After customer told me about how the system was running, I turned on the system and when we went downstairs, I found multiple vents closed. The dampers that are in the vents are original to the house so they are not marked nor do they have a handle to indicate which direction the damper is facing. I showed customer how the Line in the screw where a flathead screwdriver would go coordinates with the direction of the damper. We opened all the vents and I let system run for over 20 minutes. Furnace is running properly at departure.