Originally posted by RookRAKCould be....
What's puzzling is that generally, everything is "working", but the house is slowly losing the battle against temperature. The furnace runs, I can feel the pipes in the baseboard heating, they are warm (though not as warm as I think they should be). My primary guess is that the water is not circulating fast enough (maybe that little control valve is stuck).
If it's a circulation issue, the pipes near the furnace will be a lot hotter than the ones further downstream. You can probably do a touch test to figure that out, just be careful....
How old is the furnace? How old is the water circulating pump?