The filling loop on a boiler is used to fill the boiler, radiators and all the heating pipework (known as “the heating system”) with water.
If the water gets out of the heating system the pressure will go down. For some boilers this will cause a fault, others will keep going but will not work properly or be damaged by it.
The pressure in the heating system is topped up using the valves in the filling loop.
After use the valves on the filling loop should be closed, the metal braided filling loop removed and the valves capped. The majority of people don’t do this so there is always a chance that the valves can be opened accidentally or be faulty. This can allow too much pressure to develop in the system which may cause the metal pipe near the boiler to overflow.
The first step is to turn the taps so they are in the off position. If its a round tap turn it clockwise. If it has a lever turn it so that is not in line and at 90 degrees with the body of the valve. If it has been accidentally turned on you will hear the water flowing and hear the flow stop when you turn the valve to the correct position.
The filling loop below is open
The filling loop below is closed
It doesn’t need to be as pronounced as this to cause the pressure to go up, even slightly open is enough.
To fix this you need to turn both valves (or one if you only have one) off. This should make the metal pipe outside stop overflowing.
The metal overflow pipe should only overflow if the pressure is above 3. So when the pressure goes back down below 3 the water should stop.
If it doesn’t then you might need a new safety valve as they are notorious for not resealing after they have opened. This is a job for a gas safe engineer not DIY.