Window Coil Parts Needed

Hi Joey. This is a 15-175 Pivot Shoe, which uses Series 152 Coil Balances. Look for stamps on the springs to determine what spring weights you need. If you're uncertain, give me the exact weight of the window sash and we can work it out for you. For supports and covers, use the 15-145, 15-144 and 15-143 for each coil.

Four 7 pound springs would equate 28 pounds of support, which is a little too strong for your 21 pound sash. Instead, you may want to consider using one 5 pound spring and one 6 pound spring on each side. That would be 22 pounds of support, which is still a little strong but not by that much.

Too strong is bad, yes. If the springs are too strong then the window will be harder to close. It could even drift open all on its own.

It doesn't matter if the 6 goes on top of the 5 or vice versa, but whichever way you pick make sure it's the same on both sides so that their operation is symmetrical.

Too weak would just give you the opposite problem. When it comes to balance spring strength, you have to think of it like Goldilock's porridge. Not too strong, not too weak. It has to be just right.

In your case, you have an odd numbered sash weight, so it won't be possible to get it exactly right just by the nature of how weight is calculated. You can't get an even number by cutting an odd number in quarters. So you just have to get as close as you can. A total spring support of 22 pounds should be just fine for a 21 pound sash. That's just a 1 pound difference. A total support of 18 pounds, which you suggest here, may also be fine, but you're starting to stray from the original weight. That's a 3 pound difference. Maybe not the end of the world, but the further you get from 21 pounds the worse your performance is going to be.