Combi Water pressure

It sounds like you may have a very slow leak somewhere. However at that rate you may not be able to find it (i.e. the water evaporates before you ever get to see it).

See Ed's sealed system FAQ:

The system will usually carry on working up until the point that the boiler detects low pressure and refuses to fire (or goes to lockout). You may find that it is happy to run with the nominal pressure at 1 bar. Doing that may also reduce the rate of loss. Have a look at the owners manual for guidance on the recommended system pressure.

Thanks for your quick reply John.

A classic here is weeping around the spindles of radiator valves. Basically what can happen is that this only occurs when warm and then the heat evaporates the water before it drips.

Try taking off the caps and looking for signs of staining as the tell-tale sign

Also check the discharge valve at the boiler. Sometimes they weep as well from being faulty or not seated properly. A few turns can sometimes re-seat them.

I'll give that a go. Many thanks for your suggestions.

In message , RussG writes

Could be the 3 bar pressure relief valve which discharges to the outside of the house. Sometimes called safety valve.

You could also try a Fernox/Sentinel internal leak sealer. For that level of leakage the result would be dramatic.

I think if that operated the drop in pressure would be after much more dramatic that slowly over a month.