This behaviour is often linked to restricted circulation. If hot water cannot move freely through the system, the boiler reaches temperature very quickly and switches off to protect itself. The heat never has time to spread fully through the radiators.
Radiator flow plays a big part here. If some radiators heat too aggressively, the boiler can satisfy itself early while cooler rooms remain underheated. This imbalance is commonly seen in homes where upstairs rooms stay cold while downstairs warms first, which is covered further in why upstairs is often warmer than downstairs.
Pressure also matters. Low system pressure reduces circulation strength, making it easier for the boiler to reach cutoff temperature prematurely. If pressure regularly drops, it’s worth understanding the cause, which is explained in this pressure guide.
If the boiler consistently shuts off before radiators warm properly, checking circulation and pressure will usually be more productive than replacing parts. A broader overview of how these issues interact is covered in the main WarmGuide pillar.