What causes air in a hydronic heating system?

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What a Hydronic Heating System Is (and Why Air Is a Problem)

A hydronic system heats your home by circulating hot water from a boiler through baseboards, radiators, or in-floor loops instead of blowing warm air through ducts. These systems are common in Metro Vancouver's older character homes, townhomes, and some condos, especially properties built before forced-air furnaces became standard. Because the system relies on a closed loop of water under pressure, any pocket of air that gets in disrupts the flow. Water conducts heat efficiently; air does not, and it also compresses, which interferes with the pump's ability to circulate water evenly through the loop.

Common Causes of Air in a Hydronic System

Air rarely enters a sealed hydronic system in one dramatic event. It usually accumulates gradually from a handful of predictable sources, several of which are made worse by Metro Vancouver's mineral-heavy municipal water and the age of local housing stock.

• Fresh make-up water: tap water naturally holds dissolved oxygen, and every time the system loses pressure and refills, more dissolved air enters and later separates out as the water heats.
• Small leaks or loose fittings: a slightly loose valve packing, union, or bleed valve can let air seep in on the suction side of the circulator pump even without a visible water leak.
• Incomplete purging after installation, repair, or radiator replacement: air pockets left in the piping during commissioning will migrate to high points over time.
• Low or unstable system pressure: if pressure drops below the manufacturer's minimum, dissolved gases in the water are more likely to come out of solution as air bubbles.
• A failing or undersized expansion tank: this tank is designed to absorb the water's expansion as it heats; if its air bladder fails or it's too small, pressure swings pull air out of solution more aggressively.
• Corrosion inside older cast-iron radiators or steel piping, which can produce hydrogen gas as a byproduct — a sign of a chemistry problem, not just a mechanical one.

Signs Your System Has Air in It

• Radiators or baseboards that are cold at the top but warm at the bottom
• Gurgling, knocking, or bubbling sounds from pipes or radiators, especially on start-up
• Uneven heating between rooms or floors
• The circulator pump running noisily or seeming to work harder without producing heat
• Frequent need to top up system pressure on the boiler's gauge

What Homeowners Can Safely Check Themselves

Bleeding air from individual radiators is one of the few hydronic maintenance tasks a homeowner can reasonably do without a technician, provided the system has cooled down first.

1. Turn off the heating system and let radiators cool to avoid scalding.
2. Locate the bleed valve, usually a small square-socket or slotted screw near the top of the radiator.
3. Hold a cloth or small container underneath, then use a radiator bleed key (or flathead screwdriver, depending on the valve type) to open it slowly.
4. Listen for a hissing sound as air escapes; close the valve as soon as a steady stream of water appears with no more hissing.
5. Check the boiler's pressure gauge afterward — bleeding radiators lowers system pressure slightly, and it may need topping up to the manufacturer's recommended range.

If you bleed the radiators and the same units keep filling with air again within days or weeks, that repeat pattern points to an ongoing source — a leak, a failing expansion tank, or a pressure problem — rather than a one-time air pocket, and it's a sign to call a professional rather than keep bleeding it yourself.

What Requires a Licensed Technician

Anything beyond bleeding an individual radiator generally requires a qualified technician, and in BC, any work involving the gas supply line, gas valve, or venting on a gas-fired boiler legally requires a Technical Safety BC licensed gas fitter.

• Diagnosing and repairing pinhole leaks, loose unions, or failed valve packings in the piping
• Testing and replacing a failing expansion tank or its air bladder
• Adjusting or recalibrating system fill pressure and the pressure-reducing valve
• Installing or servicing an automatic air separator or micro-bubble air eliminator on the main loop
• Flushing the system and treating water chemistry if corrosion or sludge is contributing to gas formation
• Any repair that involves opening the gas train, combustion chamber, or venting on the boiler itself

What to Expect From a Professional Visit

A CANRO technician diagnosing recurring air issues will typically check system pressure against the manufacturer's specification, inspect the expansion tank's precharge and bladder condition, pressure-test visible piping and fittings for slow leaks, and check the automatic air vent or air separator near the boiler for proper function. If the system is older and prone to repeat air problems, they may recommend installing a dedicated air separator or microbubble eliminator, which continuously removes dissolved air from the water as it circulates. Any diagnosis includes an upfront, written quote before work begins — there's no guessing on price once the cause is identified.

Preventing Air From Coming Back

Frequently asked questions

Reviewed by the CANRO Heating & Cooling service team — July 02, 2026. CANRO holds Technical Safety BC gas licence LGA0043870 and boiler licence LBP0046450, employs Red Seal certified technicians, and carries WorkSafeBC and liability insurance. 4.9★ from 85+ Google reviews · serving Metro Vancouver since 1999 · (604) 359-1081 · info@canroheat.ca.