Length of balance channel

A home owner from Winnipeg canada says:
My sash is missing balance channels on both sides. I've determined that a S380 will work as a replacement but I don't know how long it should be. The sash is 21 high and weighs 15 lbs. it looks like a 16, 17, 18, or 19 in length could work. What determines the optimum length.

Thanks

David
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Tom from SWISCO responded:
Can you give me the height of the visible glass for the sash these balances will support? That will allow us to calculate the best length. Thanks!
A home owner from Winnipeg says:
Thanks Tom.

The total height of the sash is 21 inches of which the visible glass portion is 16 inches. I'm thinking that the optimum length is determined with the goal of having the spring fully supporting the sash in the completely open position. Is that the correct?

David
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Tom from SWISCO responded:
Perfect, thanks. Chances are you will need a 17" long channel balance. For a 15 pound sash, I also recommend the 1640 stamp option.
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