Spiral lift and how to attach to window bottom

The balance system is our 3/8" tilt non-tilt balances Series 400. You will need the S400-30 and S400-31, both with red tips to support a sash between 10-22 lbs. Keep in mind that the balances supporting one sash should be the same length. The balance bottom bracket would be the 17-052. Unfortunately, your window sashes aren't made to be removed. We have a guide to assist you, which you can view here. See our generic instructions for replacing these non-tilt balances below:

To install these balances, you would drop them from the top of the sash into the channel. Screw the balance tube at the top of the track. Now raise the sash and have someone hold it up while you bring up the spiral to the bracket, attach the 75-005 tool and turn the spiral clockwise three turns and hook it to the sash bracket. Repeat the other side and try the sash. If sash creeps down, raise and hold up the sash and give the balances another turn or two.

You shouldn't need two different lengths but you mentioned both 30" and 31" lengths, which is why we mentioned both options. If just the housing tube measures 30" and the 31" measurement includes the spiral, then you would need the S400-30 for both sides. 

Also, these types of balances require a bracket for installation. It seems like your window uses a bracket similar to the 17-052 that attaches to the inside corner of the sash and is difficult to see. 

All spiral balances like yours require a bracket, otherwise, there would be nothing to anchor the spiral to the bottom of the sash. Your bracket isn't clearly visible with the sash still installed but it would be attached to the inner corner of the sash that faces the window jamb. 

Happy holidays to you too!