Radiators Cold at Bottom but Warm at Top? Here's the Fix

Why the bottom of a radiator goes cold

Central heating water slowly corrodes the steel inside radiators, producing a black, iron-rich sludge called magnetite. Because it is heavier than water, it settles along the bottom of the radiator, blocking the flow of hot water through the lower channels. The top still heats up, so the radiator feels half working, which is exactly what it is.

It is worth being clear on the difference between the two classic radiator faults. Cold at the top with a warm bottom means trapped air, and bleeding the radiator fixes that in two minutes. Cold at the bottom with a warm top means sludge, and no amount of bleeding will shift it. Ground floor radiators and those furthest from the boiler tend to suffer first, because that is where circulation is weakest and sludge settles most easily.

What you can check yourself before calling anyone

A few checks cost nothing and help you describe the problem accurately if you do need an engineer. Feel each radiator when the heating has been on for at least half an hour and note which ones have a cold patch, and how big it is. One sludged radiator is a different job to a whole sludged system.

• Check the boiler pressure gauge, it should sit around 1 to 1.5 bar when cold. Low pressure causes poor circulation that can mimic sludge.
• Bleed the radiator anyway to rule out air. If water comes out straight away and the cold patch remains, air is not the problem.
• If you have thermostatic radiator valves, make sure the valve is fully open and the pin underneath the head is not stuck down.
• Look at the water when bleeding. If it comes out black or grey rather than clear or slightly yellow, that confirms sludge in the system.

How the problem actually gets fixed

For one or two affected radiators, an engineer can sometimes remove the radiator, take it outside and flush it through with a hose. That is a relatively quick job and often sensible in older properties where a full system flush feels like overkill.

Where several radiators are affected, the usual answer is a chemical flush or a power flush. A chemical flush circulates a cleaning agent through the system for a period before draining and refilling, while a power flush uses a dedicated pump to push cleaner and fresh water through at higher velocity, dislodging settled sludge. Expect a power flush on a typical three bedroom house to take most of a day and cost somewhere in the region of £300 to £600, depending on the number of radiators and how badly fouled the system is. Very old or fragile pipework sometimes rules a power flush out, because dislodging debris can expose weak joints, and a good engineer will tell you that before starting rather than after.

Whatever method is used, the system should be refilled with a corrosion inhibitor afterwards. Skipping the inhibitor is how the sludge comes back within a couple of years.

Stopping it coming back

Sludge is preventable, which is why modern installations rarely suffer the way older ones do. A magnetic filter fitted on the return pipe to the boiler catches magnetite as it circulates, before it can settle in radiators or, worse, in the boiler heat exchanger. Fitted retrospectively, one typically costs around £100 to £250 including labour, and most boiler manufacturers now expect one to be present for warranty purposes on new installations.

Inhibitor levels should be checked periodically, ideally at your annual boiler service, and topped up whenever the system is drained for any reason. If your boiler is due a service anyway, it is worth asking the engineer to test the system water at the same time. Catching corrosion early is far cheaper than a flush, and considerably cheaper than the pump or heat exchanger failures that badly sludged water eventually causes.