Series 159 Coil Balance Quote

Hello there. Coil balance spring weights are denoted by how much weight each individual coil can support. So a spring stamped '10' means you have a spring that can support 10 pounds of weight. You want the same amount of support on both sides of the sash. So if you had a 10 pound spring on both sides of the sash, that would equal 20 pounds of support. The video below explains more. 

Now, you state the sash weighs 17 pounds. That would be a bit unusual if you have 10 pound springs. So long as you are within 1 pound of the exact sash weight you're good, but clearly that wouldn't be the case if you had 20 pounds of overall support for this window. Please double-check the sash weight. Did you ever have trouble closing this window or keeping it closed, prior to the balances failing? If not, most likely the sash weighs closer to 20 pounds. 

Now, as far as what springs you need for the Series 159 conversion, there's an added wrinkle, because the Series 159 springs come in at X-1/2 pound increments. For instance, the S159-10 actually supports 10.5 pounds of weight. If your window weighed exactly or just a little over 20 pounds, that would work just fine, since you'd be dealing with 21 pounds of support. But it's best to confirm the sash weight you need to support before proceeding.