When a radiator only heats on one side, the room never feels properly warm, even if the heating has been on for hours. I’ve had this happen in places I’ve lived before, and each time the symptoms were fairly similar. One side of the radiator warmed as expected while the other stayed noticeably cooler, almost as if the hot water wasn’t reaching the full panel. It’s the kind of issue that looks minor at first, but it affects the entire heating balance in the room.
The reason this happens is usually tied to the way water is moving through the radiator. If the flow isn’t even, the water enters on one side and loses momentum before it reaches the opposite end. In one of my houses, I thought the radiator itself was faulty, but it came down to the lockshield valve being slightly too restricted. Once the flow was adjusted, the radiator filled properly again and both sides began heating evenly.
Another time, the cause was sludge inside the radiator. It wasn’t enough to block the whole bottom like the typical sludge issue you see online, but it settled in a way that slowed the water as it travelled across. The first half of the radiator would heat normally, while the other half stayed cooler. Removing the radiator and flushing it cleared a surprising amount of buildup, and after that the heat spread consistently from end to end.
There are cases where air can contribute to the problem as well. Air usually affects the top of a radiator rather than one side, but if the panel isn’t completely full or if there’s trapped air in a higher point in the pipework feeding it, the circulation inside the radiator becomes uneven. Bleeding it fully is still worth doing, because it confirms whether air is playing a part.
I also noticed that this issue became more obvious when the overall system wasn’t balanced. The radiators closest to the boiler would take in most of the flow, leaving the ones further along with weaker circulation. When I balanced the system properly, this radiator began heating more evenly because it finally received enough flow for the water to travel across the entire panel. If you haven’t done this before, the balancing method I used made a noticeable difference: How to Balance Radiators Properly.
If the boiler pressure has been unstable recently, that can influence how radiators heat as well. When pressure drops, circulation becomes weaker, and radiators that rely on stronger flow often show the symptoms first. I had another radiator behave like this when my boiler was losing pressure, and it settled once the pressure issue was sorted. The guide that helped me understand that problem is here: Why Your Boiler Keeps Losing Pressure (And the Fixes That Actually Worked for Me).
If you’ve checked the lockshield, bled the radiator, and the pressure is steady, but the radiator still only heats on one side, then flushing the radiator is usually the most effective next step. It’s straightforward to do, and it’s the only way to remove any internal buildup that might be affecting circulation. Once the flow inside the panel is clear, the radiator normally goes back to heating across its full width.
If you’re working through heating issues like this and trying to improve the overall warmth of your home without overspending, the main guide that ties everything together is here: How to Keep a UK Home Warm for Cheap (Complete Guide). It helped me look at the broader picture instead of treating each radiator as an isolated problem.
