This is perhaps the simplest and most common problem to fix. It just means that there is air trapped in your radiator, which stops the hot water from circulating around the radiator effectively. In order to get that air out you have to bleed the radiator.

Step 1

Turn off the pump, so you don't end up pushing more air into the system.

Step 2

At the top right or top left corner of the radiator you should find a square brass nut. This can only be turned using a radiator key, which you may have in the house, or which can be bought inexpensively from a DIY/hardware shop.

The valve will normally be powered by electricity - effectively, it is an electro-magnet that becomes charged and then pulls the valve one of two ways. Turn off the electrical power to the system, there should be an obvious switch somewhere, and then try turning it on again.

Step 3

Put a bucket on the ground under the end of the radiator you're working on. Take the key and turn it about 45-90 degrees anti-clockwise, or until you hear the air hissing out. Let it hiss away until dirty water begins to squirt into your bucket.

Step 4

Tightly close the valve, and check it later to ensure there is no leakage.

Radiator is cold at the bottom and warm at the top

If the radiator is cold at the bottom and warm at the top and this problem persists, then there may be some sludge at the bottom of the radiator and it may need flushing out. For this, the radiator will need to be taken off the wall, flushed out with water and refitted. This can be done by yourself if you follow the correct instructions and have a basic understanding of how your heating system works but if you have not done it before or are unsure of the procedure, call a plumber or engineer out.

If several of your radiators have this problem and it is not solved by flushing the radiator out then the problem may be more serious. This could be caused by a fault with the circulating pump, blocked pipes or radiators or a badly designed system. You will need to call a professional out to look at this problem further as it may require the whole system to be flushed through or repairs to be carried out.

Finally

If your radiators are working correctly but are not hot enough or are too hot then you can alter the temperature by either adjusting the controls on your boiler or for more modern radiators with their own temperature controls, simply turn up or down accordingly.