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Heating help

Radiators cold at the bottom, or the top?

Where the cold patch sits tells you what's wrong. Cold at the top is air, and you can fix that yourself in ten minutes with a bleed key. Cold at the bottom or the middle is sludge, and no amount of bleeding will shift it. Here's how to tell them apart and what to do about each.

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Smell gas? Stop here. Open windows and doors, don’t flick switches, and call the National Gas Emergency line free on 0800 111 999 before anything else.

What’s going on

Cold at the top: air in the system

Air rises, collects at the top of the radiator, and stops hot water filling that space. Common after the summer off-season, after topping up pressure, or after any work on the system. Bleeding it out is a normal bit of home maintenance and completely safe.

Cold at the bottom or middle: sludge

Sludge is rust and debris from inside your radiators and pipes. It's heavier than water, settles at the bottom, and blocks the flow. It builds for years, quietly making every room slower to heat and your gas bill bigger. The fix is a chemical or power flush of the system, plus a magnetic filter to stop it coming back. That is a job for an engineer, not a bleed key.

One radiator cold, or the whole house slow

One cold radiator is often a stuck thermostatic valve pin or a balancing issue. Every radiator lukewarm points at circulation: a tired pump, a blocked filter, or sludge through the whole system.

How to bleed a radiator (for cold-at-the-top)

  1. 1Turn the heating off and let the radiators cool. Bleeding with the pump running pulls more air in.
  2. 2Put a cloth and a small bowl under the bleed valve at the top corner of the radiator.
  3. 3With a bleed key, turn the valve slowly anticlockwise about a quarter turn. You'll hear air hissing out.
  4. 4Close it the moment water dribbles out steadily.
  5. 5Check the boiler pressure afterwards and top up to about 1.2 bar if it's dropped.

When to stop and call me

  • The cold patch is at the bottom or middle. That's sludge, and bleeding won't touch it.
  • You're having to bleed the same radiator every few weeks. Air shouldn't keep getting in, and constant topping up feeds corrosion.
  • The water that comes out when bleeding is black or brown. That is sludge.
  • Radiators upstairs are hot but downstairs are cold, or the reverse. Circulation or balancing, an engineer job either way.
  • Every room heats slowly and the boiler seems to work hard. A flush and filter often transforms an older system.

I’m Jamie, a Gas Safe registered engineer (No. 977838) in Burgess Hill. I handle boiler and heating repairs in Burgess Hill and across Mid Sussex, Mon to Fri with some evenings.

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FAQ

Common questions

Depends how bad the sludge is and how the system is put together. A chemical clean sorts lighter cases. Heavier blockages need a power flush. I'll say which yours actually needs after a look, and fit a magnetic filter so you don't pay for this twice.

It varies with system size and how blocked things are, so I don't quote without seeing it. Message me a photo and a rough idea of your setup and I'll give you a straight price before any work.

Damaging. It wears the pump, blocks the boiler's heat exchanger, and shortens the boiler's life. Boiler warranties often expect the system to be kept clean too. Fixing it is cheaper than the parts it ruins.

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Send me a few details, and a photo of the boiler if you can. I'll get back to you quickly with a clear, no-obligation quote and honest advice.

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  • Gas Safe registered · 977838
  • Burgess Hill & Mid Sussex
  • Mon–Fri
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