Central heating systems - often identified problems
The main reason for this problem is that the pilot light has gone out. The reasons why the pilot light isn’t starting are: the timer failed or the system won’t engage because the thermostat registers a lower temperature.
In contact with freezing temperatures the pipes that are not insulated properly may get damaged.
There are various causes for a knocking noise made by the radiator. The pipes may have collected trapped air or the water tank lacks water. To release the trapped air from the pipes you will need to bleed them. Usually you have a valve for this that you only need to open.
If the bottom and top of the radiator are not evenly heated, one cause can be that sludge has gathered at the bottom of the radiator. When you don’t succeed in solving the problem by flushing water inside the radiator then you will need a professional to inspect a blocked pipe.
A problem with boilers used in central heating systems is when pressure drops. This may be caused by a leak of water or by removing air from a bled radiator. You can solve the pressure problem by adding water through a manual “filling loop”