What causes a radiator to be cold at the bottom
Central heating water slowly corrodes the steel inside radiators and pipes. The result is magnetite, a fine black sludge. It is heavier than water, so it drops to the bottom of each radiator, where it builds up in a layer and stops hot water flowing across the lower part of the panel. The top stays hot because water still passes through there.
A small temperature difference between top and bottom is normal. Hot water enters, gives off heat and leaves cooler. What is not normal is a clearly cold band across the bottom, especially one that is coldest in the middle and slightly warmer at the ends near the valves.
If the radiator is cold at the top instead, the cause is almost always trapped air, and you can usually fix it yourself. See our guide to a radiator cold at the top.
How to tell if sludge is the problem
- Feel the panel with the heating on. Run your hand down the radiator once it has been on for 20 minutes or so. Sludge typically leaves a cold patch in the lower middle.
- Check the colour of the water when you bleed. With the heating off and the radiator cool, open the bleed valve briefly. Clear or slightly grey water is fine. Black or dark brown water means there is sludge in the system.
- Compare radiators. If only the one radiator is affected, the sludge may be local. If several radiators, especially those downstairs or furthest from the boiler, are cold at the bottom, the whole system is dirty.
- Listen to the boiler. Kettling (a rumbling or whistling noise from the boiler) and frequent overheating lockouts can mean sludge is also restricting flow through the heat exchanger.
Some boilers show a circulation or overheat code when flow is badly restricted, such as Ideal L1. Sludge is one of the common reasons behind that code.
Is it sludge or poor balancing?
Balancing sets how much water each radiator gets. On an unbalanced system, the radiators nearest the boiler get most of the flow and those furthest away stay lukewarm all over. That is different from sludge, which gives a hot top and a cold bottom on the same radiator.
| Symptom | Likely cause | Typical fix |
|---|---|---|
| Hot top, cold band along the bottom | Sludge | Flush the radiator or the whole system |
| Cold top, warm bottom | Trapped air | Bleed the radiator |
| Whole radiator lukewarm, far from boiler | Poor balancing | Balance the lockshield valves |
| Whole radiator cold, others fine | Closed valve or stuck TRV pin | Check the valves |
Your options for clearing sludge
Flushing a single radiator
If one radiator is affected, a plumber or heating engineer can isolate it, take it off the wall and flush it through with a hose outside. It is a quick job and avoids treating the whole system. It does not deal with sludge elsewhere, though, so it only makes sense when the rest of the system looks clean.
Chemical or mains flush
An engineer adds a cleaner such as Fernox F3 or Sentinel X400 to the system, runs the heating for a set period so it loosens the deposits, then drains and refills with clean water. This suits systems with light to moderate sludge.
Powerflush
A powerflush connects a pump unit to the system and pushes water and cleaning chemical through each radiator at high flow, one at a time, while dirty water is dumped. It takes most of a day for a typical house. It is the most thorough option for heavy sludge, but it is not risk-free on very old systems, because weak joints or corroded radiators can start to leak once the sludge holding them together is cleared.
Magnetic filter
A magnetic system filter, such as the Adey MagnaClean, Fernox TF1 or Worcester Greenstar filter, fits on the return pipe under the boiler. It traps magnetite as the water circulates and is cleaned out at the annual service. It does not clear a badly sludged system on its own, but it stops the problem coming back after a flush. Most manufacturers now expect one on new installations for the warranty.
Powerflush vs magnetic filter: typical UK costs
Prices depend on the number of radiators, access, whether you are in London or the South East and whether the engineer adds a filter and inhibitor as part of the job.
| Job | Typical UK cost |
|---|---|
| Remove and flush one radiator | Typically £80 to £150 |
| Chemical or mains flush | Typically £200 to £400 |
| Powerflush, 6 to 10 radiators | Typically £400 to £700 |
| Powerflush, 11 or more radiators | Typically £600 to £900 |
| Magnetic filter supplied and fitted | Typically £150 to £300 |
If you are replacing the boiler, a flush and filter are usually included or offered, so it is worth asking before booking them separately.
Which one do you need?
The two are not really alternatives. A flush removes the sludge that is already there, and a filter catches what forms afterwards. If several radiators are cold at the bottom, the water is black when you bleed and the boiler is kettling, you need a flush first and a filter after it. If the radiators heat evenly and the water is fairly clean, a filter and fresh inhibitor on their own are a sensible way to keep it that way. On an older system with signs of weeping joints or badly corroded radiators, ask the engineer whether a gentler chemical flush is safer than a full powerflush, and whether any radiators are worth replacing at the same time rather than cleaning.
When comparing quotes, check whether the price includes the cleaning chemical, the inhibitor afterwards and a filter, and whether the engineer will check each radiator heats fully before leaving. A cheap quote that leaves out the inhibitor can cost you more in the long run.
How to stop sludge coming back
- Make sure inhibitor (for example Sentinel X100 or Fernox F1) is added after any flush or drain down. It slows the corrosion that makes sludge.
- Have the magnetic filter cleaned at every service. Our guide to boiler service costs explains what a service should cover.
- Fix leaks quickly. Every time you top up the pressure, fresh oxygen-rich water enters and speeds up corrosion. If you are topping up often, read our guide to boiler pressure that keeps dropping.
Flushing, adding chemicals and fitting filters involve draining and refilling the system, often right next to the boiler. Leave these jobs to a qualified heating engineer, and use a Gas Safe registered engineer for anything that involves the boiler itself.