window balance strength effect

Question

I have 31 year old windows that have 390 series balances and  weigh right at 15 pounds. The top of the 30 and bottom of the 40 strength ranges are 15 pounds. Existing balances are marked 40 but they are not OEM. What would be the operational comparative effects (opening/closing resistance) between the two?

Tom · Accepted Answer

If your sash is precisely 15 pounds, then you could theoretically use either. Both the 2030 and 2040 stamps are calibrated to carry that weight and you probably won't notice much of a difference.

Assuming a worst case scenario, if the spring is too light then you will find it a little harder to open. Conversely, if the spring is too strong, then you'll find the window to be a little too hard to close. In your case I don't think you have to worry about either.