It is a pressurised system - I did remember to refill it - When I
measure the pressure at the expansion vessel the pressure is 0.5 bar -
it used to be just over 1 bar.

The pipes are mainly microbor (3/8")

There is definetly no air locks in the system. Since all of the
radiators heat proberly I don't think there is a problem with the
boiler or programmer.

We have a 2 storey house

What is a filling loop??

Thanks for the reply

Rob