In article , Simon Langford wrote:
[snip]
The problem is that the 2nd floor rads are still not hot (just warm),
and the ground and 1st floor rads now have cool return pipes. If I
open up the LSVs so get hot return pipes on the ground floor, then the
2nd floor rads are even less hot.

Do you think I need a bigger boiler (it's already a pretty big one)?
Or would a faster pump help?

It seems suspicious that it's just the higher radiators that don't
get hot. Have you tried bleeding them to check that they're not
full of gas? Your central heating has presumably been idle all
summer and gas released by corrosion may have collected upstairs.

--
Jan