Like most things, a home's windows tend to wear out over time and eventually need to be replaced. Window replacement can be quite a costly undertaking, however, so you don't want to do it when it's not actually necessary. Taking a few things into consideration will help you decide if the windows need to be repaired or replaced.
Age of Windows
It takes time for a window to wear out, so windows that are less than 10 to 15 years old aren't as likely to need replacement as older windows. On the other hand, it isn't always a good idea to replace the original windows on a historical home, especially if they're made from special glass, as this could lower the value of the home. The local preservation commission may be able to help you find someone to repair the windows instead of replacing them. Another concern with older windows from before 1978 is that they may be painted with lead paint and will require a special removal process to limit exposure to this harmful metal.
Functionality of the Windows
Windows that are in good condition will function well, opening and closing easily, locking when closed, and not causing noticeable drafts. Drafts can sometimes be temporarily dealt with by adding weather stripping and caulking, but this is more of a temporary fix than a permanent one. You may want to replace windows that feel hot to the touch in the summer or cold to the touch in the winter, which is often the case with single-pane windows, which are less energy-efficient than double-pane windows. The same is true if condensation forms on single-pane windows or in between the layers of double-pane windows. Windows that allow a lot of noise in, are hard to clean, or allow ultraviolet light to come through and fade indoor furnishings don't necessarily need to be replaced, but many people prefer to do so to get the added benefits of newer windows.
If your energy bills are really high, it could be partly due to older, less efficient windows. You can have the electric company come in and do an energy audit to determine the best ways to improve energy efficiency in your home. The person who does the audit will tell you if replacing the windows would make a considerable difference or if the problem is mainly due to something else, such as not having enough insulation. Unless there is something actually wrong with the windows, such as the windows letting in drafts or the frames being cracked or rotten, it doesn't usually make sense to replace windows solely for a small gain in energy efficiency.
Contact a service like Dr Exteriors LLC for any window replacement needs you may have or for further questions.