S850 Crossbow Balance

A handy person from MN says:

After much research I've found your site and determined the S850 crossbow balance would be the best fit for my single hung tilt window that wont stay up. My current block and tackle balance measures 28.5 long with a 1" shoe. But what I'm not sure on is that the shoes on my current window are blue.

The window sash itself weighs 20lbs. And I can see in the table you show the black balance weighted at 7-21 lbs and the wight at 21-26lbs. Does that mean each balance can support that and I should go witht the blacks? Or would it best to go with the whites?

User submitted image of their window hardware.
Profile picture of Paul
Paul from SWISCO responded:
The Series 850 is close, but I would first check the diameter of your balance spring to confirm that it's 1/2". If so, I recommend the 1" wide black shoe based on your sash weight.
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