Why does the air flow heavily from my downstairs vents, but very weak from my upstairs vents?

Question

I live in a two-story townhouse where the first floor is ground-level on a slab. On the first floor, all air vents are either on the ceiling or at the very tops of the walls. Upstairs, the vents are on the floor.

When I have the heat or air conditioning on, the air flows heavily and healthily from the downstairs vents, so it's always warm in the winter and cool in the summer. Upstairs, though, the airflow is very weak, so it's cold in the winter and hot in the summer.

What could be causing this disparity of airflow? Do I need to have someone come out and do some cleaning or is there a more home-grown approach?

Answer 1

Sometimes it is necessary to adjust the fins on the downstairs vents to restrict some of the airflow so that more will make it to the upper level. This is especially true in cases where there are no dampers in the ducts of the type described by BMitch in his answer.

I have a similar problem in my own two level house where I have to adjust the vent fins two times per year to set the proper upper / lower level air flow.

Another important question for you.

Have you checked the primary furnace air filter to make sure it is clean? If this has never been changed it could be restricting the air flow design of the whole heating/cooling system. I've seen these filters so clogged up with dirt and dust that it was a wonder that the furnace fan was able to circulate any air at all!!

Answer 2

It's possible there's a blockage, a mistake in the design of the ducts, or a joint has come loose. However, given the design you describe, the first think I would look for are dampers on the ducts shortly after they leave the HVAC blower. This would be a small metal lever on the side of the duct right after it splits for the upstairs vs downstairs vents. When the lever is in line with the duct, it's open, and at 90 degrees, it's closed. If you do have dampers on your vents, I would spend some time determining the proper balance for each season, and then marking on the duct the winter vs summer position to make it easier on yourself and any future home owners.