What a window balance does and why it fails

A window balance is the spring mechanism inside your window frame that holds the sash (the moving part) in place at any height. Instead of relying on weights and pulleys like older windows, modern balances use a spring or torsion rod to counterbalance the weight of the glass and frame. When a balance fails, the window either won't stay open, slams shut on its own, or feels heavy and difficult to raise.

Balances wear out because the spring loses tension over years of opening and closing. A single window might have two balances — one on each side of the sash — and often both fail around the same time. Replacing them is a straightforward job that takes 30 to 60 minutes per window and costs $15 to $40 in parts, depending on the balance type.

Key Takeaways

  • Window balances come in three main types — spiral, block and tackle, and constant force — and you must match the exact type and size when replacing them.
  • The job requires removing the sash from the frame, which means taking off the stop molding and sometimes the parting bead, then sliding the balance out of its channel.
  • Measure the length of the old balance and note the part number or take a photo before you buy a replacement, because balances are not interchangeable across window brands.
  • You can buy replacement balances at home improvement stores, online retailers, or directly from the window manufacturer, and installation takes basic hand tools only.

Identify which type of balance you have

The three common balance types look and work differently, and you must replace yours with the same type. Spiral balances have a twisted metal rod visible on the side of the sash channel — they look like a corkscrew. Block and tackle balances use a pulley system and sit inside a rectangular housing in the frame. Constant force balances (also called cartridge balances) are compact cylinders that fit into a channel and are the most common in newer windows.

Open your window and look at the sides of the frame where the sash slides. You will see the balance mechanism either as a visible rod, a housing box, or a small cylinder. Take a photo or write down what you see. If you can't identify the type from looking, remove the stop molding (the thin trim that holds the sash in) and look inside the channel — the balance will be visible once the sash is out.

Remove the sash and old balance

Close the window and use a flathead screwdriver to gently pry off the stop molding — the thin strips of wood or vinyl on the inside of the frame that hold the sash in place. Start at one corner and work slowly to avoid splitting the wood. Once the stop is off, the sash will slide out toward you. Set it on a flat surface where it won't roll.

Look inside the channel where the sash sat. You will see the balance mechanism on one or both sides. For spiral balances, unscrew the balance from the sash frame and slide it out of the channel. For block and tackle or constant force balances, there is usually a clip or screw holding the balance in place — remove it and slide the balance out. Some balances are held by a pin that you straightforward pull out. Keep any screws or clips in a small container so you don't lose them.

Measure and buy the correct replacement

Measure the length of the old balance from end to end. Write down the measurement and the balance type. Look for a part number printed on the balance itself — it may be stamped or printed on a label. Take a photo of the old balance and the part number if one exists. This information is what you need to order a replacement.

Go to a home improvement store, a window supply retailer, or the website of your window manufacturer. Search by part number if you have it, or describe the balance type and length to a sales associate. Balances are specific to window brands and styles, so do not assume a balance from one brand will fit another. If you cannot find the exact match, contact the window manufacturer directly — they can usually tell you the correct part number from your window's serial number or age.

Install the new balance

Slide the new balance into the channel on the side of the frame where you removed the old one. The balance should fit snugly and sit flush with the frame. If it has a clip or screw, fasten it now. For spiral balances, align the screw holes and drive the screws in by hand first, then tighten them with a screwdriver. For constant force balances, make sure the cylinder is fully seated in the channel before securing the clip.

Repeat the process on the other side of the window if there is a second balance. Once both balances are installed, slide the sash back into the frame from the outside, making sure it sits in both channels. Close the window gently and test it — it should stay at any height you leave it and not feel heavy or loose. If the window feels too light or too heavy, the balances may not be the correct size. Check the part number and contact the retailer or manufacturer.

Reinstall the stop molding

Once the sash is back in place and the balances are working correctly, nail the stop molding back onto the frame. Use a finish nail (thin and small-headed) and a hammer, driving the nails through the molding into the frame. Space the nails about 16 inches apart along the length of the molding. If the original molding split or cracked during removal, you can buy replacement stop molding at a home improvement store and cut it to length with a miter saw or handsaw.

Test the window one more time by opening and closing it several times. The sash should move smoothly and stay in place when you stop moving it. If the window sticks or the sash feels uneven, check that the stop molding is not pressing too hard against the sash — you may need to adjust it slightly.

When to call a professional

If the window frame is cracked, warped, or the channels are damaged, a balance replacement will not fix the problem. Damaged frames need repair or replacement by a window contractor. If you remove the sash and cannot identify the balance type, or if the balance does not slide out easily, the window may have a design you are not familiar with — a photo sent to the manufacturer or a local window shop can help you figure out the next step.

Some older windows use weight and pulley systems instead of balances, and those require a different approach. If you see a rope and pulley inside the frame, or if the sash is held by a cord that runs over a pulley at the top, you have a weighted window and should consult a window specialist or the manufacturer before attempting repair.

Frequently Asked Questions

Can I replace just one balance if only one side is broken?

You can, but it is not recommended. Balances wear at similar rates, so the other side will likely fail soon after. Replacing both at the same time costs only a little more and saves you the trouble of doing the job twice in the next year or two.

What if I can't find the exact part number?

Contact the window manufacturer with your window's age, style, and serial number if it is visible on the frame. They can look up the correct balance part number. If you don't know the manufacturer, a local window repair shop can often identify it from the frame design and help you order the right part.

How do I know if the new balance is the right size?

The balance length must match the old one exactly — measure from end to end. The balance should also fit snugly in the channel without forcing or leaving gaps. If it is too short or too long, it will not work correctly and the window will feel unbalanced or stick.

Do I need special tools to replace a window balance?

No. A flathead screwdriver, a hammer, a tape measure, and a screwdriver are all you need. Some balances use clips instead of screws, but you can remove and install those by hand or with a screwdriver.

What if the sash won't slide back in after I remove the stop molding?

Make sure the balance is fully seated in the channel and not sticking out. Check that the frame channels are clean and free of paint or debris. If the sash still binds, the frame may be slightly warped — try sliding it in from the opposite side or gently tapping it with a rubber mallet to seat it evenly.