This often comes down to circulation. Hot water may not be moving through the radiators fast enough, or it may be favouring certain paths while bypassing others. As a result, the boiler runs, but the heat never fully develops where it’s needed.
This behaviour can sometimes be confused with boilers that stop early or shut down, but the key difference is consistency. In this case, the boiler continues working. That contrast is useful when comparing with boiler fires then switches off, where heat delivery is interrupted entirely.
Lukewarm radiators can also appear when heating and hot water functions interfere with each other. If heat output changes depending on other demands, heating only works when hot water is on explains how priority settings and system layout affect performance.
In many homes, this problem becomes more obvious after adjustments are made elsewhere, such as lowering boiler flow temperature or changing schedules. The system still works, but the margin for error disappears.
To narrow down whether the issue is system balance, flow restriction, or competing demand, the diagnostic page provides a clear starting point: House Cold Diagnostic.
The broader relationship between boiler output and radiator performance is covered in the main guide here: How to Keep a UK Home Warm for Cheap.