The key thing to remember is this: a radiator should heat evenly from the inlet side across to the outlet side. If one side is stone cold, something is stopping hot water from making its way across. It’s almost always one of three things — sludge, restricted flow, or a valve issue.
1. If the Inlet Side Is Hot but the Other Side Is Cold
This is the most common version. The pipe where hot water enters gets properly warm, the first section of the radiator heats up, and then the heat just… dies halfway. That’s a classic sign that flow is being restricted as it tries to move across to the other side.
The two biggest culprits are:
- a partially closed lockshield valve
- a build-up of sludge inside the radiator
Start with the simplest fix — check the lockshield. If it’s barely open, water enters the radiator but doesn’t travel across fully. Opening it slightly often wakes the whole radiator up.
2. Sludge Build-Up Can Block Half the Radiator
Sludge doesn’t always settle evenly. Sometimes it gathers on the outlet side and slows the flow so much that hot water can’t reach that part. The radiator then behaves like two separate pieces — warm on the inlet half, cold on the other side.
The giveaway signs:
- the cold side feels heavier or damp underneath
- the radiator takes longer to heat than others
- the radiator cools down quicker after the heating turns off
If this is the case, you might need a flush — but before going that far, balancing the system properly can sometimes push enough flow through to get the radiator fully working again.
If your issue is cold at the top instead of one side, the article you need is here:
Radiator Warm at Bottom but Cold at Top
3. A Stuck TRV Can Restrict Flow Across the Radiator
If you have a thermostatic radiator valve (TRV), the pin underneath can get stuck in a half-closed position. That means water enters the radiator but doesn’t flow strongly enough to travel across to the far side.
Take the TRV head off and gently press the pin. It should spring up and down freely. If it barely moves, it’s restricting flow. Freeing that pin can bring the whole radiator back to life within minutes.
4. Low Boiler Pressure Makes Flow Weak — First Half Heats, Second Half Doesn’t
Low system pressure affects flow everywhere, but the effect is most obvious in radiators furthest away from the boiler or in tight pipe runs. The radiator may heat slightly on the inlet side but never fill with enough force to warm the rest.
Check your boiler gauge. If it’s under 1 bar, top it up to around 1.2–1.5. Once pressure is restored, the radiator will usually heat more evenly.
5. Air Pockets Can Sit in One Side of a Radiator
This is less common but it does happen. Air sometimes collects not at the top, but along one internal channel. The radiator will feel hot on one side and cool on the other even though the top seems normal.
A full bleed might not fix this. What usually works better is turning off all other radiators and forcing flow directly through the problem radiator. The increased pressure often pushes the trapped air out.
6. Check the Pipes — They Tell You Where the Issue Is
Feel both the inlet and outlet pipes. This is one of the simplest ways to diagnose the problem:
- Inlet pipe hot, outlet pipe cold: flow is blocked or restricted
- Both pipes warm but radiator still uneven: sludge or internal air pocket
- Both pipes cold: no flow at all — different issue entirely
When you understand this, you can solve one-sided heating way faster.
7. The Quick Fix Order
Here’s the order I usually follow to fix a radiator that’s hot on one side and cold on the other:
- Open the lockshield slightly
- Check and free the TRV pin
- Bleed the radiator properly
- Force flow by turning off other radiators temporarily
- Check boiler pressure
Most of the time, one of these gets the radiator heating evenly again.
8. Want Your Whole Home Heating System Running Properly?
A single radiator not heating properly puts extra load on your boiler and makes the home heat unevenly. Once you fix these smaller issues, the house warms much quicker and stays warm longer.
If you want the full guide on heating your home efficiently without wasting money, here’s the one to read:
