In the summer when running the AC I can get significantly more air flow from the 2nd floor registers by closing some of the registers on the 1st floor. Now that I'm running the furnace I still experience the same lack of airflow to the 2nd floor registers as I do in the summer. Is it OK to close some of the 1st floor registers? I'm thinking that doing so may cause more heat to build up inside the furnace. I realize there is a limit to how many I can close before suffocating my system, but should I be concerned about closing maybe 4 out of 9 registers on the 1st floor?