How to Bleed a Radiator: A Step-by-Step Guide
If your radiators are hot at the bottom but cold at the top, the most likely cause is trapped air. Air gets into your central heating system over time and rises to the top of your radiators, preventing hot water from filling them completely. The fix is simple: you need to bleed them.
This is one of the easiest home maintenance jobs you can do yourself. It takes about five minutes per radiator and requires no specialist tools.
What You Will Need
- **A radiator bleed key** — available from any DIY store for about £1, or often included with a new boiler
- **A cloth or small towel** — to catch any drips
- **A small container** — a cup or bowl to catch water if needed
Step-by-Step Instructions
Step 1: Turn On Your Heating
Turn your heating on and let the radiators warm up fully. This makes it easier to identify which radiators have trapped air — they will have a noticeable cold patch across the top while the bottom is hot.
Go around your home and check each radiator by running your hand along the top. Make a note of which ones need bleeding.
Step 2: Turn Off Your Heating
Before you start bleeding, turn the heating off and wait 10 to 15 minutes for the radiators to cool slightly. You do not want to be working with scalding hot water. The system also needs to stop circulating so that the air stays in place.
Step 3: Locate the Bleed Valve
The bleed valve is a small square fitting, usually found at the top corner of the radiator. It is typically on the opposite side to the thermostatic valve. You will see a small square peg inside a round surround — this is where the bleed key fits.
Step 4: Bleed the Radiator
Hold your cloth under the bleed valve to catch any drips. Insert the bleed key into the valve and turn it slowly **anticlockwise** — about a quarter to half a turn. You should hear a hissing sound as the trapped air escapes.
Keep the key in position and wait. The hissing will continue for a few seconds. As soon as water starts to dribble out steadily with no more air, turn the key **clockwise** to close the valve. Do not over-tighten it — just enough to stop the water.
Step 5: Check Your Boiler Pressure
After bleeding one or more radiators, check the pressure gauge on your boiler. Releasing air from the system reduces the pressure, and if it drops too low your boiler may not fire. The gauge should read between **1.0 and 1.5 bar** when the system is cold.
If the pressure is below 1.0 bar, you will need to top it up using the filling loop — a small braided hose or valve usually found underneath the boiler. Open the valve slowly until the gauge reaches about 1.2 bar, then close it.
When to Call a Professional
Bleeding your radiators is a normal part of home maintenance and perfectly safe to do yourself. However, if you find that:
- You need to bleed the same radiators every few weeks
- The problem returns within days of bleeding
- Your boiler pressure keeps dropping even after topping up
- Radiators are cold at the bottom but hot at the top (this indicates sludge, not air)
Then there may be an underlying issue with your heating system that needs professional attention. A system flush or a check of the expansion vessel may be needed.
If you are in the Wirral area and your radiators are not performing as they should, JF Plumbing & Heating can diagnose and fix the issue. Get in touch and we will have your home warm again in no time.