Window still sags with my S600-25

Hello, John. Before you buy anything, can you tell me how many times you charged the balance before you installed it? These balances don't have very much tension at all until you wind the spiral rod. In the case of the S600-25 you want to give it 3-4 clockwise turns. You will feel tension start to build.

If you did all that and your window is still sagging, then your next step should be to weigh your sash on a scale and refer to the weight chart on the S600-25 store page. That chart tells you how much weight each color tip supports. That way you just pick whatever works best, no need to guess.