Guide · Central heating
Where a radiator is cold tells you a lot about what's wrong. Here's how to read the signs, what you can safely do yourself, and when it's a job for a heating engineer.
With the heating on for half an hour or so, carefully run your hand over the radiator — it can get hot, so go gently. Is it cold at the top and warm below? Warm at the top and cold at the bottom? Cold all over? Is it just one radiator or several? The pattern points to very different causes.
Air rises, so when it gets into a heating system it collects at the top of radiators, stopping hot water filling them. You might also hear gurgling. The fix is usually to bleed the radiator, which lets the air out.
On a sealed system (most combis and system boilers), letting air out also lets pressure out. If the gauge has dropped below the level in your manual, repressurise using the filling loop. Our boiler losing pressure guide explains how. If a radiator needs bleeding again and again, air is getting in somewhere, and that's worth having checked.
If a radiator is hot at the top but cold along the bottom, the likely cause is sludge. Over the years, heating systems produce a black, magnetic sludge from corrosion inside radiators and pipes. It's heavy, so it settles at the bottom of radiators and blocks hot water from reaching that part.
Bleeding won't help here. The fix is to clean the system. Depending on how bad it is, that might mean flushing individual radiators, or a full system clean using chemical cleaners and sometimes a powerflush machine that circulates water at high flow to shift the debris.
Once the system is clean, a magnetic filter fitted on the pipework near the boiler catches debris as the water passes through, and it's cleaned out during each service. An inhibitor added to the system water slows corrosion. Most manufacturers expect both when a new boiler is fitted, so they are standard parts of a good installation.
If only one radiator stays cold all over, check that both valves are open — the thermostatic valve on one end and the lockshield valve, usually under a plastic cap, on the other. A thermostatic valve pin can also stick in the closed position.
If radiators furthest from the boiler are cooler than those nearby, the system may need balancing: adjusting the lockshield valves so hot water is shared out evenly. If lots of radiators are cold, or the whole house is slow to warm, the problem might be the pump, a zone valve, or the boiler itself.
Sometimes upstairs radiators heat and downstairs ones don't, or the other way round. On older systems, especially those with a tank in the loft, this can point to circulation problems or a pump that's failing. It's the sort of thing that needs testing with the system running.
Bleeding a radiator is a safe job for most homeowners. Anything involving draining down the system, taking radiators off the wall, powerflushing or working on the pump and valves is best left to an engineer.
We handle system cleaning, radiator replacement, balancing and controls under our central heating service. If you're thinking about a new boiler anyway, a system clean is part of the job — see our boiler installation page. Get in touch with a description and a photo of the radiator, and let us know which of the above you've already tried.
Common questions
Clear answers before you arrange a visit.
Only when you notice cold patches at the top or gurgling. If you need to do it often, have the system checked for the cause.
Usually not. Cold at the bottom is normally sludge, which needs the radiator or system cleaning.
Often a closed or stuck valve. If both valves are open and it's still cold, it may be sludge or a balancing issue.
No. It depends on how much sludge there is. Sometimes cleaning chemicals and a filter are enough.
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