To extend the life of your furnace, do not close off more than 20% of the registers in your house. This can cause high resistance and unnecessary heat build up in your furnace and premature failure. A proper furnace is sized for your whole house, when you block off some of the ducting the furnace is unable to adjust and will retain the heat it was designed to generate. When the weather is extremely cold you want to be able to depend on your furnace. If your furnace breaks down and you need repair please call Air-tech at (847)639-5336..serving Cary and McHenry County for over 30 years, a family owned and operated local business.