A radiator that is warm at the bottom but cold across the top is almost always trapped air collecting at the radiator's highest point, and bleeding it with a radiator key, following the manual, usually resolves it within minutes.
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A radiator that is warm at the bottom but cold across the top is almost always trapped air collecting at the radiator's highest point. This is the reverse pattern from magnetite sludge, which settles at the bottom instead. Bleeding the radiator with a key, per the boiler's manual, usually resolves it within minutes.
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Air is lighter than water and rises to the highest available point inside a closed hydronic system, so a radiator that stays cold across the top while warm at the bottom is almost always an air pocket sitting right where the bleed valve sits. This is the opposite pattern from magnetite sludge, which settles at the bottom and blocks flow there instead, leaving the top warm. Since air is the cause rather than a solid blockage, this is usually the more straightforward of the two problems to fix without a technician.
Some air entering the system after a repair, a pressure top-up, or at seasonal start-up after the heating system sat idle over the shoulder months is entirely normal and typically clears with one bleeding session. On hillside Maple Ridge properties with more elevation change lot to lot, the radiator or zone sitting highest in the loop is often the first to show trapped air, which is a useful clue if you have several radiators and only the upper ones seem affected. If air keeps coming back to the same radiator after it has been properly bled, that points toward a continuous source, such as a small leak, a failed automatic air vent, or a fill valve overfilling the system, rather than a single pocket that just needed releasing.
When bleeding does not hold and the same radiator needs it repeatedly, a technician checks for a slow leak at fittings or valve packings, tests the automatic air vent, and confirms the fill valve is not continuously adding water. Air that keeps coming back is a symptom pointing to something else rather than a stand-alone problem.
If a radiator needs bleeding more than roughly once a month outside of the normal seasonal start-up, or if system pressure keeps dropping between bleeds, that is a reasonable point to stop repeating the fix and have a technician trace the underlying source.
A radiator that needs bleeding once after a repair or at seasonal start-up is normal and nothing to worry about. One that needs bleeding every couple of weeks, though, is telling you air is being continuously introduced, and the usual sources are a small pinhole leak letting in a trace of air along with the lost water, a failed automatic air vent that has stopped releasing air on its own, or a fill valve adding more water than the system needs and carrying dissolved air in with it.
Whether the cause is a leak or a faulty vent or fill valve usually comes down to whether system pressure is also dropping between bleeds. A pressure drop points toward a leak, while steady pressure with recurring air points more toward the air vent or fill valve.
Older cast-iron radiators, common in character homes near Haney and Websters Corners with original systems, tend to hold more internal water volume and can take slightly longer to fully bleed compared to newer steel panel radiators found in more recently renovated spaces, simply because there is more water and internal surface area for air to hide along. This is not a functional problem, just a difference worth expecting, and it is one reason a technician might use a slightly different bleeding sequence for an older cast-iron system versus a newer panel radiator.
If your home has a mix of both radiator types, noticing the older cast-iron units need bleeding somewhat more often than the newer panels is a normal pattern rather than a sign something is specifically wrong with those particular radiators.
Try bleeding the affected radiator once, slowly, and check whether water flows steadily afterward with no more hissing. If it clears and stays clear for a few days, no further action is needed. If it needs bleeding again within a week or two, or system pressure has visibly dropped on the gauge since the last check, mention that combination specifically when booking a visit.
Noting which radiator or radiators are affected, and whether they share a zone or sit at a similar height in the house, gives a technician a useful starting point before they arrive.
It is worth noting that a radiator with a decorative cover or enclosure can sometimes make it harder to hear the hissing sound that signals air escaping during bleeding, so removing or opening the cover, if it is designed to do so, makes the process easier to judge accurately rather than guessing when the valve has fully cleared.
Safety note
Any fault on a gas boiler that touches ignition, flame, venting, or pressure should be diagnosed by a Technical Safety BC licensed gas fitter rather than reset repeatedly. If you ever smell gas, leave the home first, then call FortisBC at 1-800-663-9911.
Yes, bleeding a radiator with a key, following your boiler's manual, is a routine homeowner task and clears most trapped-air cases within a few minutes.
On hillside properties, the radiator or zone sitting at the highest point in the loop tends to collect air first, so it is normal for one to need it more often than the rest.
No, sludge produces the opposite pattern of cold at the bottom with warmth at the top and does not respond to bleeding at all, while this cold-at-top pattern is air and usually clears easily.
A single round of bleeding across the house after the system has been idle for the shoulder season is normal; needing it much more often than that points to a continuous air source worth checking.
Try again slowly and recheck the valve is fully seated afterward; if it still will not clear or keeps returning quickly, that is worth having a technician look at rather than repeating indefinitely.
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