Why am I loosing pressure on my boiler:

This can be a number of reasons, which are listed as follows:
1. You may have a leak on any part of the heating circuit (Radiator valves, bleeds, heating piping, or inside the boiler).

2. Check to see if you have water dripping out of your relief pipe, which is what sticks out of your wall on the outside of the property (Unless it terminates at the boiler into a tundish), if so, this might indicate an issue with your relief valve or expansion vessel.

3. An indication that you may have a faulty expansion vessel is to visually look at the pressure gauge when there is a demand for heating. If after a couple of minutes, the dial rises past the number 3, this will then start relieving water through your outside relief pipe, when cooled down, the gauge will indicate low, which means you will fill up and re pressurise on a regular basis, which is not good for your heating system, as the system will start rusting from inside, thus causing damage to components.