Hi, we have a single heater in the attic with two separate thermostats; one upstairs and one downstairs. When the house was new and the downstairs thermostat was selected to heat it seemed there was an 80% flow to the downstairs and appx. 20% flow upstairs. The system seemed to heat only the downstairs portion of the house. Now regardless of whether the upstairs or downstairs thermostat is selected it seems that the airflow is always 80% upstairs and only 20% downstairs. If I attempt to heat only the downstairs to 70 degrees the upstairs gets to over 100 degrees since nearly all the airflow is now going to the upper portion of the house and the heater runs continuously attempting to heat the downstairs thermostat to 70 deg. Is there a flow diverter valve that is stuck in the system or is there something else that is causing all the airflow to be directed to the upper floor. Everything else on the system works well. (we also get the same problem during the summer with the AC on with nearly all the airflow going to the upper floor). House is appx. 10 years old.

Thanks!

Sounds like you have a power damper system to direct the flow of air from the furnace to the location calling for heat. These systems can be complex and there are many different systems out there. I suggest you call a qualified service person for a On site service call to find out the exact part that is defective. If you want to try and service it yourself all I can offer is a Good Luck since those are field installed and I have no documentation to refer to for service.