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Channel Balance sizing

A contractor from Delaware says:
I have used you guys for parts before and they were just what I needed. Now I am fixing some windows that have a different size balance (length and wieght number)on each side of most sashes. The sashes I have worked on so far wiegh 10.25# and are 29.75"H x 18.5"W double pane aluminum replacement windows from the 60's. I have no idea which is the right size balance.
These windows have 390 series Balances with stamped numbers of 2640's and 2840's and 2520's and 2620's in them. I'm pretty sure the balances should be 28" because with the ends the 2840's fit like they should. This old house has 66 windows of various sizes and I'm affraid they may all have been similarly repaired over the years (just putting any balance in any window). If you could provide a chart of the weights each size 390 Balance would carry I'd appreciate it. That way I can start over and make sure the windows all have the right balance in place then repair or replace the balances as needed. Thank you in advance
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Donna from SWISCO responded:
Hello,
You wrote "I'm pretty sure the balances should be 28" because with the ends the 2840's fit like they should." The 2840 stamp option is for a 29" length balance.
A 29 inch Series 390 with stamp option 2840 will carry a window sash weight range of 23 through 28 pounds.

If you have a sash that weighs 10.25 pounds, and currently has 29" Series 390 balances, you should replace both balances with the 2810 option, this will carry a window sash weight range of 7 through 12 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.

If you are unsure of the balance series you selected please post detailed photos for us to research.
A contractor from Wilmington DE says:
I know the balance style is the series 390 I'm just not sure which size balance for which Sash weight. I can figure out the length but what is the load range of each balance? You say the 10 can carry 7-12# what about the 20's, 30's,40's. I have several sizes of sashes that I'm working with am I going to have to post each sash weight so you can tell me the proper balance or can you just give me a chart or list of the weight ranges each balance carries? I need to get the balances ordered but can't do so until I know which is the proper one for each size sash. Thanks
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Donna from SWISCO responded:
I do not have a chart, I would be happy to provide all the needed information to you, I just need to manually look up each product for you internally. Normally, it is best to match up to the original length and stamp as intended by the window manufacturer; I understand you feel the balances you are trying to replace have been incorrectly chosen and replaced over the years.
Here are the 30" weight ranges for the Series 390 to get you started, please let me know if you require other length weights as well:

S390-29
2810 will carry 7 - 12 lbs
2820 will carry 12 - 16 lbs
2830 will carry 16 - 23 lbs
2840 will carry 23 - 28 lbs
2850 will carry 28 - 32 lbs

S390-30
2910 will carry 7 - 12 lbs
2920 will carry 12 - 16 lbs
2930 will carry 16 - 23 lbs
2940 will carry 23 - 28 lbs
2950 will carry 28 - 32 lbs

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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