For a heating system to work properly the water pressure should remain constant. Modern combination boilers are pressurised systems and water will enter the system from the mains cold water supply through a manual filling device, known as a ‘filling loop’. Adding water via the filling loop to the system after a drop of pressure will help bring your system to the correct pressure again.
There are several factors which may contribute to a loss of pressure in your heating system. It could be as a result of a water leak or air removed from the system when the radiators have been bled or removed and replaced. You should check the pressure after bleeding or removing the radiators and if you notice a drop of pressure then the system will need to be topped up with water.
You can check the pressure of the system by using the pressure gauge which you should find on your boiler. You should check this gauge regularly to make sure it is at the correct pressure. It should be between 1 and 1.5 bar. It should not drop below 0.5 or rise above 3.0 bar.
If you find your pressure low it is very simple to repressurise the system yourself a quick youtube search of the make of your boiler will show you exactly how to do it in minutes, below is an example!