If your radiator is hot at the top but cold at the bottom, the most likely cause in a UK heating system is sludge buildup settling along the base of the panel. That lower section is where water movement slows and debris naturally collects over time. Less commonly, weak circulation, poor system balance, or a partially restricted valve can produce a similar pattern. It is rarely an air issue when the top is already hot.
If you are unsure whether this is an isolated radiator fault or part of a wider heating imbalance, working through a house cold diagnostic first helps you confirm the pattern before committing to deeper work.
Why the bottom goes cold while the top stays hot
In a standard UK wet central heating system, hot water enters the radiator and spreads across internal channels before returning to the pipework. When the radiator is functioning properly, the whole panel warms gradually from top to bottom. If the radiator is hot at the top and cold at the bottom, heat is entering but not circulating evenly through the full body of the radiator. You often feel this as a cold band along the base, where the bottom of the radiator is not heating even after the system has been running for a while.
Over time, corrosion inside steel radiators produces iron oxide particles. These combine with limescale, installation debris, and small fragments from valves and pipework to form a dark, heavy sludge. Because this material is denser than water, it sinks. The lowest point of the radiator is therefore the first place it settles, which is why “radiator cold at the bottom” is such a common symptom in older UK systems or systems that have gone years without proper inhibitor maintenance.
Water flow is also slowest along the base of the panel. As hot water enters and rises, the upper channels fill first. The lower section relies on continuous movement to displace cooler water. When sludge collects, it reduces the cross-sectional area available for flow. That restriction increases resistance, meaning hot water loses momentum before reaching the bottom fully. The result is a clear temperature divide: hot above, cold below. In real terms, you can end up with a radiator hot at top cold at bottom even though the boiler is running normally, because the restriction sits right where the radiator needs steady circulation to heat the lower panel.
This is mechanically different from a radiator that is cold at the top. In that case, air has risen and displaced water, preventing the upper section from filling. When the top is hot and the bottom is cold, the radiator is already full of water. The problem is not missing water, but obstructed movement within it, which is why the cold band along the base tends to persist rather than “come and go” like an air pocket.
How sludge forms in the first place
Most UK heating systems contain mild steel components. As oxygen enters the system, small amounts of internal corrosion are inevitable. Even sealed systems allow tiny oxygen ingress over years through maintenance work, refilling, or microscopic leaks. Without adequate inhibitor levels, corrosion accelerates. This is the slow background process behind the classic “bottom of radiator not heating” pattern, especially in homes where the system water has never been properly cleaned and protected.
Each time the heating runs, fine particles circulate. In areas of low turbulence, especially the bottom of radiators furthest from the pump, particles settle. Systems that have not been flushed in many years gradually accumulate this sediment until circulation is affected. The symptom often starts subtly as slower warm-up and ends with a radiator cold at the bottom that never catches up to the top.
Homes where radiators take longer than expected to heat up often show early signs of restricted flow before the cold-bottom pattern becomes obvious.
Confirming sludge rather than air
If bleeding the radiator releases air and restores full heat, the issue was trapped air. If bleeding produces only water and the bottom remains cold, the cause is almost certainly internal restriction. This is why “radiator cold at bottom after bleeding” is such a useful clue: bleeding has confirmed water is present inside the radiator, but the movement of that water through the lower channels is still restricted.
Another useful clue is pipe temperature. When sludge is present, the flow pipe entering the radiator will be hot. The return pipe may be noticeably cooler because hot water is not circulating effectively through the full panel. With air-related faults, the temperature difference behaves differently and often improves immediately after bleeding.
If only one radiator shows the symptom, the restriction is often localised. If several radiators are cold at the bottom, the issue may be system-wide sediment buildup or weak pump performance. In practice, multiple radiators showing the same cold band along the base is a stronger sign of broader contamination, while a single radiator hot at top cold at bottom can be a local collection point on that circuit.
Repeated bleeding that temporarily improves performance but does not fix the lower cold band usually signals deeper circulation imbalance rather than a simple air pocket.
Practical confirmation before removing the radiator
Before you go as far as removing a radiator, it helps to confirm the pattern carefully during a normal heating cycle. Give the system around ten to fifteen minutes from cold start, then check whether the radiator is warming progressively or whether it develops a persistent cold band along the base. A radiator that is simply slow can feel cooler at the bottom early on, but it should steadily close the gap as circulation continues. When sludge is the driver, the bottom of the radiator not heating tends to remain obvious even as the top becomes fully hot.
The simplest confirmation is the hand test. With the heating running and the radiator actively warming, place one hand near the top third and the other along the bottom edge. If the top is properly hot while the bottom stays noticeably cooler, you are dealing with a genuine “radiator hot at top cold at bottom” gradient rather than a minor warm-up delay. The stronger the contrast, the more likely internal restriction is present.
