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S385-34 Weight range and more

A home owner from Julian, CA says:
2 Questions.
1. Balance is stamped 33 3 HQ Caldwell -- Sash weighs 28#, this is on the cusp between 3340 & 3350. Is it better to higher (3350) or lower (3340)? Can I get both and return the ones I don't use?

2. Another window -- looks identical. Same dimensions. Installed at the same time when house was built (1997. Balance is stamped 33 4 AQ Caldwell. Wassup? I'd like to get balances for both of these windows. Do I get the same one as the other window?
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Tom from SWISCO responded:
Hello! I wonder if the "33 3" balances were a more recent replacement. They are definitely too weak to support your sash.

How well does the window with "33 4" stamped balances work? If you've never had any issues with it, then I'd say it's a pretty good reference, assuming it's identical in every way. The 3340 is probably a safe option.

If you still aren't sure, then you may want to triple check the sash weight. Usually the weight will be a couple ounces one way or the other. If it's slightly under 28 pounds, go with 3340. If it's slightly over, 3350 may be what you want.
A home owner from Julian, CA says:
Followup to my last question -- I misled you -- the sash that weighs 28# has the balance stamped "33 4". The sash with the balance stamped "33 3" weighs 20.4#. they look identical -- same dimensions and manufacturer, installed at the same time when house was built. the difference is that the 28# one is on the first floor. The 20.4# one is on the second floor -- directly above the 28# one. Is this normal? Is there a structural reason why an identical-looking window would be 8# lighter? Would I get 3330's for the lighter sash to replace the 33-3 balances I have?
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Tom from SWISCO responded:
It's possible the heavier window has double strength glass, which would account for the extra weight. Either way, hard numbers don't lie. If one sash is heavier than the other, I would definitely go with the appropriate stamp for either weight regardless of how identical the windows seem.
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