Every sliding sash window needs something to counterbalance the weight of the sash — otherwise the window would slam shut the moment you let go, or be too heavy to lift in the first place. For over 300 years that job was done by cords, pulleys and cast-iron weights hidden inside the box frame. From the mid-20th century onwards, a cheaper alternative appeared: the spring balance (also called a spiral balance).
If you’re repairing or replacing sash windows in a period property, sooner or later someone will suggest swapping the cords and weights for spring balances. Here’s how the two systems compare, and why we almost always recommend keeping the original mechanism.
How the traditional cord-and-weight system works
In a traditional box sash window, each sash hangs on two cords. The cords run over brass or cast-iron pulleys set into the pulley stiles at the top of the frame, and down into hidden weight pockets inside the box, where they’re tied to cast-iron or lead weights that exactly counterbalance the sash.
The result is beautifully simple: the sash stays wherever you leave it, glides up and down with fingertip pressure, and the whole mechanism is serviceable through a small pocket cover in the frame. For a refresher on how the pulleys fit into the rest of the window’s anatomy, see our guide to sash window parts.
Strengths:
- Longevity. The weights and pulleys are effectively permanent — many are still working after 150+ years. Only the cords wear out, and re-cording is a routine, inexpensive repair.
- Smooth, balanced operation. A correctly weighted sash feels weightless, regardless of how heavy the sash is. This matters when retrofitting double glazing, because the weights can simply be increased to match the heavier glass.
- Repairability. Every component is accessible and replaceable. Nothing is sealed, riveted or moulded shut.
- Authenticity. Listed buildings and conservation areas often require the original mechanism to be retained — and buyers of period homes notice the difference.
Weaknesses:
- Needs a deep box frame to house the weights, so it can’t be fitted into a slim modern frame.
- Re-cording requires partially dismantling the window — a job best done by someone who knows what they’re doing.
- Snapped cords left unrepaired lead to sashes being wedged or nailed shut, which is how many windows end up painted shut and rotting.
How spring balances work
A spring balance replaces the cords, pulleys and weights with a pair of slim tubes, one each side of the sash. Inside each tube a spiral rod and torsion spring provide the lifting force. The tubes screw into the frame, the sash clips onto the bottom of them, and the spring tension is adjusted (by twisting the rod) until the sash holds its position.
Spring balances were developed for modern, slimline windows that have no box to hide weights in, and they’re standard on most new uPVC and aluminium “sash-style” windows as well as many off-the-shelf timber replacements.
Strengths:
- Slim frames. No weight pockets needed, so the frame can be much shallower — the main reason manufacturers use them.
- Cheap to fit in new manufacture.
- Reasonable performance when new and correctly tensioned.
Weaknesses:
- Limited lifespan. Springs fatigue, lose tension and eventually fail — typically within 8–15 years, far sooner than a pulley wheel. When they fail, the sash drops or won’t stay open.
- Fiddly to match and replace. Balances come in dozens of lengths and tension ratings; the correct replacement must match the exact sash weight, and discontinued sizes are a recurring headache.
- Poor tolerance of weight changes. Re-glaze the sash with heavier glass and the balances must be replaced with stronger ones — you can’t just add another pound of lead as you can with weights.
- Not period-correct. In a Victorian or Georgian box frame, blanked-off pulleys and surface-mounted spring tubes are obvious, and the conversion is usually irreversible without new pulley stiles. Conservation officers generally won’t approve it on listed buildings.
Side by side
| Cords, pulleys & weights | Spring balances | |
|---|---|---|
| Typical lifespan | Pulleys & weights: indefinite; cords: 20–40 years | 8–15 years |
| Repair | Re-cord, reuse everything else | Replace the whole balance |
| Operation | Smooth, truly counterbalanced | Stiffer, degrades as springs fatigue |
| Frame required | Deep box frame | Slim frame |
| Heavier glazing later | Add weight | Replace balances |
| Period authenticity | Original | Modern substitute |
Our recommendation
If your windows are original box sashes, keep the cords and weights. The mechanism is the most durable counterbalance system ever fitted to a window, and problems that look terminal — snapped cords, seized pulleys, sashes painted shut — are routine restoration work. As with rotten timber, the answer is almost never to rip out the original: it’s to repair it. Swapping weights for spring balances in a box frame saves little money up front and costs you smooth operation, repairability and character for decades afterwards.
Spring balances have their place: slim modern frames, secondary buildings, or budget replacements where there is no box frame to work with. But fitting them into a period window is a downgrade dressed up as a modernisation.
Snapped cords or failing balances? We can help
We re-cord, re-balance and overhaul sash windows across Hertfordshire — including adjusting weights when retrofitting slim double glazing, and converting tired spring-balance windows back to proper cords and weights where the box frame allows. Get in touch for a free assessment, or call us on 01727 638 999.