An air-locked radiant floor loop shows up as cold or lukewarm patches on an otherwise working boiler system, and it's usually a purging fix.
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Quick answer
An air lock in a radiant floor loop happens when trapped air collects at a high point in the tubing or manifold, blocking water flow through that circuit and leaving part of the floor cold or lukewarm while the rest of the system heats normally. It's typically resolved by purging that specific loop at the manifold, and it tends to recur if the original fill and purge wasn't done thoroughly or if the system has recently been drained for repairs.
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Radiant floor tubing runs in long, looping paths embedded in a slab or under a subfloor, often with several rises and dips as it snakes through a room, and any high point in that path is a place air can collect if it isn't fully purged out during the initial fill. Air is lighter than water, so it migrates upward and gets stuck at these high points rather than being carried along with the flow.
A manifold, where multiple loops split off from the main supply and return, is also a common spot for air to gather, especially in a loop that runs less often than its neighbours, such as a rarely used bathroom zone.
These symptoms overlap with a partially closed manifold valve or a failed actuator, so a technician typically checks flow at the manifold before assuming it's air, since the fix is different for each cause.
Purging a single radiant loop usually means isolating that loop at the manifold and running water through it at a higher flow rate than normal operating conditions, pushing trapped air out through a purge valve or air separator rather than letting it settle back into the tubing. This is done loop by loop rather than for the whole system at once, since air tends to hide in whichever loop has the least flow.
On a system with a properly installed air separator or automatic air vents at the high points, ongoing air issues are less common, but even a well-designed system can trap air after any work that involves opening the loop, such as adding a fixture or repairing a leak.
Occasional air after a repair or top-up is normal and expected. Air that keeps coming back on a system that hasn't been opened up recently is a different story, and it often points to a small air leak somewhere in the loop, a failing air separator, or make-up water constantly being introduced through an automatic fill valve because the system is losing pressure elsewhere. Fresh water constantly entering a loop brings dissolved air with it, which then comes out of solution as it heats.
If purging fixes the symptom but it returns within weeks rather than months, that recurring pattern is worth investigating rather than just purging repeatedly, since it usually means something else is feeding the air problem.
You can safely observe which room or loop feels cold and roughly how long the pattern has been happening, and if your manifold has visible flow meters, you can note whether one loop's indicator looks stuck compared with the others. Actually opening purge valves and manipulating boiler pressure is where it becomes technician work, since doing it incorrectly can introduce more air or affect system pressure.
Radiant and radiator loops share water (and often pressure) with a natural-gas boiler, so changes to the boiler's fill pressure, relief valve, or combustion venting are not homeowner work. That belongs to a Technical Safety BC licensed gas fitter. CANRO holds gas licence LGA0043870 and boiler licence LBP0046450. If you ever smell gas near the mechanical room, leave the house first and call FortisBC's emergency line, 1-800-663-9911, from outside.
Safety note
Radiant and radiator loops share water (and often pressure) with a natural-gas boiler, so changes to the boiler's fill pressure, relief valve, or combustion venting are not homeowner work. That belongs to a Technical Safety BC licensed gas fitter. CANRO holds gas licence LGA0043870 and boiler licence LBP0046450. If you ever smell gas near the mechanical room, leave the house first and call FortisBC's emergency line, 1-800-663-9911, from outside.
It isn't a safety hazard on its own; it mainly shows up as uneven heating in one area, though a technician should still check that boiler pressure and the relief valve are behaving normally while diagnosing it.
That pattern often points to trapped air or restricted flow in that specific loop at the manifold, rather than a problem with the boiler itself, which is heating the other loops fine.
Purging involves isolating a loop and managing flow and pressure at the manifold correctly, which is best left to a technician, since doing it incorrectly can push air deeper into the loop or affect overall system pressure.
Recurring air often means fresh water is continually entering the system through an automatic fill valve because of a small pressure loss elsewhere, and that underlying leak or fill issue needs to be found rather than just purging repeatedly.
Yes, any drain-down reintroduces air into the loops, so a proper purge afterward is expected maintenance rather than a sign something went wrong.
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