Radiators Cold but Boiler On? Common Causes Explained
If your boiler is running but your radiators are staying cold, you are not alone. This is one of the most common heating issues we see in West London homes, especially at the start of colder weather. While the boiler may appear to be working, the heat is not circulating properly around the system.
Below are the most common reasons this happens, what you can safely check, and when it is time to call a professional.
Air trapped in the heating system
One of the most frequent causes of cold radiators is air trapped inside the system. When air builds up, it prevents hot water from circulating fully, leaving radiators cold or only warm at the bottom.
You may notice:
Radiators warm at the bottom but cold at the top
Gurgling or bubbling noises
Bleeding radiators can sometimes resolve this. However, if the problem keeps returning, it may indicate a deeper issue such as pressure loss or air being drawn into the system.
Sludge or debris in radiators
In many older properties across West London, heating systems can build up sludge, also known as magnetite. This thick, dark residue settles in radiators and pipework, blocking the flow of hot water.
Common signs include:
Radiators cold at the bottom
Some radiators heating while others stay cold
Boiler working harder than usual
Sludge cannot be fixed by bleeding radiators. It often requires a professional clean, such as a power flush, to restore proper circulation.
Faulty pump or motorised valve
The heating pump is responsible for moving hot water around the system. If it is failing or blocked, the boiler may fire up but the heat does not reach the radiators. Similarly, a faulty motorised valve can prevent hot water from being directed to the heating circuit, even though the boiler is on. These components should only be checked or replaced by a qualified engineer.
Heating system needs balancing
If some radiators heat up fully while others remain cold, your system may be out of balance. This means hot water is flowing unevenly around the property. Balancing a heating system involves adjusting radiator valves so heat is distributed evenly. This is common after new radiators are installed or system work has been carried out.
Boiler or control issues
In some cases, the issue may be linked to boiler settings, thermostats, or heating controls. Incorrect settings or faulty controls can cause the boiler to run without delivering heat effectively to the radiators. Low boiler pressure can also affect circulation. Most systems should sit between 1 and 1.5 bar when cold.
When to call a Gas Safe engineer
If your radiators remain cold after basic checks, or if the issue keeps coming back, it is best to have the system inspected professionally. Heating issues can worsen over time and place additional strain on your boiler. A Gas Safe registered engineer can identify whether the problem is air, sludge, faulty components, or system imbalance, and recommend the safest long term solution.
West Gas Service regularly diagnoses heating circulation issues across West London and surrounding areas, helping restore reliable heating and comfort to homes and businesses.
If you need help, call 07866 567112, email ralphwestgas@gmail.com, or submit a form and a qualified Gas Safe engineer will be in touch.
