When the steam boiler burner is not running and there is no steam being sent up, swap the vents that are on the side of the radiators between the upstairs and downstairs radiators. They screw out and in. Then observe the difference. The vents control the amount of steam let into each radiator by the amount of air let out during each heat cycle. Vents come with different size holes to do this. The amount of heat given to each radiator depends upon adjusting the vents to the amount of heat needed in each room.