How to Bleed a Radiator Properly

Cold spots on radiators mean trapped air. Bleeding releases it, letting hot water fill the whole radiator. It's a simple job that makes a real difference to heating efficiency.

When to Bleed

Signs you need to bleed:

• Radiator cold at the top, warm at the bottom
• Radiator takes longer than others to heat up
• Gurgling noises from radiators
• Whole radiator not reaching temperature

Air enters the system gradually. Bleeding once a year (before winter) is good practice.

What You Need

• Radiator key (or flat screwdriver for modern valves)
• Cloth or small container
• Old towel to protect carpet

Step by Step

1. Turn Off the Heating

Let the system cool slightly. You don't want to burn yourself on hot water, and bleeding works better when the system isn't circulating.

2. Identify the Bleed Valve

Usually at the top corner of the radiator. A small square-drive fitting. May be covered by a cap.

3. Prepare for Water

Put towel below the valve. Hold cloth against valve ready to catch drips.

4. Open the Valve

Insert radiator key and turn anticlockwise - quarter to half turn. You'll hear hissing as air escapes.

5. Wait for Water

Keep the valve open until water starts to dribble out - this means the air is gone. May take 20-30 seconds.

6. Close the Valve

Turn clockwise to close as soon as water appears. Don't overtighten - just snug.

7. Wipe Up

Clean any water. Check valve isn't dripping.

8. Check Boiler Pressure

Important step many skip. Bleeding releases pressure from the system. Check the pressure gauge on your boiler. If it's dropped below 1 bar, you need to repressurise.

Repressurising

If pressure is low after bleeding:

1. Find the filling loop (flexible hose with valves, near boiler)
2. Open the valves to let water in
3. Watch the pressure gauge
4. Stop at about 1.5 bar (or per your boiler manual)
5. Close the valves

If you're unsure, consult your boiler manual or a heating engineer.

Multiple Radiators

If bleeding multiple radiators:

• Start with the radiator furthest from the boiler
• Work back towards the boiler
• Check and top up system pressure after doing all

If Bleeding Doesn't Help

If radiator still has cold spots after bleeding:

• Sludge buildup - may need flushing
• Thermostatic valve stuck - try turning it fully open
• System needs balancing - different issue, adjust lockshield valves
• Radiator blocked internally - may need replacing

Prevention

Inhibitor in the system reduces corrosion that creates sludge and gases. If your system wasn't flushed and inhibited when installed (or hasn't been in years), consider having this done professionally.

Find tools in our plumbing and heating tools collection.