How to Bleed a Radiator: A Step-by-Step Guide
If your radiators in {location} have cold spots — particularly at the top — they almost certainly need bleeding. Trapped air prevents hot water from filling the radiator fully, reducing its efficiency and making your boiler work harder than it needs to. The good news is that bleeding a radiator is simple, free, and takes about five minutes per radiator.
What You'll Need
- A radiator bleed key (available from any hardware or DIY store for around £1–£2, or often included with your boiler manual)
- A cloth or small towel
- A small container to catch any water
Step-by-Step Instructions
Step 1: Turn On Your Heating
Switch your central heating on and let all the radiators heat up fully. This builds pressure in the system and makes it easier to identify which radiators have trapped air.
Step 2: Identify Problem Radiators
Carefully feel each radiator (they'll be hot, so use the back of your hand at a safe distance). A radiator that needs bleeding will typically be hot at the bottom but noticeably cooler at the top. Some may not heat up at all.
Step 3: Turn Off the Heating
Before bleeding, switch the heating off and wait 10–15 minutes for the radiators to cool enough to handle safely. The system should still be pressurised but not dangerously hot.
Step 4: Bleed the Radiator
- Locate the bleed valve — a small square fitting at the top corner of the radiator
- Place your cloth and container beneath the valve
- Insert the bleed key and turn it anticlockwise — just a quarter to half a turn
- You'll hear a hissing sound as trapped air escapes
- Once the hissing stops and water begins to dribble out steadily, close the valve by turning the key clockwise
- Don't open it too far — the valve can come out completely, and hot water will follow
Step 5: Check Your Boiler Pressure
After bleeding radiators, your boiler pressure may have dropped. Check the pressure gauge on the front of your boiler — it should read between 1.0 and 1.5 bar when the system is cold. If it's dropped below 1.0, you'll need to repressurise the system using the filling loop (a small braided hose or valve underneath the boiler).
Step 6: Turn the Heating Back On
Switch the heating on again and check that the previously cold radiators are now heating evenly from top to bottom.
How Often Should You Bleed Radiators?
Most plumbers recommend bleeding your radiators at least once a year, ideally in autumn before you start using the heating regularly. If you notice cold spots developing more frequently, it could indicate a more significant issue such as a faulty pump, sludge build-up, or a leak drawing air into the system.
When to Call a Professional
If bleeding doesn't solve the problem, or if your radiators need bleeding very frequently, it's worth getting a qualified plumber in {location} to inspect your system. You may need a system flush (also called a powerflush) to remove sludge and debris, which typically costs £300–£500 depending on the number of radiators.
Bleeding your radiators regularly keeps your heating running efficiently, reduces your energy bills, and helps your boiler last longer. It's one of the easiest maintenance tasks you can do yourself.