This behaviour is often linked to how other radiators are drawing heat. As the system warms up, radiators closer to the boiler or with wide-open valves can change the balance of flow. Once those radiators reach temperature, the distribution shifts and some radiators receive less circulation than they did at the start.
Valve behaviour can also contribute. A thermostatic valve may react early to local temperature changes, reducing flow sooner than expected. The radiator doesn’t switch off completely, but the reduced circulation causes it to cool gradually even though the boiler is still active. This pattern is related to cases where radiators heat unevenly across the system, which is explained here: Radiators Heat Unevenly Across the House.
System balance plays a major role in preventing this. When radiators are not balanced, the flow constantly shifts during a heating cycle, leading to inconsistent temperatures. This is especially noticeable in homes with multiple floors or long pipe runs. A related flow issue can also cause radiators to be warm at the top but cooler elsewhere, as explained here: Radiator Cold at the Bottom.
If it’s not clear whether this is a balance issue or something more localised, the diagnostic page helps identify the likely cause based on how the system behaves: House Cold Diagnostic.
Once circulation is stabilised, radiators tend to maintain a steady temperature throughout the heating cycle. This improves comfort and prevents unnecessary boiler cycling, which ties into the broader goal of running heating systems efficiently, covered in the main guide here: How to Keep a UK Home Warm for Cheap (Complete Guide).
