How to Bleed Radiators: Step-by-Step Guide

Is Your Radiator Cold at the Top?

When a radiator heats at the bottom but stays cold at the top, there's air trapped inside. Bleeding releases this air and restores proper heating. It's a five-minute job that anyone can do.

Signs You Need to Bleed

• Radiator warm at bottom, cold at top
• Takes longer to heat up than it used to
• Gurgling or trickling sounds from radiator
• Some radiators hot while others are cold

What You'll Need

• Radiator key (or flat screwdriver for newer valves)
• Cloth or small container
• That's it!

Radiator keys are available in our hand tools section and at any DIY store.

Step-by-Step Bleeding Process

Step 1: Turn on the heating

• Run the heating until all radiators are warm
• This helps identify which need bleeding (cold at top)
• Turn the heating off before bleeding

Step 2: Start with radiators furthest from boiler

• If you're doing multiple radiators, start with those furthest away
• These typically have more air
• Work back towards the boiler

Step 3: Locate the bleed valve

• Usually at the top corner of the radiator
• Small square nut or slotted screw
• Opposite end from the TRV usually

Step 4: Protect surroundings

• Hold cloth under the valve
• Water may spray initially
• Protect carpet and walls

Step 5: Open the bleed valve

• Insert radiator key into valve
• Turn anticlockwise about half a turn
• You'll hear hissing as air escapes
• Don't open too far - just enough for air to escape

Step 6: Wait for water

• Keep valve open until hissing stops
• Water will start to dribble out
• As soon as you get steady water (no spitting), close the valve

Step 7: Close the valve

• Turn clockwise until snug
• Don't overtighten - can damage the valve
• Wipe up any spilled water

Step 8: Check boiler pressure

• Bleeding releases water as well as air
• Check the pressure gauge on your boiler
• Should typically be 1-1.5 bar
• If low, top up via filling loop

Topping Up Boiler Pressure

If pressure drops after bleeding:

1. Locate filling loop (usually under boiler)
2. Open valves on filling loop
3. Watch pressure gauge
4. Close valves when pressure reaches 1-1.5 bar
5. Some systems have keyed filling links

Troubleshooting

No air or water comes out:

• Valve may be blocked - try loosening more
• May need to tap valve gently
• Valve may need replacing

Radiator still cold after bleeding:

• May be sludge not air - needs flushing
• Check TRV is open and working
• Check lockshield valve is open
• System may need powerflush

Keep having to bleed same radiator:

• Air is getting in somewhere
• May indicate system problem
• Could be faulty expansion vessel
• Worth getting heating engineer to check

Black water comes out:

• Sign of corrosion/sludge in system
• System would benefit from flush
• Add inhibitor after flushing

How Often to Bleed

There's no set schedule, but:

• At the start of heating season is a good idea
• After any work on the heating system
• Whenever radiators feel cold at the top
• Some systems need it monthly, others never

When to Call a Professional

• Pressure keeps dropping without explanation
• Multiple radiators failing despite bleeding
• Black sludge indicating system contamination
• Leaks you can't stop
• Boiler problems after bleeding

The Bottom Line

Bleeding radiators is basic maintenance anyone can do. It takes minutes, requires minimal tools, and can significantly improve your heating efficiency. Make it a habit at the start of winter, and your radiators will thank you with even heat all season.