Window is too heavy to lift.

Thanks for posting. Just to clarify terms here, the metal channel with the nylon fittings is called a channel balance; the window sash is actually the movable frame with the glass. Now, did the window only start having trouble? Or have you always had trouble keeping it open? May we see the nylon fitting on the other end of the balance as well? We need to see both to identify what you need. 

Confirm that the metal channel is 27" long, not including the nylon fittings at either end. If so, you need either the S380-27 or S385-27. The only difference between these two balances is the depth of the bottom nylon shoe, or the 'heel-to-toe' measurement, so compare both with your old hardware to see what matches best. 

If you've already had trouble keeping this window open, then it's possible that at some point this window was fitted with the incorrect balances. It's not advisable to just pick a stronger stamp option though without knowing exactly what weight range your sash falls into. Obtain the exact weight of the window sash, then match that exact weight with one of the weight ranges listed alongside each stamp option on the shop page.