Window Balance Identification

A home owner from Washington says:
I am trying to figure out which spring balances I need for a single hung vinyl window (they might be Jeld Wens).

I have an existing window whose movable pane assembly dimensions are Width 28.5" and Height 23.5" and whose spring balances measure Length 21", Width 1/2" (actual width, is not 9/16's actual), and are marked "20 3 LP Caldwell". The top shoes look like your 19-005 and the bottom shoes are either a 15-005 or 15-004.

I need spring balances for an identical window whose measurements are slightly larger: this windows movable pane assembly measures Width 30" and Height 25.5". What balances do you think I need for this window? 22"?

How do I know if the bottom shoe is a 15-005 Series 380 assy) or 15-005 series 385 assy)?
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Tom from SWISCO responded:
Hey, thanks for posting. I've narrowed this down to two possible channel balance Series: either our Series 380 or our Series 385 balances.

Now, as you can see, there is only one difference between these two Series, and that is their bottom shoe. The Series 380 uses the 15-005 while the Series 385 uses the 15-004. I can't quite tell what you have based on your pictures, though. Please carefully compare their dimensions to your own hardware and see which is a better match.
15-004 and 15-005
A home owner from WA says:
Thanks Tom,

Yeah, I had narrowed it down to those two Series' too...!!

But the measurements you supplied are exactly what I needed to help figure out the differences.

Part 2 of my question: Do you have any thoughts on if I need slightly higher-rate-spring spring balance assemblies for the slightly larger window? I am able to figure out by measuring the sides of the movable pane if 21" balances are better sized for this window, and I guess the spring rate would depend on the window's weight.

If the smaller 'reference window' I am looking at uses a 20-3 and the window that I need the balances for needs a 21" balance and is a couple pounds heavier, should I use a 21-3 or a 21-4?

Profile picture of Paul
Paul from SWISCO responded:
It's best to weigh the sash in question to determine the appropriate option to use, as these are very delicate systems and it has to be as precise as possible. We have a list of each stamp option on each individual balance page, along with each weight that those stamp options represent. Once you know your sash weight, you can just cross-reference it to that chart.
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