Spiral balance inquiry

A quick learner from US says:
?Hello,
I am interested in retrofitting the lower sashes of the double hung windows in a seasonal cottage with spiral balances. The windows include two sizes, the lower sash on one is 17 1/2" tall, with 14" tall glass and weighs 3.5 lbs, while the second size is 27 3/4" tall, with 24" glass height and weighs 8.5 lbs.
I have the tools needed to mill the channels for each sash. Can you recommend the correct balances for these windows, and include any necessary mounting hardware? Thanks.
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Tom from SWISCO responded:

Sure, for old wood windows like this we usually recommend the Series 500 spiral balance. To determine the proper length, you want to add 3" to the visible glass height for the bottom sash and just 1" for the top sash. So in your case, you want an S500-17 for the bottom sash on the first window. You didn't mention if the second window was a top or bottom sash, but assuming it's a bottom sash you want an S500-27.

You'll also need something to connect the sash and the spiral balance. That would be the 17-012 bracket. A 75-005 charging tool may also be useful for you.

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