How do I replace a spiral balance on a sash window?

Telling a spiral balance sash from a corded one, matching a replacement on length and load, and winding the new one to the right tension so the sash stays where you put it.

Difficulty: intermediate
Time: 1 hr 15 min
Read: 4 min
Safety: caution

Short answer

A spiral balance is a twisted steel rod inside a tube fitted into the frame jamb, used on modern timber and uPVC vertical sliders instead of cords and weights. Replace in pairs, match the tube length and the spring load to the sash weight, and set the tension by winding the rod before hooking it to the sash.

Vertical sliding sash windows come in two quite different families. Traditional boxed sashes use cords running over pulleys to weights hidden in the frame. Modern timber and uPVC vertical sliders use spiral balances instead: a spring-loaded twisted rod in a tube let into the jamb. If your sash window has no box and no pulleys, it is the second kind and cords are irrelevant to it.

Safety

Spiral balances are springs under tension and they will spin violently if released carelessly, which can injure a hand or an eye. Always de-tension a balance with the correct tensioning tool or by controlled release, wear eye protection, and never let go of a wound balance.

What you'll need

A spiral balance tensioning tool
Eye protection
A second person to hold the sash
Replacement balances, matched as a pair
Screwdrivers and a small pry tool

Step by step

Confirm what you have.Look at the jamb beside the sash. A spiral balance shows as a narrow tube with a twisted rod visible inside it, usually behind a slim cover strip. A boxed sash has a pulley wheel near the top of the jamb and a hollow box behind. The repairs have nothing in common.
Support the sash before releasing anything.A sash whose balance has failed is already unsupported on that side. Have a second person hold it, or prop it, before you start work.
Release the balance from the sash.The balance's shoe or tip hooks into a fitting on the sash stile. On most designs the tip is unhooked with a screwdriver or by winding down the tension first. Do not simply pull it out — a tensioned balance will spin.
De-tension carefully.Use the tensioning tool to take the wind out of the rod in a controlled way. If you have no tool, this is the point at which to get one; improvising with pliers is how people are injured on this job.
Remove the sash.Most vertical sliders tilt inwards or lift out once both balances are released. Follow the manufacturer's method — many have small release catches at the top of the sash. Two people, and lay the sash somewhere safe.
Take the old balance out and record everything.The tube length, the rod diameter, the spring colour code or stamped marking, the type of tip and shoe, and the fixing arrangement. Photograph it all. The colour code identifies the spring load and it is what you must match.
Weigh the sash if you have any doubt.Bathroom scales. Balances are rated to carry a range of sash weights, and a pair rated too light will not hold the sash up while a pair rated too heavy will push it up on its own.
Replace in pairs.Always. A new balance on one side and a tired one on the other gives a sash that racks and jams, and the old one will fail within a year anyway.
Fit the new balances and set the tension.Fix the tube into the jamb, then wind the rod with the tensioning tool to the number of turns the manufacturer specifies for that sash weight, and hook the tip to the sash shoe while holding the tension.
Test and fine-tune.The sash should stay put anywhere in its travel, neither creeping down nor rising. If it drops, add a turn to each balance; if it rises, take one off. Always adjust both sides equally.
Check the sash still locks and seals.Balance tension affects where the sash sits. Check the meeting rail alignment, the lock engagement and the seals before finishing.

If it doesn't work

Sash slams down when released

Cause: One or both balances have lost tension or the spring has failed — Fix: Replace as a pair. A failed balance is not economically repairable and re-tensioning a broken spring does nothing.

Sash creeps up on its own

Cause: Over-tensioned, or balances rated for a heavier sash — Fix: Take a turn or two off each side. If it still rises, the spring rating is wrong for the sash.

Sash jams diagonally as it slides

Cause: Unequal tension between the two sides — Fix: Set both balances to the same number of turns, then adjust equally. Racking is almost always a mismatched pair.

The balance keeps unhooking from the sash

Cause: A worn shoe or the wrong tip type — Fix: Replace the shoe as well as the balance. They are sold together for exactly this reason.

Questions people ask

Can I convert a spiral balance window to cords and weights?

No. There is no box in the frame for weights to hang in. The frame construction is fundamentally different.

What does the colour on the spring mean?

It is the manufacturer's load rating code. Different makers use different colours, so it identifies the spring within a range rather than universally — take the old balance or a clear photograph to the supplier.

My uPVC vertical slider uses something else entirely.

Some use constant-force or block-and-tackle balances in a different arrangement. The principles are the same — match, replace in pairs, set tension equally — but the parts are not interchangeable.