Home Improvement : Repairing a ‘Guillotine Window’ : Repairs: When a double-hung window crashes down, it can cause injuries. Usually the cause is a defective sash balance. Here’s how to fix it.

Do you have “guillotine windows,” double-hung wood-frame windows that come crashing down every time you open them? Do you have windows in the house that cannot be opened fully because they snag on a thin cable that hangs loose at the side of the frame?

Both problems are caused by a broken cable or spring from the sash balances, the mechanisms that balance the weight of the window sashes to keep them from falling.

Not only are broken sash balances a constant source of annoyance, they are also a considerable safety hazard. Emergency room statistics indicate that many broken hands and fingers (especially childrens’) result from wood windows falling due to broken balances.

The good news is that in most cases, window sash balances can be replaced by a novice in less than an hour once you have the proper part.

To get started, you have to determine the correct weight sash balance to buy.

Begin by taking the window measurements. Imagine both upper and lower sash were taken out of the frame, leaving a rectangular opening in the wall. What would the dimensions of the rectangle be?

Then, determine whether the window requires one balance or two. With the lower sash fully closed, look into the window track just above the top of the sash.

You will see a thin metal plate on either the left or the right indicating a one-balance window, or you will see a thin plate in both the left and right tracks, indicating a two-balance window.

Now, with the dimensions of the window and knowledge of whether one or two sash balances are required, you can go to most any hardware store and obtain the proper weight replacement sash balance.

The replacement of the sash balance requires removing the sashes from the window frame.

With a hammer and chisel, remove either the left or right “stop,” the long thin molding against which the lower sash slides up and down. Begin at the top or bottom and tap the chisel blade into the joint between the stop and the window jamb.

As the stop begins to separate from the jamb, carefully pull it away farther using the chisel blade as a pry bar. Pry the stop at several spots along its length to prevent its breaking. Once free, set the stop aside and remove any remaining nails.

With the stop removed, the lower sash can be lifted slightly and swung out of the frame. If it is still connected by the cable at the bottom, use the chisel blade to pry the cable bracket nail from the lower edge. Grab the cable bracket and allow it to retract slowly into the sash balance now exposed midway up the window track.

Set the lower window sash aside and begin removing the upper sash. Note the rounded molding that runs the length of the window jamb adjacent to the upper sash. This is the “parting bead” and also should be removed using a hammer and chisel in a similar way in which the stop was taken off.

With the parting bead set aside, the upper sash is free. Swing it out of the frame and detach the sash balance cable if it still connected under the lower edge.

Now that both upper and lower sashes have been removed from the frame, the sash balance itself is easily replaced. Simply pry it out of the window jamb with the claws of a hammer and discard it.

With a pair of pliers grab the cable brackets that lay on the inside portion of the new balance and swing them around so they will be on the outside.

Insert the sash balance into the window jamb opening and secure it in place with the nails provided.

Now comes the kay to the success of this repair, reassembling the window.

Reinstall the upper sash first. Put the non-cable bracket edge of the sash into the track and swing the other edge toward the frame. Grasp the cable bracket exposed on the outside of the sash balance with a pair of pliers and pull it down far enough to clear the bottom of the sash.

Continue swinging the sash as far as possible into the frame. Allow the cable bracket to retract until it contacts the bottom edge of the sash. Orient the bracket on the wood according to the outline left by the old bracket and secure it in place with one of the nails provided.

Test the upper sash for free movement, and if it is OK, replace the parting bead into its groove and secure it with two or three 1 1/2-inch finishing nails driven directly through the top of it.

Repeat this procedure for the lower sash. Attach the cable bracket under the sash, swing the sash into place, and test it for free movement.

Before reattaching the stop, place it next to the lower sash and secure it with four or five evenly spaced 1 1/2-inch finishing nails driven about half way in.

Retest the operation of the lower sash to assure that it is smooth. Remove and adjust the nails if necessary. Once you are satisfied that it operates properly, drive the nails in fully. Touch up any chipped paint on the casing or stop. Lubricate the window tracks with a thin coat of Vaseline if necessary.

That’s it. No longer will you test your frustration level and risk broken bones when you want some fresh air.