Cannot close the window, maybe wrong option?

A home owner from Charlotte, NC says:
September 7, 2017
My window is 8 lbs, but I ordered 770 28D (was thinking stronger might be better). Installation was easy, but then I cannot close the window. The window just bounce back when I push it down. Is it because balance is too strong? should I have ordered 28B? or there might be other issue? (The window worked well before with the old balance).
Another question, I have another window which is 17.5 lbs, should I order 28B or 28C? Thanks.
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Casey from SWISCO responded:
September 8, 2017
Thanks for choosing SWISCO.com. Does your old balance have any stamps or engravings on it? A 28D supports a 23-28 pound sash. If your sash is only 8 pounds, a 28B may also be too strong. Can you confirm the sash weight?
A home owner says:
September 11, 2017
Many thanks for prompt reply.

The old balance has "2800 BSI LC" on it. The sash (that I installed the new balance and kept open) weighs 8 lbs.

I have another window that the sash weights 16 lbs, and also has "2800 BSI LC" on it. I haven't ordered the balance for this one yet. Which option should I choose?

If the balance is too strong for the sash weight, will it cause it open?
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Tom from SWISCO responded:
September 15, 2017
Sorry for the wait, this took some research. We will be able to replace these balances for you, but it will need to be custom ordered since your sash is so light. I will email you a special custom balance order link and price quote for two of these immediately.

As for the 16 pound sash, we recommend our 28B option.
28" Series 770 Balance
28" Series 770 Balance
S770-28
$12.70
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