you can’t see may still cause hidden water damage to floors, ceilings, or framing over time, and pressure that won’t hold warrants professional investigation. A HeatLand technician can pressure-test the loop and pinpoint leaks that are hard to spot, stopping both the water loss and the air ingress at the source.

Failing Expansion Tank and Air Vents

A hydronic system has components specifically designed to manage air and pressure, and when they fail, air problems follow. The expansion tank absorbs the pressure changes as water heats and expands; if it becomes waterlogged or its internal bladder fails, system pressure swings wildly, which can pull air in and cause repeated pressure loss and bleeding. A waterlogged expansion tank is a very common cause of chronic air and pressure issues. Automatic air vents and air separators are meant to continuously purge air from the water; if a vent clogs, sticks, or fails, air that should be removed instead accumulates in radiators and high points. Manual bleed valves that are corroded or left slightly open can also let air seep in. These components are inexpensive relative to the trouble they cause, but diagnosing which one has failed takes experience – the symptoms overlap. A licensed technician can test the expansion tank’s charge, inspect and replace air vents and separators, and confirm the system’s automatic air removal is working so you’re not stuck bleeding radiators manually every heating season.

Internal Corrosion and Gas Production

Not all ‘air’ in a hydronic system is actually air from outside – some is gas generated inside the system by corrosion. When oxygen reacts with steel radiators, iron components, and pipework, it produces rust and can release hydrogen gas, which collects just like air and causes the same cold-top radiators, gurgling, and repeated bleeding. Systems that have had oxygen repeatedly introduced (from leaks and top-ups) or that lack a proper corrosion inhibitor are especially prone to this. If you bleed a radiator and the gas that comes out has a distinct smell or the water is black and sludgy, internal corrosion is likely advanced. Left unchecked, corrosion produces sludge that blocks radiators (cold at the bottom), damages the pump, and shortens boiler life. The solution combines removing the existing air and sludge – often via a power flush – with adding a quality inhibitor and fixing the oxygen source so corrosion doesn’t restart. Serving Metro Vancouver and the Fraser Valley since 1999, HeatLand can flush a corroded system, restore protection, and address the root causes of air so your hydronic heating runs quietly and efficiently again.

Quick Checklist

• Check whether the system needs frequent pressure top-ups
• Inspect joints, valves, and the pump for weeps or drips
• Test the expansion tank for waterlogging or bladder failure
• Confirm automatic air vents and separators are working
• Check for black, sludgy water indicating internal corrosion
• Ensure a corrosion inhibitor is present at the correct level

Frequently Asked Questions

Reviewed by the HeatLand Heating & Cooling team — licensed HVAC contractor serving Metro Vancouver & the Fraser Valley since 1999. This article is for general guidance only. If you smell gas, suspect a carbon monoxide leak, or are unsure about any electrical or gas component, stop troubleshooting immediately and call a licensed technician or 911.