This pattern often shows up in systems where flow is slightly restricted. Hot water enters through the inlet valve, warms the first section of the radiator, and then struggles to travel across the full width. As a result, the area closest to the valves feels hot, while the rest of the radiator never quite catches up.
One common reason is partial restriction at the lockshield valve. If it’s opened too little, the radiator receives just enough flow to warm the inlet side but not enough to distribute heat evenly. This can happen after balancing work or accidental adjustments. Slightly opening the lockshield often improves heat spread without affecting the rest of the system.
Another contributing factor can be internal build-up. Sludge doesn’t always block the entire radiator. Sometimes it narrows the internal channels just enough to slow circulation. In these cases, the radiator may appear to work, but the usable heat output is much lower than it should be. A similar restriction pattern is explained in this guide on radiators that only heat at the bottom: Radiator Only Gets Warm at the Bottom.
Thermostatic radiator valves can also play a role. If the TRV pin isn’t opening fully, flow is reduced from the start. The radiator may still warm near the valve, but the reduced flow prevents heat from spreading properly. This overlaps with issues seen when a radiator only heats on one side, which is covered here: Radiator Cold on One Side.
If you’re unsure which part of the system is limiting flow, the quickest way to narrow it down is to work through the heating diagnostic tool. It helps identify whether the issue is likely balance-related, pressure-related, or local to the radiator: House Cold Diagnostic.
Once flow is restored and heat spreads evenly, the radiator should feel consistently warm across the panel. Fixing this kind of partial heating improves overall comfort and reduces the strain on the rest of the system. The wider context of improving radiator performance and heat retention is covered in the main guide here: How to Keep a UK Home Warm for Cheap (Complete Guide).
