This issue usually points to an imbalance in the heating system rather than anything mechanical being broken. Once you understand why it happens, the fix becomes much simpler.
1. The Moment You Realise It’s a Flow Competition
The radiator that only heats when others are off is basically losing the fight for hot water. Stronger or closer radiators take the majority of the circulation, and the weaker one doesn’t get enough flow until the others stop using it.
In my house, the radiator closest to the boiler was wide open on its lockshield. It was getting more heat than it needed, leaving the problem radiator with almost nothing.
2. Lockshields Decide Which Radiators Win
Once I adjusted the lockshield on the greedy radiator, everything changed. The problem radiator started heating even with all the others on. The difference wasn’t subtle — it was instant.
This is why balancing makes such a huge difference. You’re not fixing the radiator — you’re fixing the system around it.
If you want the step-by-step balancing method, here’s the guide:
How to Balance Radiators Properly
3. TRVs Can Restrict One Radiator Without You Noticing
Another time I had this issue, the TRV pin on the problem radiator wasn’t opening fully. The radiator heated fine when everything else was off because that was the only time it got any real pressure. When the system was running normally, the partial restriction meant it barely heated.
Removing the TRV head and checking that the pin moved freely sorted it out.
4. Sludge Can Cause “Selective Heating”
If a radiator has partial sludge inside, it might heat weakly under normal conditions but heat properly when the circulation is stronger — which happens when other radiators are turned off.
It’s a subtle sign but a surprisingly common one.
If your radiator also heats unevenly, this might help:
Radiators Heat Unevenly Across the House
5. The Fix That Worked for Me
Balancing the radiators (especially turning down the strong ones) completely solved the problem. After that, all radiators heated consistently together — no more turning off half the house just to get one room warm.
For more home-warming guidance, here’s the main guide I always refer back to:
How to Keep a UK Home Warm for Cheap (Complete Guide)
