And once you know the flow path and what can interrupt it, diagnosing the issue becomes ridiculously straightforward.
1. First Check: Are the Pipes Going to That Radiator Hot or Cold?
This is the easiest way to narrow the issue down in under 10 seconds.
- If both pipes are cold: no hot water is entering → valve / pressure / flow issue.
- If one pipe is hot and the other cold: hot water is entering but not circulating → blockage / TRV / lockshield.
- If both pipes are warm but radiator is cold: radiator likely full of air.
This single check tells you where to look next.
2. TRV Pin Stuck — The Most Common Reason One Radiator Won’t Work
If the radiator has a thermostatic radiator valve (TRV), 70% of the time this is the cause. The head turns normally, but the actual pin underneath is jammed shut. That pin controls whether water enters the radiator. If it’s stuck down, the radiator is closed off completely — even if the rest of the system is perfect.
Remove the TRV head, gently tap the pin, and make sure it pushes in and springs back up. If it suddenly pops free, you’ve just solved the entire problem.
Radiators with stuck valves behave perfectly normal on the outside — until you take the head off and see what’s going on underneath.
3. Lockshield Valve Closed Too Much
The lockshield controls how much water flows THROUGH the radiator. If the last person who bled or balanced the system tightened it too much, or if it got knocked during cleaning or decorating, that radiator gets almost no flow.
Remove the plastic cap and turn the lockshield about a quarter turn open. Don’t overdo it — small changes make a huge difference.
If the radiator starts warming within 5–10 minutes after opening it slightly, you’ve found your issue.
4. Air Trapped in the Radiator (Even If You Already Bled It)
A radiator can hold air in places bleeding won’t immediately catch. The classic sign? The radiator is cold, but both pipes are at least warm — meaning hot water is trying to move through but can’t fill the radiator properly.
Bleed it again, but this time:
- Bleed with the heating ON
- Close nearby radiators to force more flow into this one
Both of these tricks push stubborn air pockets out much more effectively.
5. Boiler Pressure Too Low to Push Water to That Radiator
This is especially common for radiators on higher floors or at the end of long pipe runs. If system pressure is too low, the boiler will still heat the house — but certain radiators won’t get enough flow to heat at all.
Check the boiler gauge. If it’s sitting below 1 bar, top it up to around 1.2–1.5. That alone often wakes up the “dead” radiator.
6. Partial Sludge Blockage in the Radiator
Sludge doesn’t only cause “cold bottom” problems — it can also block one radiator completely while others are fine. Sludge builds up in radiators that haven’t had inhibitor added in years, or in systems with old pipework.
Signs your radiator is sludge-blocked:
- inlet pipe gets hot but radiator stays cold
- radiator feels noticeably heavier than others
- bleeding gives air, then nothing, then air again
If you suspect sludge, balancing or forcing flow through the radiator can help temporarily, but a flush might be needed later.
If your issue is one side hot and one side cold, that article is here:
Radiator Cold on One Side
7. Pump Weakness or Incorrect Flow Direction
A weak circulation pump won’t usually stop every radiator — just the ones furthest from the boiler or in awkward pipe runs. If you’ve noticed slow heating across the house lately, or strange gurgling noises, the pump might be losing efficiency.
But before assuming that, always rule out valves and air first — they’re far more common.
8. What Usually Fixes It (Real-World Order)
If one radiator isn’t working while others are fine, do things in this order:
- Check TRV pin is moving freely
- Open the lockshield a little
- Barely open the TRV (just to check flow)
- Bleed the radiator with heating on
- Check boiler pressure
- Force-flow by turning off other radiators briefly
In most homes, it’s fixed within 10–20 minutes once you hit the right step.
9. Want to Warm the House More Evenly?
One dead radiator can make the rest of the house feel colder and forces the boiler to work harder. Fixing it isn’t just about comfort — it’s about efficiency too.
If you want the bigger picture on heating your home properly without wasting money, here’s the full WarmGuide breakdown:
