Replacing the springs in your Chair or Settee isn't as difficult as it sounds and the springs are really cost effective.
Zig Zag Springs (also known as Serpentine springs) are used a lot in modern furniture. The springs are stocked in standard sizes but can be easily cut with a hacksaw if you need to adjust the size.
Tension Springs are manufactured with plastic covers, and if the cover splits through wear or over stretching the bare wire can cause damage to the cushion covers.
Double Coned Coil Springs x 9 gauge. On average a 5" Coil Spring would be used in a chair seat.