Hi,
My house was built in 1990. I have no air coming from the registers in the upstairs bedrooms. My ducting is in the attic. I have examined it several times for air leaks and haven't been able to find any. Is it possible that the wrong size ducts were used for the upstairs, significantly reducing the airflow, and if so, how do I tell.

Mike

Start at the air handler, where the fan motor is located, and confirm the fan is running. I assume you have air flow from registers on the lower floor.

Follow the duct work in the basement, if you can, to see if the duct going to the second floor is open, or otherwise broken, not allowing air to flow upstairs.

Also, you may have a manual or even a motorized damper, stopping the air flow. A manual damper is found by finding a small lever anywhere in the length of the duct, usually close to the air handler. If the handle is perpendicular to the air flow direction, the damper is closed. Turn the handle 90 deg till it is level with the air flow direction.

A motorized damper will not have a handle, but will have a small box on the side with control wires going to it, many of the popular ones have an orange cover. This is a bit more involved to check and repair, wires may be broken anywhere, stat could be bad, relay could be bad, damper motor could be stuck or bad.

Either of these dampers can be located in the attic also.

Or the duct work can even be separated somewhere in the walls.

Hope this helps