Warm at the bottom, cold at the top is trapped air, and it is one of the few boiler symptoms most homeowners can fix themselves in a few minutes.
Tell us what you need—we’ll call you back fast.
Quick answer
A radiator that is warm at the bottom but stays cold across the top is almost always trapped air sitting at the highest point inside the radiator, blocking hot water from reaching that section. This is the opposite pattern from magnetite sludge, which produces warmth at the top and cold at the bottom. Bleeding the radiator with a radiator key, following the boiler manual, usually resolves it within a few minutes, though air that keeps returning points to a separate underlying cause.
Need a licensed gas fitter for your gas boiler? Call CANRO at (604) 359-1081 — residential natural-gas heating and hot water across Metro Vancouver since 1999.
Air is lighter than water, so any air that enters a hydronic heating system, whether from a refill, a repair, or dissolved gas coming out of solution as water heats, naturally rises and collects at the highest point it can reach within each radiator or along the pipework. Inside a radiator, that means the top section fills with a pocket of air instead of water, and since air does not carry heat the way water does, that section stays cold while the lower portion, still filled with circulating hot water, warms normally. This is the direct opposite of the sludge pattern, where a heavy sediment settles at the bottom instead, which is why the location of the cold spot is such a useful diagnostic clue on its own.
A modest amount of air after any system refill, a repair, or the first run of the heating season is completely normal and expected, since air is nearly impossible to avoid entirely during those events. Fresh municipal water also carries dissolved gasses that come out of solution once inside the warm system, which is one reason even a system that has not been drained can occasionally develop a small air pocket. If air keeps returning to the same radiators within days or weeks of bleeding, rather than settling down after one or two bleeds, that points toward a continuing source: a small leak drawing in air where it lets water out, a failed automatic air vent, or a fill valve introducing more fresh water than it should.
One bleed per radiator after a known event, a refill, a repair, or the seasonal start-up, is normal and usually sufficient. If the same radiators need bleeding again within a couple of weeks with no clear reason, or if pressure keeps dropping between bleeds, that combination suggests something is continuously introducing air or losing water, and repeated bleeding will only manage the symptom rather than fix the underlying cause. At that point it is worth booking a technician to trace the source rather than adding it to a recurring seasonal chore.
Safety note
Let the radiator cool before bleeding to avoid scalding, and stop immediately if water sprays rather than trickles, since that can indicate higher pressure than expected. If bleeding coincides with any gas smell near the boiler, leave the home and call FortisBC at 1-800-663-9911.
A well-sealed system might only need bleeding once at the start of the heating season, or not at all in a given year. Needing to bleed the same radiator repeatedly within weeks is not normal and points to a continuing air source.
Yes, this is one of the more common tasks homeowners handle themselves safely, provided the radiator has cooled somewhat and you follow the boiler manual's guidance on refilling pressure afterward.
If you hear no hissing and no water at all, the valve may already be clear of air, or in rare cases the valve itself may be stuck; a technician can check if repeated bleeding attempts show no result.
A persistent air problem can affect circulation and, if severe or combined with a related leak, contribute to corrosion inside the system, so it should not be ignored even though it feels like a minor nuisance.
Air often collects preferentially at the highest points of a system or in radiators with a slightly awkward pipe connection, so it is common for one or two radiators to show the symptom while others stay unaffected.
Bleeding releases a small amount of water along with the air, which does lower pressure slightly, so checking and topping up the pressure gauge afterward, per your manual, is an expected part of the process.
Family-run, TSBC-licensed (gas LGA0043870 · boiler LBP0046450), serving Metro Vancouver homes since 1999. Residential, natural gas only.
Call (604) 359-1081 Email info@canroheat.ca“Canro does an amazing job! Super professional and skilled with everything—boilers, furnaces, and AC. Always a pleasure working with them!”
“Five stars for Canro. We needed a replacement of a 30-year-old boiler. A new high-efficiency unit was installed—clean, professional work.”
“I can’t speak highly enough about George, who was recommended for a boiler service call for my parents’ home. George is excellent.”
25+ years keeping Metro Vancouver homes comfortable. Honest advice, upfront pricing, work that lasts.