This type of behaviour often repeats in a clear pattern, which helps distinguish it from a faulty radiator or a complete blockage.
Radiator gets hot then goes cold
When a radiator gets hot and then goes cold again, it’s almost always related to uneven flow rather than trapped air or a failed component. At the start of a heating cycle, the boiler pushes hot water through the system at full force. Radiators with weaker circulation heat initially, but once the system stabilises, they lose out to radiators with stronger flow and begin to cool.
This is closely related to situations where radiators stop receiving heat partway through a cycle, which is explained in more detail in this guide on radiators stopping mid-cycle.
Lockshield valves restricting flow
A lockshield valve that is too tightly closed can create exactly this pattern. The radiator heats during the initial surge of hot water, but once flow evens out across the system, the restricted radiator can no longer stay warm.
Slightly opening the lockshield often allows enough continuous flow for the radiator to remain hot throughout the heating cycle.
TRVs closing too early
Thermostatic radiator valves respond to room temperature, not radiator temperature. If a TRV senses the room warming quickly, it may close the radiator even though the rest of the system is still calling for heat.
In some cases, the TRV pin underneath the head doesn’t move freely, limiting how much hot water can pass through. This allows an initial burst of heat but prevents sustained circulation.
Air pockets affecting circulation
Air doesn’t always sit neatly at the top of a radiator. It can collect in ways that allow hot water to enter at first, then shift and restrict flow as the system runs. This can cause the radiator to warm briefly before fading.
Bleeding the radiator properly can resolve this if air is contributing to the issue.
Low boiler pressure causing competition between radiators
When boiler pressure is slightly low, radiators closer to the boiler tend to receive most of the flow. Others heat initially while pressure is highest, then cool as circulation evens out.
Restoring pressure to the correct range can improve flow enough for affected radiators to stay warm.
Sludge disrupting sustained heat
Sludge doesn’t always block a radiator completely. In some cases, it restricts certain internal channels so that hot water reaches the radiator at first but can’t continue circulating properly.
This creates a “heat then cool” pattern that repeats each heating cycle. If the radiator also feels uneven in temperature, sludge is more likely to be involved.
If a radiator only heats when the boiler is running at full output, that’s a related issue covered here:
Radiator Only Heats When the Boiler Is On Full
Understanding the pattern
A radiator that heats and then goes cold is usually signalling inconsistent flow rather than a complete failure. Common fixes include adjusting the lockshield valve, ensuring the TRV pin moves freely, bleeding the radiator, and checking boiler pressure.
Once circulation is steady, the radiator remains warm throughout the heating cycle instead of fading partway through.
Improving overall heating performance
A single radiator behaving like this affects more than one room. It forces the boiler to run longer and reduces overall heating efficiency. Resolving flow issues helps heat distribute more evenly across the home.
For a broader explanation of how circulation, balance, and heat loss work together in UK homes, this is covered in:
How to Keep a UK Home Warm for Cheap (Complete Guide)
