A handy person from Bonita Springs, FL says:
February 13, 2016
I have Alenco double hung windows. Can I use 2840 window balance to replace a 2830?
Tom from SWISCO responded:
February 15, 2016
It would depend on the weight of your sash. I see you were looking at our S385-29, is that what you have? If so, refer to the chart of options and weight ratings we show on the S385-29 store page. If you were to weigh your sash, that chart can tell you exactly what option you need.
I would not recommend getting an option for heavier sashes than what you're working with. A balance that's too strong will not work properly and could even damage your window. We advise only getting the exact strength you need.