Our combi boiler in the loft (less than 3 years old Vaillant EcoTec) is heating the upstairs radiators nicely, but the downstairs ones are lukewarm to cold.
I have opened all the radiator valves fully downstairs, and reduced the upstairs ones to try to increase pressure to downstairs. This worked for a few weeks, but again downstairs have become cool. Opening the upstairs ones fully makes the downstairs ones dead cold.
The pressure on the boiler reads 1.7 bar. Increasing it to 1.9 doesn't help and I don't want to go any higher. The thermostat is now set to 23°C and downstairs radiators still don't get beyond lukewarm. All the radiators have been bled, there is no air trapped.
One thing to note, and I don't know how related it is, but when we turn on any hot water tap, all the radiators get hot, even if the heating is off, even the downstairs ones!