So your radiator is warming up evenly but just not getting hot enough. This time there’s not enough hot water going through the radiator, or the water is not hot enough. Let’s see what can cause this.
Not enough hot water flowing through implies a blockage:
The TRV may not be responsive or the pin stuck in position. Try removing the TRV head and moving the pin a few times.
The lockshield valve at the other end may be closed too much, preventing enough hot water from circulating in the radiator.
Partial blockage elsewhere on the pipework. Close off all other rads to allow the pump to push water through this rad only and dislodge any blockage.
No blockage but most of the hot water is flowing to other radiators instead by taking the path of least resistance. The heating system will require balancing, i.e. allow the right flow of hot water through each rad using the lockshield valve. This is likely the issue if some rads are very hot and other cooler, in particular those at the end of the circuit.
The pipe size is too small and cannot deliver enough hot water.
Another possible reason for the radiator not getting hot is that the water reaching it is not hot in the first place.
This can be because the water temperature from the boiler may lukewarm in the first place. In this case, other radiators will not become very hot either, although those closer to the boiler may get hot enough, especially if the system has not been balanced.
The radiator piping may be incorrect, for instance the return pipe from one radiator may be used as the flow pipe to another radiator. In this case cooler water will be delivered to the radiator.
So your radiator does become very hot but takes a very long time. This can be due to some of the reasons in #3 where the rad didn’t get hot. So a partial blockage would deliver less water but over time this will be enough to get the radiator to temperature. Similarly, the heating system may need to be balanced to deliver more hot water to the radiator.
What it won’t be though are any issues with the TRV or thermostat causing the flow of hot water to be cut off too early. The radiator would not get hot eventually otherwise.
This means that hot water cannot circulate through the radiator. This is often due to a complete blockage in the system:
Are both valves at each end of the radiator sufficiently opened?
Is either valve defective or blocked? For a TRV, unscrew the head and push the pin in and out a few times.
Too many radiators on the system; the pump is not powerful enough to circulate water throughout. Test this by closing all other rads so water is only pumped through this one. This will also help clear any blockage albeit it might just be moved somewhere else.
Pipe bore size is too small; this is especially the case if the radiator that doesn’t heat up is the last one on the circuit. This is also coupled with a less than powerful pump or too many rads or both.
Instead of a blockage, the hot water may never get there in the first place due to fault piping. The radiator may be connected at both ends to the same flow pipe or the same return pipe so that water never actually gets to flow through it.
So the radiator is red hot but the room never gets warm enough. It’s not a technical problem with the heating system then, as the radiator is doing its job and heating up. The problem is likely at the design stage:
The radiator may not be big enough to heat up the room.
The radiator seems big enough relative to the size of the room but as room is not insulated, it is losing heat faster than it can heat up. Either insulate it or fit an even bigger radiator.
The radiator may look big and be big but may not emit heat efficiently. This is particularly the case with cast iron rads and designer rads. Replace with a bigger rad or one with a more efficient design.
The radiator has been installed where heat can escape the room, for instance near an external door that’s frequently opened. Most of the heat escapes the room rather than heat it up.
So the room is small, there’s no external door or other way for the heat to escape, the radiator is massive, there’s plenty of insulation and the rad is red hot. Why is the room still cold? Are you blocking air circulation through the radiator? Radiators, unlike their name, work by warming up the air that flows through its fins. If you cover the radiator with clothes or curtains, or place sofas, wardrobes or other furniture in front, you are stopping air from circulating not just through the rad but also throughout the room.