If you have access to an IR thermometer, it can make this clearer without guesswork. Measure a point near the top, a point halfway down, and a point close to the base. In a healthy radiator, those readings converge as the system settles. With sludge, the top remains high while the base stays stubbornly lower, creating a stable gradient that does not smooth out. That stable temperature gradient is what people describe as a radiator cold at the bottom even though the radiator is clearly working at the top.
It also helps to understand how the gradient behaves in sludge versus imbalance. When system imbalance is the main issue, a radiator may heat unevenly at first because it is being starved of flow. However, once it receives enough circulation, the radiator usually warms more evenly from top to bottom. With sludge, the radiator can receive flow and still fail to warm the base because the restriction is inside the panel itself. That is why balancing may improve overall warmth yet leave a cold band along the base if the radiator is internally contaminated.
Restricted flow versus system balance
Restricted internal flow from sludge differs from poor system balance, though both affect circulation. With imbalance, stronger radiators closer to the boiler draw more flow, leaving weaker ones under-supplied. The radiator may heat unevenly not because it is blocked, but because it is being starved.
After internal cleaning, proper balancing becomes essential. The method is explained in how to balance radiators. A radiator that is clean but poorly balanced can still feel underperforming, but it is more likely to be uniformly lukewarm rather than consistently hot at the top with a cold band along the base.
Could it be a stuck valve instead of sludge?
Sometimes a valve restriction can imitate parts of the same symptom, so it is worth separating “radiator cold at the bottom” caused by internal sediment from “bottom of radiator not heating” caused by limited flow entering the radiator. A thermostatic radiator valve can stick if it has not moved for a long time, especially at the end of the heating season. When the TRV pin does not lift properly, the radiator receives less flow, which can make it warm weakly and unevenly.
A partially closed lockshield can do something similar. If it has been tightened too far during past balancing, or knocked during decorating, the radiator can be starved enough that the top warms first and the bottom never catches up properly. In valve restriction cases, the radiator often feels generally under-supplied, and the temperature difference across the radiator can change more noticeably when other radiators open and close.
The key difference is that valve restrictions are upstream limits, while sludge is an internal barrier. With a valve restriction, improving the valve opening usually changes the radiator’s behaviour quickly because you have increased the amount of hot water entering and leaving the panel. With sludge, you can have a good hot feed and still keep a cold band along the base because the internal channels at the bottom remain narrowed by settled debris.
Escalation: what level of intervention is justified
If a single radiator is affected and others behave normally, removing and flushing that radiator individually is often sufficient. This involves isolating it, carrying it outside, and forcing clean water through until discharge runs clear. In many homes, this restores full lower heat immediately and resolves the radiator hot at top cold at bottom pattern because you have physically removed the sludge that settled along the base.
If multiple radiators show similar cold-bottom symptoms, or if dark water appears throughout the system, a chemical system flush may be appropriate. Cleaning chemicals circulate through the pipework and loosen sediment before being drained. This can help when more than one radiator is cold at the bottom because the contamination is not isolated to a single panel.
A full powerflush is usually reserved for systems with heavy contamination, repeated blockages, or signs of pump strain. When sludge significantly restricts flow, the circulation pump works harder to maintain movement. Over time this can produce uneven heating, noisy pipework, or weak performance across upper floors.
If the pump struggles or several radiators cool at the bottom and top unpredictably, the system may require professional assessment rather than repeated local fixes.
Can you fix a radiator cold at the bottom without powerflushing?
Yes. In many cases, especially where only one radiator is affected, a targeted flush of that unit resolves the problem. Powerflushing is not automatically required. It becomes relevant when sediment is widespread or repeatedly returns after local cleaning.
Is a radiator cold at the bottom dangerous?
It is not dangerous in itself, but it reduces efficiency. A partially heated radiator delivers less usable warmth, meaning the boiler runs longer to reach the same room temperature. Over time, this increases wear and running costs.
Why is only one radiator cold at the bottom?
Single-radiator cases usually mean localised sludge accumulation. Radiators at the end of a circuit or on upper floors often experience slower flow, making them natural collection points for debris. Once cleaned and correctly balanced, they typically return to normal operation.
Bringing it together
A radiator that is cold at the bottom but hot at the top is almost always signalling internal restriction from sediment buildup. Bleeding will not resolve it because the radiator is already full of water. Confirm the pattern, assess whether the issue is isolated or system-wide, then escalate proportionally from single-radiator flushing to broader cleaning only if justified.
Improving circulation and removing internal restrictions not only restores heat output but reduces unnecessary boiler runtime. If you are working through multiple heating issues at once, understanding how heat retention and system efficiency interact is covered in How to Keep a UK Home Warm for Cheap.
The next logical step is simple: confirm whether the problem is local or widespread, flush proportionally, then balance the system properly so restored flow is shared evenly.
