Radiators are hot at the bottom but cold at the top

Problem

There is a cold patch at the top of a hot radiator.

Cause

Air in the hot water system has built up and become trapped in the radiator.

What you can do

Releasing air trapped in the radiators (‘bleeding the radiators’) keeps them working efficiently.

  1. This can be done when the radiator is hot or cold. Cold is safer!
  2. Buy a radiator key in a hardware shop. They are not expensive.
  3. To release the air, find the radiator bleed valve. It will be at the top of the radiator, at one of the ends. It has a square end which the key fits over. The ones in Marcia Road also have a slot for a screwdriver. DO NOT use a screwdriver: it can break the valve.
    Radiator air vent and key

    Marcia Road bleed valves also have a plastic part with a hole in the side. You can see it in the photo. (Click photo to enlarge.) The hole is where the air and some water will come out.

  4. Turn the hole to point away from the wall. It is good to have a cloth or paper towel handy for the next part as some water will come out.
  5. Slowly turn the key anticlockwise (like turning on a tap). It may be hard to turn at first. After a few turns you will hear the sound of air hissing out of the valve. STOP TURNING.
  6. After a few seconds, depending on how much air is trapped, the hissing sound will change and water will start to come out. Quickly stop it by screwing the key clockwise (like turning off a tap). Screw the valve in until it stops turning but do not screw it too tight or it may snap off. How did I find that out, I wonder?
  7. Warning: This may sound scary, but it really is not. In fact, when you have done it once, it is so easy that you may become over-confident and turn the valve too many times, to speed it up. If you do, it may fall out completely, and all the heating system water will come squirting out! You have been warned!

The whole radiator will now get warm when the heating is on.

Extra tip

You could get a plumber to bleed the radiator, but you will be paying a lot for a very small job. However, if air gets trapped in the radiator again quickly (i.e. within a few days or a week) do get a plumber to check it. You may have a leak somewhere which is letting air into your heating system. Leaks are always bad news for the floors, walls and ceilings below.