Window Balance — Double-Hung Sash Spring Types and Repair

A window balance is an internal spring or coil mechanism inside a double-hung window frame that counterbalances the sash weight and holds it open at any position.

Window Balance diagram — labeled parts and installation context

What It Is

A window balance replaces the rope-and-pulley-and-counterweight system used in older wood windows. In modern vinyl and aluminum windows, the balance is a self-contained cartridge that fits inside the window frame jamb channel. As the sash is raised, the balance spring stores tension; when the sash is lowered, the spring releases tension. This stored energy offsets the sash weight, allowing the sash to remain in any open position without sliding down.

Balance systems also provide the inward force that keeps the sash pressed against the weatherstripping and the frame, which is important for both air sealing and security. A failed balance allows the sash to move freely and may allow the sash to fall — a safety hazard, especially in upper-story windows accessible to children.

Window balances are proprietary components. The correct replacement balance depends on the window manufacturer, the sash weight, the sash height, and the specific balance type. There is no universal replacement balance — using the wrong one results in a sash that either slams shut or cannot hold its position.

Types

Spiral balances (also called tube balances or clock-spring balances) use a coiled flat spring inside a cylindrical tube that attaches to the sash at the bottom and to the frame at the top. Block-and-tackle balances use a pulley arrangement with a spring that provides more consistent force across the travel range of the sash. Constant-force balances use a coil of flat stainless steel that uncoils as the sash rises, providing nearly constant force at any height — used in very heavy or oversized sashes. Coil balances are a variation of the spiral type and are used in narrower jamb channels.

Where It Is Used

Window balances are found in virtually all modern double-hung and single-hung windows in residential and light commercial construction. They are inside the jamb channels on each side of the sash — a typical double-hung window has two balances for the lower sash (one on each side) and sometimes two more for the upper sash depending on the design. They are not used in casement, awning, or sliding windows, which use a different support mechanism.

How to Identify One

If your double-hung window sash will not stay open, drifts down on its own, or is very difficult to raise or lower, a failed balance is the most likely cause. On vinyl windows, the balance is visible as a plastic or metal cartridge in the jamb channel when the sash is tilted in for cleaning. Spiral balances look like a narrow metal tube with a spiral rod at one end. Block-and-tackle balances look like a small pulley assembly.

Replacement

Most modern double-hung windows allow the sash to be tilted in from the inside for cleaning and balance replacement without removing the sash from the frame. Tilt the sash in, lift it out of the balance shoes, and slide the shoe and balance out of the channel. Identify the balance by the manufacturer, balance type, and balance code stamped on the tube or cartridge — the code indicates the spring tension, which must match the sash weight. Order an exact replacement from the window manufacturer or a window parts supplier. Slide the new balance into the channel, hook the sash back onto the shoe clips, and test the sash for smooth travel and hold.

Window Balance — FAQ

How do I know if my window balance is broken?

The most obvious sign is a sash that will not stay open and slides back down when released. Other signs include a sash that is unusually difficult to raise or lower, a sash that feels like it is binding in the jamb channel, or a visible broken or disconnected balance component when the sash is tilted in. Spiral balances occasionally break at the attachment point where the coil connects to the shoe or the jamb anchor.

Can I replace a window balance myself?

Yes, on most modern double-hung windows. The sash tilts inward for cleaning access, which also allows the balance to be reached without removing the sash from the frame. The main challenge is identifying the correct replacement balance — you must match the manufacturer, balance type, and balance tension code precisely. Order from the window manufacturer's parts department or a specialty window parts supplier using the code stamped on the old balance.

What do the numbers stamped on a spiral balance mean?

Spiral balance codes typically indicate the balance length and the spring tension. For example, a code of 2420 might mean 24-inch length with tension code 20. The tension code corresponds to the weight of the sash — heavier sashes require higher tension codes. Using the wrong tension code results in a sash that either slams shut or drifts down, so matching the original code is critical.

Why does my window balance keep breaking?

Repeated balance failure is usually caused by a sash that is heavier than the balance is rated for, by a balance exposed to extreme temperature swings that fatigue the spring, or by a balance that was incorrectly installed with excessive pre-tension. If balances fail repeatedly on the same window, consider whether the replacement tension code is correct for the actual sash weight, or have the window evaluated by the manufacturer's service representative.

Do I need to replace both balances at the same time?

It is generally recommended to replace both balances on the same sash at the same time, even if only one has failed. Balances wear at similar rates, and the remaining balance is likely close to failure as well. Replacing both ensures even tension on both sides of the sash and avoids having to repeat the job shortly after. The cost difference is small since you are already doing the work.