A radiator that stays cold across the bottom while warm on top is a sign of magnetite sludge settling low inside the radiator and blocking flow there, the reverse of a trapped-air pattern, and it requires flushing rather than bleeding.
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Quick answer
A radiator warm on top but cold across the bottom points to magnetite sludge settling low inside the radiator and blocking flow, the reverse of a trapped-air pattern. Bleeding does nothing to fix it since no air is involved. A power flush, either of the single radiator or the whole system depending on how widespread it is, resolves it.
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Where the cold spot sits on a radiator tells you what is causing it. Cold across the bottom with warmth up top means whatever is blocking flow has settled to the lowest reachable point, which is the typical behaviour of magnetite sludge, a fine black iron-oxide byproduct that builds up inside an older hydronic system over years of internal corrosion. It is the reverse of trapped air, which rises and produces cold at the top with warmth at the bottom instead. Since there is no air involved in a cold-bottom pattern, bleeding the radiator does nothing to fix it, and repeated attempts just delay dealing with the actual blockage.
Older character homes around Haney and Websters Corners running original cast-iron radiators for decades have had a long time to accumulate internal sludge, particularly if the system has never been power flushed. Homes on municipal supply benefit from Metro Vancouver's relatively soft reservoir water, though internal corrosion inside an aging system is its own separate process and continues regardless of the source water quality. Rural properties in Ridge Meadows on private wells can see it move somewhat faster where harder water interacts with older iron and steel components over the years, but in every case the underlying mechanism producing the sludge is the same.
| Pattern | Likely cause | Bleeding helps? |
|---|---|---|
| Cold at top, warm at bottom | Trapped air | Yes, usually resolves it |
| Cold at bottom, warm at top | Magnetite sludge settled in the radiator | No, does not clear the blockage |
| Cold throughout, boiler firing normally | Closed valve or fully blocked radiator | No, needs a valve check or flushing |
One cold radiator can often be resolved by isolating and flushing just that unit, while several affected radiators across the house usually point to a full power flush of the whole system. Adding a magnetic filter afterward, and checking it periodically, is the practical way to slow the rate sludge rebuilds, since some level of internal corrosion continues in an older system no matter what.
If only one radiator shows the cold-bottom pattern, flushing that single unit is often sufficient, but if several radiators around the house show it, or the boiler has started making unusual noises or the system needs more frequent pressure top-ups, that combination points toward sludge circulating more broadly rather than settling in just one place. In that situation, a full power flush of the whole loop, not just the affected radiators, tends to be the more durable fix.
A system that has never been flushed and is more than ten to fifteen years old is a reasonable candidate for a full flush even before several radiators show symptoms, simply as preventive maintenance, since sludge buildup tends to accelerate once it starts restricting flow in more than one spot.
A power flush uses a pump to push a high volume of water, sometimes with a cleaning chemical added, through the system at a higher flow rate than normal operation, dislodging settled sludge from radiators and pipework so it can be captured and removed rather than simply pushed from one radiator to another. The process typically takes several hours depending on how many radiators and zones are involved, with a technician working through the system section by section to confirm sludge is actually being removed.
Refilling afterward with a corrosion inhibitor added to the fresh water is standard practice, since that additive slows the same internal corrosion process that produced the sludge in the first place, extending the time before another flush becomes necessary.
Feel each radiator in the house from top to bottom and note which ones show the cold-bottom pattern versus which heat evenly, since that map is genuinely useful for deciding between a single flush and a full system flush. Also note roughly how long ago the system was last flushed, if you know, or whether it has ever been flushed since the boiler was installed.
That information, even approximate, helps a technician arrive with the right equipment and set realistic expectations for the visit rather than assessing everything from scratch on site.
It is also worth checking the colour of the water if you ever drain a small amount from a radiator's bleed valve, since water that comes out dark grey or black, rather than clear or slightly rust-tinted, is a fairly direct confirmation of sludge and is useful information to mention when booking a flush.
Homeowners sometimes ask whether adding a chemical cleaner to the system water on their own, without a full flush, is a reasonable middle step, and generally it is not recommended without a technician's guidance, since the wrong product or dosage can react poorly with certain metals in an older system and cause more harm than the sludge itself was causing.
A radiator that has been flushed once but shows the same cold-bottom pattern again within a year or two, rather than after several years, points toward an unusually active corrosion process worth having a technician investigate further, since that faster recurrence is not the typical pattern and may indicate something specific to that system worth addressing directly.
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.
Trapped air produces the opposite pattern, cold at the top with warmth at the bottom, and clears with bleeding, while sludge settles low and blocks the bottom, which bleeding cannot fix.
It won't hurt to try once, but if the pattern is clearly cold at the bottom and warm at the top, bleeding is very unlikely to help and a flush is the more direct fix.
That depends on how many radiators show the pattern; a single affected radiator can sometimes be flushed on its own, while multiple ones point toward a full system power flush.
It can build somewhat faster on properties with harder well water interacting with older iron components, though the sludge itself comes from internal corrosion that happens in any aging system over time.
An inline magnetic filter installed after the flush, checked and cleaned periodically, is the practical way to slow how quickly sludge accumulates again.
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