A chair that sinks is not a comfort annoyance, it is a fit failure that gets worse over the day. Seat height is the adjustment everything else is set from: OSHA’s workstation guidance puts the forearms roughly parallel to the floor at the keyboard with the shoulders relaxed, and a chair drifting downward from that position quietly raises your shoulders and bends your wrists back, hour by hour, without ever announcing that it has moved.
The good news is that the cause is nearly always one component, it is inexpensive, and it is replaceable without special skills.
What is actually inside
The vertical adjustment on a task chair is a pneumatic gas cylinder, sometimes called a gas lift or gas spring. It is a sealed steel tube containing compressed nitrogen and a piston. Pressing the lever opens a valve; your body weight or the spring pressure moves the piston; releasing the lever closes the valve and the column holds position because the gas has nowhere to go.
Sinking means the seal has stopped holding gas on the load-bearing side of that piston. Seals wear. This is a consumable part with a finite life, not a defect that implies you bought badly.
Diagnose before buying anything
If height stays set but movement makes noise, a squeaky chair is a different fault; use that guide to locate the source of the sound.
Four faults produce similar symptoms, and only one of the four is something other than a cylinder replacement. Run these checks in order.
Does it sink only under load? Sit down without touching the lever and watch the seat over a minute or two. If it descends under your weight but holds indefinitely when the chair is empty, that is a leaking cylinder seal. This is the common case.
Does it drop instantly the moment you sit? An immediate drop to the bottom of travel, rather than a slow settle, usually means the cylinder has lost its charge completely. Same fix, same part.
Does it move when you brush the lever? If the height only changes when something contacts the lever or the underside of the seat, and it otherwise holds fine, the fault is in the release linkage, not the cylinder. The lever cable or paddle has gone out of adjustment, or the mechanism has shifted on its mounting bolts and now holds the valve slightly open. Check the four bolts securing the mechanism to the underside of the seat, and check that the lever returns fully to its rest position. Tightening those bolts or reseating the cable fixes this at zero cost.
Does it refuse to rise but hold its height? A cylinder that has lost pressure will still hold a low position under weight while no longer having the force to lift an unloaded seat. Also a cylinder replacement.
Only the third case is not a cylinder. Establishing which one you have takes two minutes and it is worth doing before you order a part.
The repair options, ranked
1. Replace the gas cylinder
The only fix that restores height adjustment. Replacement cylinders are widely available, generic across a large number of chairs, and cheap relative to the chair.
The removal is the part people find surprising: the cylinder is not bolted in. It is held by a friction fit on a tapered cone at both ends, one into the base, one into the seat mechanism. Years of load drive that taper tight, so it has to be driven apart rather than unscrewed.
The sequence:
- Turn the chair over and remove the casters and the base if it makes access easier.
- Free the base from the cylinder. Penetrating oil into the joint, left to sit, then repeated taps with a rubber mallet on the base hub around the cylinder. Do not strike the cylinder body itself.
- If it will not release, a pipe wrench gripping the cylinder body with the base supported gives you leverage to twist the taper free. Twisting breaks a stuck taper far more reliably than pulling.
- Separate the seat mechanism from the cylinder the same way.
- Fit the new cylinder into the base, drop the seat on, and sit on it. Your weight seats the taper. No adhesive, no fasteners.
Safety, and this one is not optional: the cylinder is a pressure vessel. Do not cut it, drill it, grind it, or put it in a fire, and do not attempt to open one. Dispose of the old unit as pressurised waste according to local rules.
2. A clamp on the column (fixed height, no adjustment)
A hose clamp tightened around the exposed inner column, or a length of PVC pipe split lengthwise and fitted over it, physically stops the seat travelling below that point.
This works, costs almost nothing, and it is genuinely reasonable as a stopgap. But it converts the chair into a fixed-height chair, and it locks in whatever height you set it at. That is only acceptable if the height is right for you and your desk, so set it deliberately rather than wherever the clamp lands. The ergonomic chair and desk sizer will give you the target to clamp at.
Treat it as temporary. CCOHS’s position on office seating is that adjustability is what makes a chair suitable for a given person, and a permanently fixed chair has given that up.
3. Warranty claim
Worth checking before either of the above. Gas cylinders are frequently covered, sometimes on a shorter term than the chair frame, and the manufacturer will often ship a replacement cylinder without requiring the chair to go anywhere. Read the exclusions rather than the headline warranty length.
Buying a replacement that fits
Use the chair manufacturer’s replacement-part guidance before ordering. Identify your exact chair model and the original cylinder’s part number where available, then ask the manufacturer to confirm each specification below.
Class and stroke. Check these as separate specifications; do not infer stroke length from a class label. Confirm the required cylinder class and stroke in the manufacturer’s documentation.
Dimensions and actuation. Match the specified taper and diameters, collapsed and extended lengths, and actuation type. Ask the manufacturer to confirm the resulting seat-height range rather than choosing by stroke alone.
Load requirements and compatibility. Match the chair manufacturer’s load requirements and order only a replacement confirmed as compatible with your exact chair model. Do not treat a matching dimension or a higher advertised cylinder rating as confirmation of compatibility.
Making the next one last
Cylinder life is mostly a function of load and cycles, and neither is fully in your control. Two things are:
- Do not use the chair as a step stool. Standing on a seat puts a shock load through a component sized for seated weight, and it is a common cause of early failure.
- Buy above your weight, not at it. The weight rating on a chair is the figure the cylinder, base and mechanism were engineered against. Sitting near the top of that rating every day is what the rating is for, but sitting well under it is what makes parts last.
Once the chair holds height again, it is worth resetting the whole fit rather than returning to whatever position the failure left you in. Fitting an ergonomic chair to your body and your desk covers the order to make the adjustments in, starting from the keyboard rather than the floor.
For other adjustment or repair questions after replacing the cylinder, browse the chair setup and maintenance guides.
If the chair turns out to be worn beyond the cylinder, the specifications worth insisting on next time are in how to choose an ergonomic chair, and the minimum-seat-height trap that catches shorter sitters is covered in ergonomic chair vs gaming chair.
