Window Balance Identification

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)?

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?

Thx,
RS