Covered springs?

Hello! It sounds like you need new balances. Your exact model hasn't been made in a long, long time, but we use a modern product on older wood windows like this and it works very well. See our Series 500, which is a 9/16" diameter balance assembly for non-tilt windows, as well as the 17-012 connector bracket. This would go on the bottom corners of the sash and give the Series 500 something to clip onto.

To determine size, you're going to need to measure the height of the visible glass and add 3" if you're repairing the bottom sash, or 1" if you're repairing the top sash. So for example if the visible glass height on the bottom sash is 20", you want a 23" long balance. If the top sash glass is also 20", you'd want a 21" balance instead.

You'll also need to determine the weight calibration of the spring. The only way to do this is to weigh the sash on a scale. Any common bathroom scale will do. The logistics of doing this can be tricky on a window like yours so just do your best to get the weight as accurate as humanly possible. You really don't want to guess.

You don't want to measure the whole sash, just the visible glass. Can you clarify what you mean when you say that it's between 19" and 20"? You only measure the glass you see, don't worry about whatever might be obscured by the frame. You want to be precise.

Great, thank you. That area you marked in red is precisely what you want to measure. At 19-1/2", I believe you can round up to 20" without issue.

Yep, since your bottom sash's glass is 19-1/2", I think a 23" long balance will be perfectly sufficient. I usually always round up or spiral balances, so I would mentally translate 19-1/2" to 20". Then, add 3" for the wood sash's operation.

If you wanted to play it safe, you could round down to 22" instead of rounding up, and then drill a new installation hole at the top to make up the difference. That means the window won't be able to open up as wide, but you'll have more control.