Balance spring too stiff

Hello there. It sounds like you are talking about channel balances of some sort. Are these the balances that were originally installed in the window? I ask because in general we advise to always stick with the original stamp code. You only really want to go with a different stamp if the wrong balance was installed to begin with, and therefore the window was always difficult to close or keep closed. Going with a stamp that is intentionally for a lighter window weight range will not work properly--most likely it will simply break more quickly. 

A standard channel balance for a non-tilt window that is stamped 2830 will support a window sash in the weight range of 16 through 23 pounds. If you want to be safe, obtain the exact weight of the sash to see if it falls in the range.  If the sash is in that weight range, then the balances should work just fine--it is possible the track needs to be cleaned, or the balances themselves are failing. If that's the case, remove a balance from the window and send us photos of the nylon attachments at both ends, just so I know exactly what you are working with.