Are the springs on your armchair lumpy & saggy? Replace them with new biconical Coil Springs.
Coil springs are placed inside a frame by secured onto jute webbing, rails or onto a wooden base. The top of the springs compressed and tied together with jute twine, &/or sewing onto hessian. Use a large curved needle for sewing jute webbing.
What gauge of spring? Use 9g coil springs for seats, and 10g for backs.
What height spring? The height of the spring has many different factors, but here is a general rule of thumb -- measure the height of the side frame and add 10-12cm. For instance, if the frame is 8cm high, buy a spring about 18-20cm.
How much should I compress the spring? Press the spring down with your hand. When you feel the spring starting to resist, that is the height to tie it off at. Basically, you want to compress it enough to give you enough resistance on the top padding and fabric, but not so much that it starts to force the bottom out.