Radiators Cold at the Top But Hot at the Bottom — What's Wrong?

Quick Answer

Why are my radiators cold at the top but hot at the bottom?

Air trapped inside the radiator rises to the top and prevents hot water from filling the whole radiator. The fix is bleeding the radiator — releasing the trapped air through the bleed valve.

This is one of the most common central heating questions we hear from homeowners across Manchester and Salford — and the good news is it's almost always a simple DIY fix. If your radiators are warm at the bottom but cold at the top (or have distinct cold patches at the top), you have trapped air in the system that needs to be released.

Why Air Gets Into Your Radiators

Small amounts of air enter heating systems naturally over time — through fittings, during pressure top-ups, or when the system is run at low pressure. Hot water pushes air upward through the radiator where it accumulates at the top, blocking hot water from filling that section.

In newer or recently serviced systems, it's normal to bleed radiators once or twice a year. If you find yourself needing to bleed them much more frequently — every few weeks — this suggests a more significant problem such as a micro-leak somewhere in the system introducing air continuously.

How to Bleed a Radiator

Bleeding radiators is a straightforward process that takes about 5 minutes per radiator.

  1. Turn the heating system on and let radiators reach full temperature
  2. Identify which radiators have cold spots at the top
  3. Turn the heating off and let it cool for 20–30 minutes (so you don't get scalded)
  4. Hold a cloth or container under the bleed valve (at the top corner of the radiator)
  5. Use a radiator bleed key (available from any hardware shop for £1–£2) to slowly turn the bleed valve anti-clockwise
  6. You'll hear air hissing out — keep the valve open until water starts to trickle out, then close it
  7. Check your boiler pressure gauge — after bleeding, pressure may have dropped. Top up if needed to 1–1.5 bar using the filling loop
  8. Turn the heating back on and check that the bled radiators now heat evenly

When Bleeding Doesn't Work

If bleeding the radiators doesn't resolve the cold top issue — or they quickly develop air again — there may be a more significant problem. Persistent air introduction can be caused by a micro-leak (water escaping and air entering to replace it), a failing pump that's not circulating water properly, or system corrosion generating hydrogen gas.

If you're seeing black water when you bleed radiators, this indicates sludge in the system — a power flush and inhibitor treatment is the recommended fix. Contact Next Gen Boilers for a diagnosis.

Frequently Asked Questions

Do I need a special tool to bleed a radiator?

You need a radiator bleed key, available from any DIY or hardware shop for £1–£2. Some modern radiators have flat-head slots that can be opened with a screwdriver.

How often should I bleed my radiators?

Bleeding once a year at the start of winter is good practice. If you find yourself needing to bleed them multiple times a season, there's likely an underlying problem worth investigating.

Should I bleed all radiators or just the ones with cold tops?

It's good practice to check all radiators. Start with the ground floor radiators and work upward, or start from the radiator furthest from the boiler and work back toward it.

My radiators are cold all over, not just at the top — what does this mean?

Cold all over could indicate a different problem — low boiler pressure, a circulation fault, or zone valve issue. This is different from cold-top only, which points specifically to trapped air.

I bled my radiators and now my boiler pressure is low — what do I do?

Use the filling loop to top up your boiler pressure to 1–1.5 bar. Your boiler manual will explain how, or there are video guides online for your specific model.

Need emergency boiler help in Manchester or Salford? Call: 07305 687 331