A radiator that's piping hot across the top but stone cold along the bottom is one of the most common calls we get once the heating goes back on in October. It's tempting to reach for the bleed key, but that almost never fixes this particular fault. Here's what's actually going on inside that radiator and what your realistic options are.
Air rises, so trapped air collects at the top of a radiator. If your radiator is cold at the top and warm at the bottom, bleeding it is the right move. Cold at the bottom is the opposite problem and points to magnetite sludge: a black, gritty iron oxide that forms as steel radiators corrode from the inside. It's heavier than water, so it settles along the base of the radiator and blocks the flow.
You can usually confirm it without any tools. Run the heating for 20 minutes, then feel across the radiator with the back of your hand. A sludged radiator gives you a clear horizontal line, hot above and cold below, often with the cold band deepest in the middle where flow is slowest. If you take a bleed screw out and the water runs black rather than clear, that settles it.
A lot of the housing stock around Wigston, South Wigston and Oadby is 1930s to 1970s, and plenty of it still has original or second-generation steel panel radiators on an open-vented system with a loft header tank. Open-vented systems draw in fresh oxygenated water every time they top up, and oxygen plus steel equals sludge. Sealed combi systems corrode far more slowly, but they still do it if the inhibitor has never been topped up.
Water hardness plays a part too. Severn Trent supplies most of this area with moderately hard to hard water, which encourages limescale in the boiler heat exchanger alongside the sludge in the radiators. The two problems tend to arrive together, which is why a sludged system often shows up as kettling noises from the boiler at the same time.
Before assuming the worst, rule out the cheap causes. Turn the thermostatic valve fully open and check the pin underneath the head isn't stuck down: pull the head off and press the pin with pliers, it should spring back. Also check the lockshield valve at the other end hasn't been closed off, and make sure a bulky sofa or a full-length curtain isn't sitting against the radiator.
If it really is sludge, a single radiator can sometimes be improved by removing it, taking it outside and flushing it through with a hosepipe until the water runs clear. It's a messy two-person job, the water stains carpets permanently, and a radiator full of water is heavy, so lay down plenty of old towels and a plastic sheet. Be honest with yourself about whether you're comfortable draining down and refilling the system afterwards.
If two or more radiators are cold at the bottom, the problem is the system rather than the radiator, and flushing one in isolation is wasted effort. A powerflush pushes water and cleaning chemical round at high velocity to shift the deposits, and typically takes most of a day for a standard three-bedroom semi. Costs vary with the number of radiators and how bad the build-up is, but for a house of that size you're usually looking at a few hundred pounds rather than tens.
One thing worth doing regardless: fit a magnetic filter on the return pipe by the boiler if you haven't got one, and make sure corrosion inhibitor goes back in whenever the system is drained. Inhibitor is the cheapest insurance in central heating and it's the step most often skipped. If a radiator is badly pitted or already weeping at a seam, replacing it usually makes more sense than flushing it, since a corroded panel will only fail again.
No. Bleeding only releases trapped air, which sits at the top of the radiator, so it won't touch a cold bottom section. If bleeding produces black water rather than a hiss of air, that's confirmation you've got sludge.
Yes, it's not dangerous, though that radiator will give out noticeably less heat and your boiler will work harder. If you also hear kettling or banging from the boiler, get it looked at sooner rather than later.
There's no fixed interval, but most systems benefit from a chemical clean and fresh inhibitor every five to six years, or whenever radiators are removed for decorating. A magnetic filter that's cleaned at each annual service will stretch that out considerably.