coil replacement needed

Thanks for contacting us. So, if I'm understanding your post, your bottom sash has four springs supporting it (two on each side) and weighs 20.4 pounds, while the top sash has 2 springs supporting it (one on each side) and weighs 10.8 pounds.

That's easy enough to figure out. The next step after you have that information is to just divide the weight by how many springs support it. Can you just confirm the thickness of the spring? It looks like our 1/2" thick Series 160. Assuming I'm right, the bottom sash would use one S160-4 and one S160-5 on each side (4.5 lbs + 5.5 lbs = 10 lbs, 10 lbs x 2 = 20 lbs) and the top sash would use one S160-5 on each side (5.5 lbs x 2 = 11 lbs). 

The 15-191 is considered more durable than the 15-111. That's why it's labelled "heavy duty."

Sounds great. We are happy to have been able to help you get pointed down the right path to the replacements you need.