Double hung tilt out window spring broke

Thanks for posting. This looks like a 15-087 shoe, which uses Series 180 Coil Balances. To know what spring strengths to select for the coils, look for stamps on the coil tape--the number on the shoe for this particular kind of system isn't always reliable to go by. If there are no stamps on the springs, or you're uncertain, give me the exact weight of the window sash that these balances support, and I can work out what you need.

Series 180 coil balance diagram

Perfect. In that case, our S180-10 10 pound spring should be what you need. Let me know if you have any other questions.