[Archrms]

Most HVAC systems are "balanced" based upon the cfm rate of the system. If you start closing vents completely, you will alter the balance of the system, creating hot/cool spots in the rest of the house. The whole system should be re-balanced to properly compensate for the closed vent. Other factors that will affect this is the insulation value of the walls of the rooms being closed off. The return vent is the big issue, as it will suck hot air through cracks, door jambs, etc, even thought the supply vents are closed. Closing this would definitely affect the performance of the system. As others have stated, it would be best to consult a local professional that can analyze your specific system and circumstances in order to make suggestions.

Heat Pumps work great, but have diminishing returns the farther North you go.