Most modern British homes have at least one uPVC door — usually the front or the back. They are warm, low-maintenance and secure when they work, but their locking is more complicated than the single lock most people picture, which is why repairs can be confusing and occasionally expensive.
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The lock types you will find
A uPVC door usually relies on a multipoint lock rather than a single latch. Turn the key or lift the handle and a set of hooks, rollers and bolts extend along the edge of the door, locking it to the frame at several points. The parts you are most likely to hear about are:
- The cylinder. The barrel the key goes into. It is a separate, replaceable component and the usual target for someone trying to snap or pick a door.
- The gearbox. The central unit that links the handle and cylinder to all the locking points. A failed gearbox often leaves the door impossible to lock or unlock.
- The multipoint strip. The long metal rail running up the door edge, carrying the hooks and bolts to the top and bottom.
- The keeps. The metal receivers in the frame that the hooks and bolts engage with. If a door drops, these are often where the misalignment shows up.
Why uPVC locks fail
The common failures are worth knowing because they change the cost of a fix:
- Handle drops or feels loose. Often the gearbox starting to fail, or the handle fixings working loose.
- Door will not lock. Frequently a gearbox fault, a dropped door, or the door swollen in damp weather and no longer meeting the keeps.
- Key jams or turns stiffly. Usually the cylinder. If it becomes hard to turn, do not force it — a snapped key or cylinder is more expensive.
- Snapped cylinder. A well-known break-in method. Anti-snap cylinders are designed to resist it.
- Sagging door. Hinges and door alignment, which then puts strain on the whole locking system.
Replacing the cylinder
The cylinder is the simplest and cheapest part to replace, and the one most often done for security rather than repair — for example when keys go missing or a tenancy changes hands. The key measurement is the cylinder's overall length and how it is split between the inside and outside of the door; a replacement has to match, or it will not sit flush and can be leveraged. Choosing an anti-snap, anti-pick, anti-drill cylinder is worth the small extra cost. Look for one that carries a recognised standard — our guide to BS3621 and insurance-approved locks explains what those marks mean.
Replacing the whole mechanism
When the gearbox or the full multipoint strip has failed, the fix is a mechanism swap. This is fiddlier than it looks: multipoint locks are largely brand-specific and are often made in sizes that change over the years, so a matching replacement has to be sourced, and sometimes the door has to be re-aligned afterwards. That is why a mechanism replacement costs more than a cylinder change, and why a firm that quotes for "a new lock" without asking about the door is a firm to be careful with.
Ask for the make and model of the replacement, and whether the guarantee covers the whole mechanism or only the part supplied. Keep the invoice, and keep any keys that come with it.