How to determine channel balance weight rating?

I need to replace the window balances in several non-tilt aluminum windows of unknown manufacture. The house was built in 1953. I have removed the lower sash from one window. It which measures 51" wide by 22-5/8" ht. The sash weighs 29-30 lbs. The old balances are 1/2" in width and 23" in length. The balances are each stamped "30" and "2240."

According to the SWISCO website, the 2240 balance is rated for 19 - 23 lbs.

My question is - If the window uses two balances (left and right), should I calculate half of the sash weight for each balance? For example, my sash weighing 30 lbs. should require two balances, each rated for 15 lbs. SWISCO's #2230 which is rated for 12-19 lbs. would seem to apply. Or, do you always assume that two balances will be used? In which case, a 30 lb. sash would require two #2260 balances, each rated for 27-30 lbs.

I should add that this particular window, using two #2240 balances, has always been difficult to raise and lower, but I realize that might not be the fault of the balances.

I have several windows that need to be addressed. All of the windows appear to be identical but, at this point, I have removed only the one sash and balances for further examination.

Thank you for any assistance you can provide.

Dan

I should have described myself as "Slow Learner." After sending yesterday's question I realized that I had replaced the channel balances in one of the windows two years ago with your Series 380, Option 2260. That replacement went smoothly but the #2260 may be too strong for the approximately 30 lb. sash. That window sash is now easy to raise but somewhat difficult to lower. I suspect that the Option 2230 (rated 12-19 lbs.) would have been more appropriate.

HOWEVER, I now have another problem. Today, I attempted to remove the sash on another one of the problem windows. That sash is in the "down" (closed) position and I am unable to raise it to the take-out clips. I can't raise it at all. Thinking there might be a problem with the window latch, I removed the latch, but that did not help. I suspect that one, or both, of the old balances has broken in the channel in such a way that the sash is jammed in the down position. In that position it's not possible to shift the sash to the left or right in order to swing the sash out. I should add that the top sash guides are visible and do not appear to be damaged. Do you have any suggestions for removing the sash from the "down" position, or how to get it up.

Thank you again.