A moving coil, constant force, curl spring window balance assembly for installation in a hung window assembly comprising a sash is disclosed. The window balance assembly can include a carrier, a mounting bracket, a curl spring and a sash support bracket. The sash support bracket can be positioned intermediate the carrier and the mounting bracket. The sash support bracket can selectively engage at least one of the carrier or the mounting bracket. The sash support bracket is operable to supplement the counter-balance or holding force provided by the curl spring of the window balance assembly against the weight of the sash when the sash is in a raised position.