If a radiator is hot on one side and cold on the other, the issue is almost always uneven water flow rather than a faulty radiator. In most UK homes, this happens when hot water enters the panel but can’t circulate fully across it, leaving part of the radiator under-heated and the room slower to warm.
This behaviour affects more than just comfort. A radiator that isn’t heating evenly disrupts the balance of the whole system and forces the boiler to work harder for less result.
Uneven flow is the core problem
Radiators are designed to heat evenly from the inlet side to the outlet side. When flow is restricted, water loses momentum partway through the panel. One side heats normally, while the other stays noticeably cooler.
This often leads people to assume the radiator itself is failing, but in reality the restriction is almost always elsewhere.
A partially closed lockshield is the most common cause
The lockshield valve controls how much water passes through the radiator. If it is set too tight, hot water enters the radiator but doesn’t circulate strongly enough to reach the far side.
In one property I lived in, a small adjustment to the lockshield completely changed how the radiator behaved. Once the flow was increased slightly, heat spread evenly across the panel again.
This is why system balance matters so much. The correct approach is explained in How to Balance Radiators Properly.
Sludge can block circulation across the panel
Sludge doesn’t always settle neatly at the bottom of a radiator. Sometimes it builds up in a way that restricts flow across the internal channels, especially toward the outlet side.
When this happens, the first half of the radiator heats normally while the rest stays cool. Flushing the radiator often releases more debris than expected and restores even heat from end to end.
Air can contribute, even if the top feels warm
Air usually causes cold spots at the top, but trapped air in pipework feeding the radiator can still disrupt circulation inside the panel. This makes heat distribution uneven, even when the radiator appears to bleed normally.
Bleeding the radiator fully is still worth doing, as it helps confirm whether air is part of the problem.
System imbalance exaggerates one-sided heating
Radiators closer to the boiler naturally receive stronger flow. If those radiators are unrestricted, weaker radiators further along the circuit struggle to heat evenly.
Once the system is balanced, flow is shared more evenly and radiators that previously heated on one side often begin warming fully across the panel.
If several rooms behave differently, it’s worth stepping back and checking the wider pattern using the house cold diagnostic.
Unstable boiler pressure can make the issue worse
Low or unstable boiler pressure reduces circulation strength. Radiators that already rely on stronger flow are usually the first to show symptoms, including uneven heating.
If pressure has been dropping recently, this can explain why the problem appeared suddenly. The causes and fixes are covered in Why Your Boiler Keeps Losing Pressure.
When flushing is the right next step
If the lockshield is set correctly, the radiator has been bled, and pressure is stable, internal blockage becomes the most likely cause. At that point, flushing the radiator is often the only way to restore proper circulation.
Once internal flow is clear, the radiator usually returns to heating evenly across its full width.
How this fits into heating your home efficiently
One-sided radiator heating is a circulation problem, not a radiator problem. Fixing it improves comfort, reduces boiler workload, and helps the entire system run more smoothly.
For a full understanding of how these issues connect — and how to keep a UK home warm without overspending — this guide brings everything together clearly: How to Keep a UK Home Warm for Cheap.
