SWISCO - The Replacement Hardware Authority

Need help choosing option

A home owner from San Francisco says:
Dear Sir or Madam,
I would like to replace some broken channel balancers on my aluminum windows (each sliding pane is about 45x30 in). The closest match I found are the 28" Series 351.
I suppose that the option 2710, 2720, 2730, or 2740 specifies the strength of the spring, is that correct?
My current balances are stamped with "27 9 Caldwell"; what would be the corresponding option?
Could it be that 27 9 means 9 kg and 2720 means 20 lbs? Would that be the weight for 2 balancers (window weight) or per each balancer (half the window weight)?
I always found my windows too heavy to lift up, so I would like to order a stronger spring. I will weigh them before ordering if that helps.

Thank you for your help in choosing the right product,
Regis
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Donna from SWISCO responded:
Yes, the stamp options represent different strength balances.
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.

If you are unsure of the balance series you selected please feel free to reply with photos for us to research.
A home owner from San Francisco says:
Thank you for your reply. The window sash is 32.8 lbs.
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Donna from SWISCO responded:
S351-28 with stamp option 2760
will carry a window sash weight range of 30 through 33 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 says:
Hello I have windows with the stamp 2740 if I use a 2760 would there be any advantages
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Donna from SWISCO responded:
No, if you use a balance that is not the intended strength it will either sag or lift on its own. In your case, if you use a stronger strength balance the sash may rise without touching it.

If you believe the original strength stamp option 2740 is not the correct replacement, remove the window sash and place it on a scale.

S351-28 with stamp option 2740
will carry a window sash weight range of 19 through 26 pounds.

Suggested sash weights do not include the manufacturer's sash friction from use of weatherstripping. All windows vary in sash friction due to manufacturing methods and weatherstrip types, therefore all suggested sash weights are approximate.
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