A radiator that's hot at the top but stays cold across the bottom typically has iron oxide sludge settled in the lowest part of the radiator, restricting flow there...
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A radiator that's hot at the top but stays cold across the bottom typically has iron oxide sludge settled in the lowest part of the radiator, restricting flow there while the top continues to circulate normally — a common issue on older cast iron systems.
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Cold at the bottom with heat at the top is the reverse pattern of trapped air, which always collects at the highest point in a radiator. This bottom-cold pattern instead points to something denser than water sitting low in the radiator body — almost always black iron oxide sludge, sometimes called magnetite, that's built up from years of ordinary internal corrosion in the steel and cast iron components of the loop and settled wherever flow is slowest, typically the bottom of a radiator.
This is a gradual aging process rather than a sudden failure, which is why it tends to show up radiator by radiator over years rather than appearing overnight, and it's more of a maintenance issue than an emergency.
The 1960s and 1970s character homes and split-levels around downtown PoCo and Birchland frequently still run original cast iron radiators, in some cases on a boiler that's been swapped once or twice over the decades while the radiators themselves have stayed in place the entire time. Cast iron radiators hold considerably more water and have more internal surface area than modern panel radiators or baseboard convectors, giving sludge more time and more surface to accumulate on across the decades these systems have often been in service.
The newer hillside homes in Citadel Heights and Oxford Heights, mostly built with baseboard or panel radiators rather than freestanding cast iron units, see this less often simply due to system age — but it's not exclusive to old cast iron, and any system that's gone many years without a flush, particularly one that's had repeated make-up water added to cover a slow pressure loss, can develop the same issue faster than expected.
If bleeding doesn't resolve the cold bottom and any water you can see looks dark, that points clearly toward sludge and it's worth booking a flush rather than continuing to bleed a radiator that has little actual air in it.
The standard remedy is a system flush, sometimes preceded by a chemical cleaner circulated through the loop to loosen and suspend accumulated sludge before it's flushed out, followed by a fresh fill and a corrosion inhibitor to slow future buildup. Heavily affected radiators occasionally need individual attention beyond a whole-system flush if sludge has packed densely enough to resist a general flush on the first pass. CANRO holds Technical Safety BC gas licence LGA0043870 and boiler licence LBP0046450, and any work on the gas line, burner, or venting has to be done by a licensed gas fitter under those permits.
On the original cast iron systems common in downtown PoCo, a technician will typically also check older shutoff and zone valves for the same buildup, since sludge doesn't confine itself to radiators — it can affect valve seating and circulator performance on a system that's gone a long time without service.
Adding an inhibitor after a flush, and in some cases a magnetic filter near the boiler, is a reasonable way to slow recurrence, particularly worthwhile given how long many of these cast iron systems have been circulating water without a flush.
Safety note
If you ever smell gas (a rotten-egg odour), hear a hissing near the gas line, or your carbon monoxide alarm sounds, leave the home immediately, do not operate switches or the boiler, and call FortisBC's emergency line at 1-800-663-9911 from outside. Only a Technical Safety BC licensed gas fitter should open the gas train, adjust venting, or service the burner and ignition components.
Not directly — sludge is a byproduct of gradual internal corrosion over a system's life, separate from the boiler's own condition, though badly sludged water can reduce boiler efficiency and shorten circulator life over time if it goes unaddressed for years.
A basic individual drain-and-refill is possible on some radiators, but clearing magnetite sludge properly usually needs stronger circulation, sometimes with a chemical cleaner, which is more reliably done as part of a full system service.
If you don't have service records showing a flush and the house has original cast iron radiators from the 1960s or 1970s, it's reasonable to assume it hasn't been done recently, if ever, and worth asking a technician to check the water condition directly.
A properly performed flush is standard maintenance and shouldn't harm sound cast iron, though a technician typically checks for any unusually thin or weak sections first on a very old system before proceeding.
A radiator only heating part of its surface delivers less warmth for the same fuel use, so sludge buildup across several radiators in an older system can mean longer runtimes to reach comfortable temperatures, which shows up as higher gas use.
It's a device installed near the boiler that captures circulating iron sludge particles before they resettle in radiators. It's optional but a reasonable addition after a flush on an older cast iron system prone to this kind of buildup.
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