Whenever new water is added to your central heating system, it's possible for a certain amount of air to enter at the same time. Since air floats up above the waterline, it will start collecting at certain points within your heating system, including the radiators.
If air becomes trapped at the top of your radiators, you need to bleed them or have them bled by someone else. This just means that you 'bleed out' any air that is in the radiator until nothing but water remains, and it's a very easy home maintenance procedure to take care of.
Here are just a few reasons why you need to bleed your radiators.
Reduced Heating Performance
The most obvious reason to bleed your radiators is that any amount of air in the system is going to inhibit their ability to heat your home. Since it is hot water that heats your radiators, any air trapped at the top will reduce the amount of heat each radiator can give off. In fact, the easiest way to tell whether a radiator needs to be bled is by feeling along its surface when the heating system is on. It should feel equally hot from top to bottom; if a radiator requires bleeding, it will be hot at the bottom but cold at the top. Additionally, the presence of air means that water doesn't circulate out from the boiler as effectively due to changes in internal pressure.
Higher Energy Bills
If your radiators aren't able to heat your home as effectively as they used to, your boiler will need to work harder to produce more hot water. After all, the thermostat will still be making it keep your house at the same steady temperature. An inefficient system is going to require more energy, and that means you'll be looking at higher energy bills.
Your central heating system should run without making a sound; if you start to notice sounds from the pipes and around the radiators, it's likely that air has become trapped within. Any empty space within a radiator can create more sound, and you might hear your pipes gurgling because of the excess air within the system.
As you should now understand, bleeding your radiators is pretty important. Luckily enough, you can perform the procedure yourself by turning off the heating, inserting the right key at the radiator's drainage point, then slowly opening the system until water comes out instead of air. If you're unsure about doing this yourself, you can always call in a handyman.