A quick learner from ny says:
September 29, 2016
I have an anderson double hung window. 31 1/2 wide by 17/1/2 tall which replacement balance do i need to buy
Tom from SWISCO responded:
September 30, 2016
I can't say for sure. Can you give me the Width x Height" of the visible glass? That would help us determine an applicable replacement balance.
It would be even better if you could remove the balance from the window and show us the stamps printed on it. That will tell us right off what can work.