Our sliding glass doors are probably at least 20 years old. They were here when we bought the house. They seem well made, however, the room that they're in is always significantly colder than the rest of the house in winter. I think it's because of the large expanse of glass. We close off the room and don't use it in winter. I've been thinking about replacing them, but does anybody know how much of a difference it will make? We're looking into either Anderson or Pella that qualifies for the 30% tax credit. But I sure don't want to spend $800 only to find out the room isn't that much warmer. I understand I won't recoop the $800 in heat bill savings, I'm just looking to be able to use the room year round comfortably. Thanks to anyone who can give advice/opinions.