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How to Bleed a Radiator

🔧 How to Bleed a Radiator

3 min read • Published April 2026

Air trapped in radiators blocks water flow and stops them from heating properly. Bleeding them (releasing the air) takes minutes and costs nothing. This is one of the quickest DIY fixes you can do.

Do Your Radiators Need Bleeding?

Check for these signs:

  • Radiator is cold at the top, hot at the bottom - classic sign of air inside
  • Gurgling or hissing sounds coming from the radiator
  • Uneven heating across the radiator surface
  • Radiator not heating at all despite boiler working

If your radiator shows any of these signs, bleeding will likely fix it.

💡 Tip: If multiple radiators need bleeding, start with the ones furthest from the boiler and work your way back towards it. Air naturally rises to the highest radiators.

What You'll Need

  • Radiator bleed key (£2–5 from any DIY store - it's a small brass or plastic hexagonal tool)
  • Bowl or cloth (to catch water)
  • Newspapers or towels (to protect the floor)

That's it. This is genuinely a 5-minute job.

Step-by-Step: How to Bleed a Radiator

Step 1: Locate the bleed valve

On one of the top corners of the radiator (usually the opposite corner from the supply valve), you'll see a small square head or hexagonal socket. This is the air release valve.

Step 2: Prepare for spillage

  1. Place a cloth or newspaper under the valve
  2. Have your bowl or cloth ready to catch water

Step 3: Turn the valve

  1. Insert the bleed key into the valve
  2. Turn anti-clockwise (counter-clockwise) slowly - just a quarter turn or less
  3. Do NOT force it - valves can break if over-tightened

Step 4: Listen and wait

  1. You'll hear a hissing sound - that's the air escaping
  2. After a few seconds, water will start dripping out
  3. Once the hissing stops and steady water flows, air is released

Step 5: Close the valve

  1. Turn the valve clockwise to close it
  2. Tighten gently - you want it snug, not over-tight

Step 6: Check the result

  1. Feel the radiator - it should now be warm across the whole surface
  2. If still cold at the top, air remains (unlikely, but repeat if needed)

Total time: 2–5 minutes per radiator

⚠️ Warning: If water squirts out forcefully, there's pressure in the system. Close the valve immediately and wait for the system to cool and depressurize.

Which Radiators to Bleed

You don't necessarily need to bleed all of them:

  • Only bleed radiators that are cold at the top
  • Start with upstairs radiators (air rises)
  • Work downstairs if needed

Often, bleeding 2–3 key radiators solves the whole system.

How Often Should You Bleed Radiators?

  • Healthy systems: Once per year (usually at the start of heating season)
  • Older systems: 1–2 times per year
  • After boiler work: May need bleeding if pipes were opened

If Air Keeps Coming Back

If radiators need bleeding repeatedly (more than once per season), there's likely an underlying issue:

  • Leak in the system - water is escaping, allowing air to enter
  • Faulty auto-bleed valve - these are designed to release air automatically but can fail
  • System sludge - buildup can cause poor circulation and air pockets

Solution: Call a plumber for a system check and possibly a power flush.

Boiler Pressure & Bleeding

When you bleed radiators, a small amount of water escapes. This can drop your boiler pressure:

  • Check the boiler pressure gauge after bleeding
  • It should read 1–2 bar
  • If it drops below 1: Use the filling loop to restore pressure (turn the small lever under the boiler)

This is normal and nothing to worry about.

Can't Get the Valve Open?

  • It's stuck: Try a little WD-40 or penetrating oil, wait 15 minutes, then try again
  • It's corroded: Tapping gently with a hammer may help
  • Still stuck? Call a plumber - forcing it risks damaging the valve

Safety Tips

  • Water will be hot - be careful not to splash your hands
  • Don't bleed while the system is running at full heat - let it cool slightly first
  • Don't open the valve more than a quarter turn - you only need to release air, not drain the radiator
  • Wipe up any water immediately to avoid stains on walls or flooring

When to Call a Professional

  • Radiator valve is stuck or broken
  • Air keeps coming back repeatedly
  • Multiple radiators consistently stay cold
  • You're uncomfortable doing it yourself

Radiators Not Heating?

If bleeding doesn't work, there may be a deeper issue. We can diagnose and fix it fast.

Call Us Now: 02476 950 595

Key Takeaways

  • Bleeding removes air from radiators and restores heat
  • Takes 2–5 minutes per radiator with a simple bleed key
  • Do it when radiators are cold at the top but hot at the bottom
  • Start with radiators furthest from the boiler
  • If air keeps returning, there's a leak or system issue - call a plumber