A new construction Balance assembly consists of coil springs, balance shoe, and balance cover. The springs are 1/2" wide for non-egress and 3/8" wide for egress.Egress refers to a code rating in which 5.7 square feet of open area is achieved by raising the sash and not having to remove any part of the window. This is so you have room to get out in case of a fire. The size requirement is based on a firefighter in full gear.
The pivot bar is attached to the sash(the part of the window that moves up and down) and fits into the part of the balance shoe called the cam(the slotted metal piece).
The balance springs are rated to handle various weights of sash. There will be numbers stamped on the side of the spring indicating its pound rating, ie 3#,5#,6#. Replacement springs need to be the same size as the original. Too low on pound rating, the sash will slide down, too high on pound rating, the sash will raise on its own.
Springs are attached to one another through slots in the side of the springs.
The bottom spring is attached to the shoe through a slot in the shoe.
Depending on the age of your window the balance assembly you receive from the manufacturer as part of their parts only warranty may look different from what you currently have. This is OK the new styles will work. The main concern is the width of the springs. 1/2" springs can not be used to replace 3/8" egress springs.
