If your boiler filling loop is passing when closed you will find the pressure constantly rises.

What is the filling loop?

The filling loop is used to fill water into the boiler and radiators of your central heating system. One valve is attached to the mains water supply. The other is attached to your central heating system. The two valves are attached by a pipe. The pipe normally looks like a silver metal braided hose but can be different depending on the type installed. When both valves are opened the pressure on the pressure gauge rises.

Filling loop when not in use

The valves on the filling loop should be closed, the metal braided filling loop removed, and the valves capped when not in use.  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.

Filling loop position.

When the valves are turned off they are across the pipe. When they are on they are in line with the pipe. When you are finished with the filling loop both valves should be turned off and the hose removed.

Why is the pressure always rising?

If the valves are closed and the pressure is still going up it can be the filling loop valves failing to close properly. Listen closely to the valve if you can hear water flowing with the valves turned off the valves have failed and will need to be changed. If you can’t hear water flowing and are confident in your abilities remove the filling loop and see if any water is flowing.  Even a slight drip from the side attached to the mains can be enough to raise the pressure over a long period of time.

If they are passing or even dripping the valve should be changed.  The valve on the central heating side of the filling loop tends to be the most commonly changed. This is probably the difference between clean drinking water and dirty chemical filled central heating system water.

Replacing the filling loop involves turning off your mains water supply and draining some of your central heating system. A replacement filling loop looks like this.  

The filling loop below is open

The filling loop below is closed.