Milgard window sash balances

A home owner from Everett, WA says:
August 25, 2014
I need to replace some Milgard window sash balances. Your 15" Series 380 Channel Balance (Product Code: S380-15) seems to exactly what I want however, their are no markings on the old channel pieces. I don't know the weight of the window. I could take the window out and weigh it but it is in a rental I own in another city and will delay the replacing at least a week. I'm guessing the window sash weighs about 10 lbs. Should I go with the 1430 (10-12 lbs) or is it critical that I get the exact weight of the window first.
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Tom from SWISCO responded:
August 25, 2014
I do not recommend guessing. These are delicate systems and we recommend that you only exactly what you need. You really should weigh the sash before you purchase anything, or risk the windows not operating properly.
15" Series 350 Channel Balance
15" Series 350 Channel Balance
S350-15
$9.34
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