A radiator that is warm at the top but cold at the bottom is one of the most common heating complaints we see across Cheshire, especially once the heating goes back on in autumn. The good news is the cause is nearly always the same, and it is fixable. Here is what is going on inside the radiator and what to do about it.
Central heating water is constantly circulating through steel radiators, and over time the inside of the system corrodes. The result is a black, iron-oxide sludge (often called magnetite) that settles in the lowest, slowest-moving part of each radiator, which is the bottom. Hot water then flows across the top and down the sides, but cannot push through the blockage, so the bottom stays cold to the touch even when the top is piping hot.
This is different from a radiator that is cold at the top, which points to trapped air and is usually solved with a bleed key in two minutes. Bleeding will not fix a cold bottom, because sludge is heavier than water and sits exactly where a bleed valve cannot reach it. If you have bled a radiator repeatedly and the cold patch has not moved, sludge is almost certainly the issue.
Before assuming the worst, run a couple of simple checks. Feel every radiator in the house with the heating on. If only one is affected, the problem may be local to that radiator. If several are cold at the bottom, particularly downstairs radiators where sludge settles first, the whole system likely needs attention.
Check the thermostatic radiator valve (TRV) is fully open and that the lockshield valve on the other side has not been closed. Then crack the bleed valve with a cloth underneath: if the water that comes out is black or dark brown rather than clear, that is corroded metal suspended in your system water, and it confirms the diagnosis. Also check your boiler pressure gauge afterwards, as bleeding drops the pressure; most combi boilers want around 1 to 1.5 bar when cold.
For a single affected radiator, a heating engineer can close both valves, remove the radiator, and flush it through outside with a hose until the water runs clear. It is a messy job but straightforward, and often sorts an isolated cold spot for a modest call-out cost.
If several radiators are affected, the system needs a proper clean. A chemical flush involves adding a cleaning agent, circulating it for a period, then draining and refilling with fresh water and inhibitor. A powerflush goes further, using a machine to push water through at high velocity to shift stubborn deposits. In the UK a powerflush typically runs from around £300 to £600 or more depending on the number of radiators and how badly the system is fouled, so it depends on a survey rather than a fixed menu price. On older systems with very corroded radiators, an honest engineer will tell you if a flush risks exposing pinhole leaks, and whether replacing one or two radiators makes more sense.
A flush deals with the sludge that is there now, but without protection the corrosion simply starts again. Two things make the biggest difference. First, chemical inhibitor in the system water slows corrosion dramatically; it should be topped up whenever the system is drained and checked roughly once a year. Second, a magnetic filter fitted on the return pipe to the boiler catches magnetite before it settles in radiators or, worse, in the boiler's heat exchanger. Fitted alongside other work, a filter typically adds £100 to £200 including the part.
This matters for more than comfort. Sludged systems run less efficiently, so you pay more gas for the same warmth, and sludge is a leading cause of premature failures in pumps and heat exchangers. Many boiler manufacturers also expect the system to be cleaned and inhibited for the warranty to hold. Hard water across parts of Cheshire adds limescale into the mix, so keeping the system water in good condition is worth the small annual effort.
No. Bleeding releases trapped air, which collects at the top of a radiator. A cold bottom is caused by sludge sitting in the base, which bleeding cannot remove; the radiator needs flushing instead.
Usually most of a day for an average three-bedroom house, though a heavily sludged or larger system can take longer. The engineer flushes each radiator in turn until the water runs clear, then refills the system with inhibitor.
You can add a cleaner to the system yourself if you are comfortable doing so, and on lightly affected systems it can help. However, it must be drained out again after circulating, and badly sludged radiators usually still need a mechanical flush to shift the settled deposits.
Tell us what's going on and we'll give you a straight price. Talk to an engineer, not a call centre.
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