The answer to this question is very site specific and depends on several factors, such as: heating, ventilating, and air conditioning (HVAC) system design, location of the thermostat, house design, and climate. In the right situation, closing vents and/or doors to certain rooms that are not being used during the day (or even during a whole season) may reduce heating or cooling bills. However, when you close vents/doors you could also throw off the balance of the system, change the way air is distributed, or cause the thermostat to detect temperatures which make the rest of the house uncomfortable.
We recommend that you check with your local heating and cooling contractor when they come for the next system tune-up. Propose to them which doors or vents you want to close and why. They should be able to help you decide whether certain doors or vents can be closed to reduce the amount of space in your home that is heated or cooled.
If you need assistance in locating an HVAC contractor, check to see if your area is participating in the Home Performance with ENERGY STAR program, and ask them for a referral. If Home Performance is not offered in your area, consult our guidance on finding an HVAC contractor.