This usually means one thing: the radiator is only responding when the system is under maximum strain. And when that happens, it tells you a lot about how the water is moving through your pipes.
1. When a Radiator Depends on “Full Power” to Heat
When the boiler is on high, the pump works harder, the water moves faster, and the system pressure sits slightly higher. If a radiator only heats during that moment, it means it’s not getting enough flow during normal operation. It’s not about heat — it’s about circulation.
In my case, the first sign was that the radiator took much longer to heat up than the others. It would eventually get there, but only when the boiler was practically roaring. That immediately tells you the radiator is on the weak side of the system.
2. The Lockshield Being Too Tight Can Cause This Exactly
This is one of the most common reasons. If the lockshield valve is barely open, the radiator gets restricted flow. Under normal boiler power that isn’t enough to heat it, but when you turn everything up, the extra push forces more water through.
When I checked mine, the lockshield only needed a tiny adjustment. I opened it just a quarter turn and the difference was instant. The radiator started warming even on low boiler settings. It’s surprising how sensitive radiators are to that small tweak.
3. TRV Pins That Don’t Fully Lift
Sometimes the TRV (the numbered dial valve) isn’t actually opening properly, even though it feels like it is. The head turns, but the pin underneath may only lift partway. That means the radiator isn’t closed, but it also isn’t open enough to get proper flow unless the boiler is pushing hard.
I’ve had this happen before — the pin wasn’t stuck, but it was stiff enough that the radiator behaved like it was half-shut. After freeing it properly, the radiator heated normally again without needing the boiler on full blast.
4. Low System Pressure Can Cause Weak Flow to the Radiator
If the boiler pressure is low, water doesn’t circulate properly through the entire house. The radiators closest to the boiler usually cope fine, but ones further away or on higher floors start to struggle. They’ll only heat when the boiler is working at full output.
When the pressure drops under 1 bar, this becomes much more noticeable. Topping it up to around 1.2–1.5 makes a big difference, especially if this radiator is one of the last in the system.
5. Sludge Can Slow the Flow Just Enough to Cause This
Sludge doesn’t only cause cold spots — it can also restrict flow just enough that the radiator feels “weak.” When you turn the boiler up, the stronger circulation forces more water through, so the radiator heats. But under normal conditions, it barely warms at all.
This was exactly what happened with a radiator in one of my older places. It wasn’t fully blocked, but the narrow channels inside the radiator were partially clogged. The moment the system slowed down, that radiator cooled first. A proper balance helped for a while, but a flush eventually solved it completely.
6. The Quick Way to Confirm It’s a Flow Issue
If you want to know for sure whether the radiator isn’t getting enough circulation, here’s the simple test I use: turn off every radiator in the house except the cold one, then run the heating. If that radiator suddenly behaves normally, it’s a flow problem — not a boiler problem.
Every time I’ve done this, the result has been obvious. The radiator instantly warms because it’s finally getting the full push of water. That tells you exactly where to focus your attention.
7. The Fix That Works Most Often
In most homes, this problem comes down to one of the following:
- a slightly closed lockshield
- a stiff TRV pin
- low boiler pressure
- a partially restricted radiator
Sort those, and the radiator will start heating normally again without needing to blast the boiler. Once mine was fixed, I didn’t have to push the boiler to the max anymore — it warmed the room properly even on lower settings.
8. If You Want the Whole House Heating More Evenly
Uneven heating is usually a sign your system needs a bit of balancing, tweaking, and maintenance — not a full overhaul. The good news is most of it can be done without spending anything.
If you want the full breakdown on keeping your home warm efficiently without wasting energy, here’s the main guide I recommend starting with:
