Spiral window balance

A quick learner from Columbia SC says:

27 1/2 inches long. Spring portion only. I told you it was a spiral window balance from windows OIneed to repair. It is 27 1/2 inches long. I need to know what zi need to order that is comparable... I sent a photo.

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Paul from SWISCO responded:

Your balance looks close to both our Series 600 and Series 900. However, I do have a question. Was this balance encased in some sort of plastic tube? I ask because spiral balances usually are not made with an exposed spring. If it does come encased with a balance tube, we will need to know the tube diameter of it so that we can recommend the proper balance. 

A quick learner from Columbia SC says:
It had 7/16 plastic sleeve as shown in new pic. The spring was secured with a plastic end with a metal screw. Cheap construction which snapped at the screws after a number of times raising and lowering.
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Bob from SWISCO responded:

Thanks for the reply. So if the outer tube is 27-1/2" long, it's best to round up to 28". So you'd need either the S900-28 or S600-28. It's a bit tricky to measure the diameter of the tube with these old plastic tubes, but do the best you can. The S900-28 has a diameter of 3/8", while the S600-28 has a diameter of 5/8". To know what color tip option to select, weigh your window sash. You can then match that exact weight with one of the weight ranges listed alongside each color tip option on the product page.

A quick learner from Columbia SC says:
So if the sash weighs 20 pounds and only one sash moves, the window has two spiral balances. Then I divide 20lbs by 2?
Each spiral needs to support 10 lbs. Is that correct
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Bob from SWISCO responded:

No, that concept only works for coil springs. If your sash weighs 20 pounds, then you would need to purchase a spiral balance for each side of the window that can accommodate a 20 pound sash weight. 

I hope this clarifies any confusion that may have come up. Let us know if you have any further questions. 

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