29" Series 900 Spiral Balance

I'm sorry to hear about the trouble. The Yellow Tip is the strongest possible option available for the 3/8" balance, unfortunately. It's surprising that it didn't work, especially considering the Yellow Tip is way, way, stronger than what you should be using for a 20 pound sash.

The only explanation I can think of is that the S900 is not the right size balance for your window. Note that the exterior tube is only 3/8" in diameter. What is the exterior tube on your balance? Is it 3/8" or 9/16"? Regardless of how strong a spring is, a skinny 3/8" balance simply cannot hold the same kind of load that the larger 9/16" balance can.

Note that you do not want to measure the exposed spring that I see peeking out of your old balance. The exterior tube is what determines size.

Sounds good! We'll be here to assist you should you need it.