A radiator that is warm at the top but cold at the bottom is one of the most common central heating complaints we hear about in Burnley homes, especially in older terraces with original pipework. The good news is that the cause is nearly always the same thing, and in many cases you can sort it without calling anyone out.
When a radiator is cold at the bottom and warm at the top, the problem is almost always sludge, a black, muddy mix of rust and debris known in the trade as magnetite. Over years of use, the steel inside your radiators and pipework corrodes slightly, and those particles settle at the lowest point of each radiator. Hot water then flows across the top and around the blockage, so the top heats up while the bottom stays stone cold.
This is the opposite of trapped air, which people often assume first. Air rises, so an airlocked radiator is cold at the top and warm at the bottom. If yours is cold at the bottom, bleeding it will not fix anything, though it does no harm to check. Sludge is especially common in systems over ten years old and in homes where the inhibitor chemical has never been topped up.
Run your heating for twenty minutes or so, then feel each radiator carefully. A sludged radiator typically has a cold patch shaped like a shallow dome along the bottom centre, with warmth at the top and both ends where the water enters and leaves. If only one radiator is affected, it is likely a local build up. If several radiators downstairs are cold at the bottom, the whole system probably needs attention.
It is also worth checking the obvious things first, because they cost nothing to rule out.
If just one radiator is affected and you are reasonably handy, you can remove it and flush it through with a garden hose outside. Turn off both valves, drain the radiator into a shallow tray (the water will be filthy, so protect carpets), lift it off its brackets and hose it through until the water runs clear. Refit it, reopen the valves and bleed it. This costs nothing but an hour or two and usually restores full heat to that radiator.
When you refill, add a bottle of central heating inhibitor if the system has not had any in recent memory. A litre bottle costs roughly 15 to 25 pounds from any plumbers merchant or DIY shop and slows the corrosion that created the sludge in the first place.
If several radiators are cold at the bottom, the boiler is noisy, or the water that comes out during bleeding is black, sludge has likely spread through the whole system. A power flush from a heating engineer forces cleaning chemicals through the pipework at high velocity and clears the lot. In this area expect quotes of around 300 to 600 pounds depending on the number of radiators and how badly fouled the system is, so it is worth getting two or three quotes.
A cheaper middle option is a chemical clean, where cleaner is added, circulated for a week or two, then drained and replaced with fresh water and inhibitor. Many engineers will also suggest fitting a magnetic filter on the boiler return, typically 100 to 200 pounds fitted, which catches debris before it settles and protects a new boiler. To be clear, this is heating engineer territory rather than locksmith work, but as a local Burnley business we would rather point you in the right direction than leave you guessing.
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