Custom Wood Window Spiralifts
Thanks for reaching out to us, Susan. As these are custom windows I'm not sure how appropriate my advice would be, since the design of your setup may differ greatly from what I have experience with. That said, assuming that this is otherwise a typical non-tilt wood window, installation is fairly straight forward. In fact, we have a brief written guide that you may find useful which you can see here.
I'd be happy to offer more insight but I think the best place to start would be with photos. I would specifically like to see photos of the old balance, how it attached to the sash, and where it installed in the frame.
Lastly, as far as covering the balance, this is fairly common practice. Some manufacturers make special covers that snap into the opening to conceal the balance itself, but as your window is custom made that may not be an easy thing to manage. You may want to contact whatever company installed the windows to begin with to see if they sell compatible covers.
The wood windows have concave channels along the vertical edges where the lifts go. Here are pictures of the channels & a double hung (top was never operable) in situ.
Many thanks, Susan
Thanks for these photos! They're a huge help. Seeing what kind of sash this is, I think that the non-tilt Series 500 spiral balance is going to be your best bet. You will also want our 17-012 bracket, which is what will connect the bottom end of the balance to the bottom corners of the sash.
What you have to look out for now is making sure that you have enough space for these tubes to fit comfortably on either side of the sash. You also need to make sure you get the right length. The formula we use for older wood windows is taking the height of the visible glass and adding 3" for the bottom sash, or 1" for the top sash (assuming your top sash even has balances, they don't often do).
For example, if the height of the visible glass for your bottom sash was 20", you'd want a 23" long Series 500 spiral balance. Note that you do NOT measure the full height of the sash, just the visible glass. Measuring the full height won't be accurate.
This hardware isn't necessarily a guarantee, since your windows are so old, but I think they're a solid pick and I've personally had a lot of experience repairing old wood windows like this with these exact parts.