Boiler losing pressure. Leaks?

A failed expansion vessel is another possibility and would be worth checking in the first instance.

If there is a genuine leak, Tec7 Plumb is an effective leak sealer which is worth a try. You just add it to the central heating system and it reacts with the atmosphere at the point of the leak in an attempt to seal it. There are no guarantees, but it’s non invasive and worth a shot.

leak in coil of cylinder another possibility

If there is no evidence of a leak from rads or pipes look at the boiler.

A cup of water lost over a few days will bring you below operating pressure

In order of least expensive

• Faulty safety valve
• Faulty expansion vessel

Both are possibilities and the former is a common cause of boilwr pressure loss. Are you able to see or can you check if there is water dripping from the discharge pipe?

I am not a plumber but in my home the cause of pressure loss of about .5bar over 10 day was the safety valve.

thank you sir/madame- that’s the one

you can buy a leak sealer…….

Would assume so

I didnI didn't read all the comments... sounds like a leek... And it's probably under the Concrete floor downstairs because it's not showing up on the ceilings above… A