When a radiator only heats properly after other radiators are turned off, the issue is almost never the radiator itself. This behaviour usually means hot water is not circulating evenly through the system. In most UK homes, it points to a flow imbalance where stronger radiators are taking priority and leaving weaker ones starved.
In a balanced system, all radiators warm at a similar rate. When one only responds once others are shut down, it is being deprived of circulation rather than lacking heat. If several rooms feel inconsistent, the wider pattern can be explored using the house cold diagnostic.
Why turning other radiators off makes one suddenly heat
Hot water always follows the path of least resistance. Radiators closer to the boiler, or those with fewer restrictions, naturally draw more flow. When these radiators are unrestricted, they absorb most of the circulation and leave little for the rest of the system.
Once other radiators are turned off, circulation is forced through the remaining one. This makes it appear to “come back to life”, even though nothing has changed inside the radiator itself.
Lockshield valves decide how heat is shared
The lockshield valve on each radiator controls how much water passes through it. If one or more radiators are left too open, they dominate circulation and prevent others from heating properly.
This is why balancing is such an effective fix. Balancing does not repair an individual radiator — it redistributes flow so no single radiator monopolises the system. The correct method is explained in this guide to balancing radiators properly.
Partially restricted valves can exaggerate the issue
A thermostatic radiator valve that does not open fully can restrict flow just enough to cause inconsistent heating. Under strong circulation, the radiator may heat. Under normal conditions, the restriction becomes significant.
This creates the pattern where the radiator only works when system demand is reduced elsewhere.
Internal sludge can create selective heating
Sludge does not always block a radiator completely. In some cases, it reduces circulation enough that the radiator only heats when overall flow increases.
This often appears alongside other imbalance symptoms, such as rooms heating at noticeably different speeds. Related behaviour is covered in why radiators heat unevenly across the house.
System pressure affects which radiators lose out
When boiler pressure sits toward the lower end of the normal range, weaker radiators are the first to suffer. The system continues heating, but circulation is prioritised through the easiest routes.
This turns small imbalances into noticeable comfort problems, particularly in radiators further from the boiler.
Why fixing this improves comfort and efficiency
A radiator that only heats in isolation forces the boiler to work harder and wastes energy. Once circulation is shared evenly, rooms warm together and temperatures hold more consistently.
How circulation balance fits into keeping a home warm efficiently is explained in How to Keep a UK Home Warm for Cheap.
When a radiator only works after others are turned off, the system is signalling imbalance — not a faulty radiator.
