When the boiler is on but the radiators remain cold, the problem rarely comes from the burner itself. In most cases, the heat produced is simply not correctly distributed in the heating circuit.
Several causes are possible: a blocked or faulty circulator, closed thermostatic valves, insufficient water pressure, air in the pipes, or even summer mode activated on the boiler. A methodical check often allows for a quick identification of the source of the breakdown.
Start by checking the pressure on the pressure gauge (usually between 1 and 2 bars), then ensure that the circulator is running and that the radiator valves are fully open. If some radiators are only partially heating or are only hot at the top, bleeding them is likely necessary.
If in doubt, or if the boiler displays an error code, it is best to call a professional to avoid damaging the installation or compromising your safety.
In more detail, a faulty circulator prevents hot water from circulating in the radiators: the boiler temperature rises, but the heat remains blocked at the source. Low pressure, due for example to a micro-leak or forgotten water top-up, also limits proper fluid circulation. Air present in the network acts as a plug and causes cold zones, hence the importance of regularly bleeding the installation, especially after draining or a long period of inactivity.
The regulation setting also plays a key role. A poorly positioned, incorrectly programmed, or defective room thermostat can prevent the heating request from being sent to the boiler. Similarly, a "summer" or "domestic hot water only" mode may be activated without you realizing it.
In summary, cold radiators with the boiler turned on indicate a problem with circulation, settings, or pressure, rarely a total failure of the boiler itself.
