The sash weighs 13 lbs. and there are 2 springs on each side. I can't make out any numbers on any springs

Hello! I am very sorry for the long wait; we appreciate your patience during an incredibly busy season.

Take a look at the 15-199 pivot shoe and the 15-150D double-spring support. You can use two S160-03 3.5-pound springs on each side of the window for a total of 14 pounds of overall support. 

Review the specifications and let us know if you have any questions.