Window sash counterbalance with independently operable sashes

1. A window with upper and lower sashes that are movable relative to a frame of the window and each other, the window comprising:

a system selectively counterbalancing a raising or lowering movement of one of the sashes relative to the frame with a converse lowering or raising movement of the other one of the sashes such that the raising or lowering movement of the one sash causes the converse movement of the other sash, and assisting independent movement of one of the sashes relative to the other one of the sashes and the frame, the system including:

guide pulleys that rotate about respective axes, wherein the guide pulleys are disposed in the frame and the axes are static relative to the frame;

an intermediate pulley that rotates about an intermediate pulley axis; and

a sash coupling element that extends between the upper and lower sashes of the window and engages the guide pulleys and the intermediate pulley, wherein the intermediate pulley is coupled with a portion of the sash coupling element that extends between the guide pulleys.

2. The window of claim 1 , wherein the intermediate pulley axis is static with respect to the frame in a window counterbalance mode, and wherein the intermediate pulley axis is movable with respect to the frame in a sash assist mode.

3. The window of claim 1 , wherein the system is in a window counterbalance mode when the raising or lowering of the one of the sashes causes the converse movement of the other one of the sashes, wherein, a position of the intermediate pulley axis is static with respect to the frame when the system is in the counterbalance mode.

4. The window of claim 3 , wherein the system further includes a retainer configured to inhibit movement of the intermediate pulley axis with respect to the frame to maintain the system in the window counterbalance mode.

5. The window of claim 4 , wherein the retainer is configured to be removable by a window user.

6. The window of claim 1 , wherein the system further includes a block and tackle assembly configured to tension the sash coupling element, the block and tackle assembly including:

a spring coupled to the frame at a side portion of the frame that includes the guide pulleys;

a block coupled to the spring, the block being spaced apart from the guide pulleys; and

a tackle element coupled to the block and the intermediate pulley.

7. The window of claim 1 , wherein the system is in an independent sash movement mode when the independent movement of the one of the sashes occurs with respect to the other one of the sashes and the frame, wherein the intermediate pulley axis is movable with respect to the frame when the system is in the independent sash movement mode.

8. The window of claim 7 , wherein the intermediate pulley axis is configured to move away from the guide pulleys when the system is in the independent sash movement mode.

9. The window of claim 1 , wherein the raising or lowering movement of the one of the sashes causes the converse movement of the other one of the sashes through the sash coupling element, and

wherein when the system assists the independent movement of the one of the sashes relative to the other one of the sashes and the frame, the other one of the sashes is static with respect to the window frame while the one of the sashes and the intermediate pulley axis are moved with respect to the window frame.

10. The window of claim 1 , wherein the system further includes a block and tackle assembly configured to tension the sash coupling element, the block and tackle assembly including a spring, a block coupled to the spring, and a tackle element coupled to the block and the intermediate pulley, wherein tension in the spring is released as the intermediate pulley axis moves toward the block.

11. A system that assists movement of one of first and second sashes in an independent sash movement mode and that assists movement of both of the first and second sashes in a counterbalance sash movement mode, wherein the first and second sashes in a double-hung window assembly, the system comprising:

a sash coupling element coupled between the first and second sashes, the sash coupling element located at a side of a frame of the window assembly; and

a pulley system coupled with the sash coupling element, the pulley system disposed at the side of the frame of the window assembly;

when the system is in the counterbalance sash movement mode, a movement of the first or second sash causes a converse movement of the second or first sash, respectively, through the sash coupling element, the converse movement of the second or first sash assists the movement of the first or second sash, respectively, and

when the system is in the independent sash movement mode, the movement of the one of the sashes is assisted by the pulley system and the sash coupling element while the other one of the sashes is static with respect to the frame.

12. The system of claim 11 , wherein the pulley system includes a pair of guide pulleys and an intermediate pulley, wherein the intermediate pulley is coupled with a portion of the sash coupling element that extends between the pair of guide pulleys.

13. The system of claim 11 , wherein the system further includes a block and tackle assembly configured to tension the sash coupling element, the block and tackle assembly including:

a spring coupled to the frame at the side of the frame;

a block coupled to the spring, the block being spaced apart from the pulley system; and

a tackle element coupled to the block and to the pulley system.

14. The system of claim 11 , further comprising an intermediate pulley that rotates about an intermediate pulley axis, and wherein when the system is in the independent sash movement mode, the intermediate pulley axis moveable with respect to the frame.