Fix a cold radiator
Save £50–80 today
20 mins · Beginner · Saves £50–80 vs a heating engineer
Last updated: March 2025
Before you start
First work out which part of the radiator is cold — hot at the top and cold at the bottom means sludge; cold at the top with hot at the bottom means trapped air (bleeding needed).
This guide covers the two most common causes: trapped air (bleed it) and a stuck valve (open it). Both are simple fixes.
Tools needed
- ✓Radiator bleed key — most homes have one — buy for £1–3 if not
- ✓Small cloth or old towel — to catch the drip when bleeding
- !Flat-head screwdriver — buy: to adjust the lockshield valve cap if needed
Identify the problem
With the heating on, feel across the radiator with your hand. Cold at the top = trapped air — go to step 2. Cold at the bottom only = sludge (needs a power flush — call a heating engineer). Completely cold = closed valve — go to step 4.
Where beginners go wrong
Not having a cloth ready when bleeding — water squirts out without warning once the air escapes.
Leaving the bleed valve open — it only needs a quarter-turn and should be closed as soon as water appears.
Forgetting to check boiler pressure after bleeding — bleeding releases water from the system and drops the pressure.
Stop and call a heating engineer if...
The radiator is cold at the bottom — this is sludge and requires a power flush
Multiple radiators throughout the house are cold — indicates a circulation problem
The boiler pressure drops to zero within 24 hours of repressurising
Cost breakdown
What you just learned
You now understand how air and circulation affect radiator performance. You know how to bleed radiators, read TRV settings, and identify when a problem is beyond a simple fix.
What this unlocks
You can now balance all the radiators in your home, repressurise the boiler, and understand how your whole central heating system works.
✅ Completed by 5,234 people
⚠️ Watch out if you rent
Landlords are responsible for a functional heating system. Bleeding a radiator is reasonable self-maintenance. If a radiator is cold due to a faulty valve or sludge, report it in writing.