S770 Option - which one?

A home owner from Monument, CO says:
August 16, 2010
My channel balance has 27TGX stamped on it but is 28" long. Not sure which option to order (28d, 28c or 28b)? It is from a double pane double hung window 42" wide by 60" tall (sliding part of window is 42" wide by 29" tall).
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Casey from SWISCO responded:
August 17, 2010
Please remove the window sash and place it on a scale.
We need the weight of the window sash in pounds to determine the proper stamp option.
A home owner from Monument, CO says:
August 17, 2010
Following up on my customer questions and comments. I have a balance that needs replaced. It has 27TGX stamped on the channel but is 28" long. I don't know which one to order. It is on a 42" wide by 29" tall window (these dimensions are of the window that moves). See pics.
Thanks,
Eric
Customer image of their S770 channel balance.
A quick learner from Monument, CO says:
August 17, 2010
The window sash is 25 lbs.
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Casey from SWISCO responded:
August 18, 2010
S770-28 balance with stamp option 28D will carry a window sash weight range of 23 through 28 pounds.

This information is based on 2 balances per sash. Balances are designed assuming friction in the jamb weather-strip will carry a portion of the sash weight. Suggested sash weights are approximate.
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Casey from SWISCO responded:
August 19, 2010
S770-28 with stamp option 28B will carry a window sash weight range of 12 through 18 pounds;
S770-28 with stamp option 28C will carry a window sash weight range of 18 through 23 pounds.

This information is based on 2 balances per sash. Balances are designed assuming friction in the jamb weather-strip will carry a portion of the sash weight. Suggested sash weights are approximate.
A quick learner from Monument, CO says:
August 19, 2010
What are the sash weight ranges for the 28B and 28C options? I have other windows that will need new balances in the near future.
28" Series 770 Balance
28" Series 770 Balance
S770-28
$12.70
How to replace a tilt-window channel balance.
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