A common cause is a TRV pin that isn’t moving freely. Even slight resistance can limit flow enough to keep the radiator permanently warm rather than hot. This often goes unnoticed because the valve head still turns normally. Issues like this are closely related to situations where a radiator only heats on one side, which are explained here: Radiator Only Heats on One Side.
Lockshield restriction can produce the same effect. If the lockshield has been tightened too far, the radiator may never receive enough flow to fully heat, even though it technically works. This is especially common after partial balancing or DIY adjustments.
System pressure also plays a role. When pressure is borderline low, radiators at the end of the system often struggle first. They may warm slightly but never reach proper output. If pressure problems are ongoing, this guide explains how they affect heating behaviour: Boiler Pressure Drops Overnight.
To work out whether the issue is local to the radiator or part of a larger system pattern, the diagnostic tool is the quickest way to narrow it down: House Cold Diagnostic.
Once flow is restored and pressure stabilised, a radiator that previously stayed lukewarm will usually begin heating normally. Improving these smaller issues contributes directly to a warmer home and lower energy use, which is covered more broadly in the main guide here: How to Keep a UK Home Warm for Cheap (Complete Guide).
