Cold at the top, hot at the bottom is trapped air, and bleeding sorts it in minutes. Cold at the bottom, hot at the top is a different problem entirely, and it's one that a bleed key can't touch. It's caused by sludge, technically called magnetite, a black sediment made from rusting steel radiators that settles at the base of the radiator and physically blocks water from circulating through the lower section.
This guide explains what magnetite is, why it forms, why bleeding does nothing for it, and the range of fixes available, from a simple single-radiator flush right through to a full power flush for a system that's badly affected throughout.
What's actually causing it
Central heating water constantly circulates past bare steel inside radiators and pipework. Over years, that steel corrodes slowly, producing a fine black iron oxide sludge known as magnetite. Because it's heavier than water, it settles at the lowest point it can reach, which in most radiators is the bottom of the panel, furthest from where hot water enters.
Once enough has built up, hot water simply can't reach the bottom section, so heat stops there and the panel is left cold below the sludge line, no matter how long the heating runs or how much you bleed it.
Why bleeding doesn't touch it
A bleed valve only ever releases gas, air or trapped hydrogen, from the top of a sealed radiator. Sludge is a solid suspended in the water itself, sitting in the bottom of the panel, nowhere near the bleed valve. Opening it will only ever let out air or, if the radiator is completely full, a trickle of water, never the sediment causing the cold patch.
Single radiator or whole system?
If it's just one or two radiators affected, especially older ones or ones at the end of a run, a localised flush of those radiators alone can often clear it. Isolate the radiator at both valves, disconnect it, and flush it through with a hose until the water runs clear, or have an engineer do this with a proper flushing tool that agitates the sludge loose.
If several radiators across the house are cold at the bottom, or the boiler is regularly making a rumbling or kettling noise, that points to system-wide contamination, and a single radiator flush won't be enough. In that case the whole system needs attention, not just the worst-affected panels.
Flushing options
- Chemical flush: a cleaning chemical is circulated through the whole system for a period, then drained and flushed with clean water, suited to mild to moderate sludge
- Power flush: a pump forces high-flow, low-pressure water through the system, radiator by radiator, to physically dislodge heavier sludge deposits that a chemical flush alone won't shift
- Localised flush: disconnecting and hosing out one or two badly affected radiators, a reasonable middle step before committing to a full power flush
Preventing it coming back
After any flush, the system should be dosed with a corrosion inhibitor, which slows the rate the steel corrodes and keeps future sludge production down. Many systems also benefit from a magnetic filter fitted to the pipework near the boiler, which catches magnetite particles in circulation before they can settle back out in radiators, and which is emptied out during an annual service.
A power flush is generally the right call once a system is showing cold radiators in multiple rooms, visibly discoloured bleed water, or a boiler making persistent noise, since at that point localised flushing on individual radiators is unlikely to solve the underlying contamination.
Frequently asked questions
- Will bleeding a radiator make cold-at-the-bottom worse?
- It won't make it worse, but it won't help either. It's worth checking there isn't also some trapped air at the top while you're there, but the cold section at the bottom will remain until the sludge is cleared.
- How do I know if it's sludge and not something else?
- If bled water runs black or dark grey rather than clear, that's a strong sign of magnetite. Draining a small amount from the radiator's drain valve into a white bowl makes it easier to see the colour clearly.
- Is a power flush disruptive?
- It's usually done in a day, radiator by radiator, without needing to drain the whole system permanently. Some noise and temporary disconnection of each radiator in turn is normal while it's carried out.
- Do all radiators eventually get sludge?
- Older systems without an inhibitor or magnetic filter are far more prone to it, since steel radiators corrode over time regardless. Systems with modern filtration and regularly topped-up inhibitor tend to stay much cleaner for far longer.
Cold radiator not responding to bleeding?
If your radiator stays cold at the bottom after bleeding, our engineers can flush it properly and fit a magnetic filter to stop it recurring.