If radiators take a long time to build temperature, or only feel mildly warm even after the system has been on for a while, the boiler can end up doing more work without delivering better results. This is commonly seen in homes where circulation is weak or uneven. Radiators may heat eventually, but not fast enough to raise the overall room temperature.
Flow imbalance is one of the most frequent causes. When some radiators heat quickly and others lag, the boiler reaches its target temperature early and eases off before heat has spread evenly. This behaviour often sits alongside issues described in radiators that take ages to heat up.
Another clue is inconsistency across rooms. If some spaces feel acceptable while others never do, it points away from the boiler and toward distribution. That pattern is explored further in uneven radiator heating across the house.
To work out whether the limitation is coming from circulation, room heat loss, or system setup, it helps to follow a clear elimination process rather than guessing. This diagnostic page is designed for that purpose: House Cold Diagnostic.
For the full picture on improving warmth without immediately changing the boiler, everything connects back to the main guide here: How to Keep a UK Home Warm for Cheap (Complete Guide).