Have an older homart gas furnace
We went through this last year and have forgotten what to do

1st floor rads heat fine

Nothing getting to the 2nd floor

It was a simple diy fix - think husband did it by accident

It is a model 8676010 homart automatic heating by sears

Additional note -

There has been no servicing of the furnace

There has been no use of it before last couple of weeks

1st floor rads are fine