Herman Miller Aeron: pitfalls of trying to buy second hand

The Herman Miller Aeron has been available on the market since 1994 and since then there have been a few subtle changes to the parts since then; the chairs have been referred to as the mark I (production runs from 1994 - ~ Aug 2004) and mark II (production after Aug 2004). All of this matters to avoiding some subtle pitfalls if you are looking at a second hand Aeron chair from various office clearance companies or ebay.

The chair has many options for configuring the type mesh and lumbar support but the main configuration that can't change involve the support for full front and rear tilts (fully loaded) or just rear tilt. The chair can also be configured to have no arms, fixed position arms or up/down adjustable arms: the arms can also be fixed or can be pivoted in 3 positions (in/out/middle).


Below are some of my experiences from trying to find my own Aeron chair, and subsequently fully refurbishing a 1999 chair, which may be useful to anyone trying to determine if an Aeron bargain is worth the risk.
_{The dates that are noted are based on my experience of a range of chairs in my office and also from speaking to various UK Herman Miller dealers and as such they may not be wholey accurate.}

Apartment Therapy have a nice introductory article from 2013 detailing how you can refurbish an Aeron chair: it's presented to be a fairly easy task. For reference, judging from the Apartment Therapy pictures it would appear that they demonstrate a late 2004-2006 chair based on hip bolts and seat link bolt head but more on this later.

Herman Miller provide an Aeron service instruction manual (~2006) that is very useful for reference.

^{(c) Herman Miller}
Let's look at some of the components and trying to date the chair. If you're lucky, there will be factory sticker on the chair or on the inside of the tilt mechanism (that's the block underneath the seat pan where the gas lift connects). However, looking online you're unlikely to be able to see this so we need other methods.

Seat Pan

The seat pan is removable and is connected to the frame via a hip bolt at either side of the chair. The seat pan is also connected via 2 seat links underneath the seat - these are held in place via a nut and a threaded bolt with ether a 4mm head (older chairs pre ~2006) or a T-27 head.

The seat pan's embedded threaded inserts, that particularly on older chairs, can rotate as you try to unscrew the hip bolts to remove the seat pan. Herman Miller's own service manual notes this and offers a 'solution'. If the threaded bolts come out with the hip bolt screw then separating the threaded insert from the bolt at this stage is problematic and difficult. In this situation you need to find yourself replacement hip bolt if you are trying to replace the seat pan, otherwise you can screw the threaded insert back into place.

^{(c) Herman Miller}
The seat pan is probably one of the things that you are most likely to replace, whether its due to rips in the mesh fabric or the mesh has become too saggy/less supportive or because the mesh stinks. I found attempts of ridding then mesh of smells was impossible so be aware of smokers or pets or even over zealous perfumed previous owners.

I had all the problems of stinking/less supportive mesh and also a threaded insert coming lose from a 1999 chair, identified via a sticker buried on the seat back frame.

Furthermore, the original seat pan was more prone to cracking around the hip pivot as it was not re-enforced (see the leading image). The redesigned seat pans can be identified by the ridges either side of the hip pivot:
^{Redesigned re-enforced seat pan}
New replacement seat pans are ~140GBP.

Hip Bolts/Pivots

As these are mounted in the frame, the older bolts (pre June 2003) can be identified by the glossy plastic around the 6mm bolt head; these bolts are in fact 2 separate egg shells that allow the bolt to move when mounted in the chair and the shells are prevented from slipping out of the chair frame via a metal retaining ring. When examining the chair from above, looking at the hip bolt connecting to the seat pan, you can identify the older bolts with the round glossy area that protrudes past the seat frame towards the seat pan. You may also notice the retaining ring on the inside.

^{(c) Herman Miller}
The newer pivots have a matt/powder finish around the bolt head and when mounted in the chair frame, you can notice 2 side and one top "ridges" that are part of the guides that ensure the hip pivot are inserted correctly. These guide ridges aren't visible on the older hip pivots once mounted. The newer pivots are a single piece that does not allow the bolt to become free from the housing/bushing unlike the previous version which consists of 5 seperate pieces (metal bolt, metal retaining ring, 2x 1/2 plastic bushing housing and metal bolt-bushing) and also does not require a retaining ring.

^{new hip pivot on the left, older hip pivot + bolt with (matt) retaining ring on right. Notice the older hip bolt is connected to a seat pan (silver/shiny) threaded insert that is not flush with the seat pan and the threaded insert may be prone to separating}
To remove the old style pivot from the seat frame you will need to pry open the retaining ring to free the plastic shell/bushing from the seat frame.
^{old on the left, new on right - see the rabitt ears/holes of the retaining ring securing the glossy plastic 1/2 shell bushing housing}

The old and new hip pivots are interchangeable in the seat frame. However if you are sourcing replacement avoid the older style pivots - aside from being a pain to fit into the frame due to the necessary retaining ring (secures the plastic shell/bushhing together) I've seen a few cracked plastic bushing housings on chairs in the office that eventually result in the seat pan not being secured to the frame and rendering the chair unusable.

Replacement hip pivot bushings are ~10GBP on ebay.

Arm Rests

If the chair has factory fitted adjustable up/down arms then you can use this to determine the age of the chair. The original adjustable arms had a thumbwheel lock compared to the flip lock that was introduced ~ Aug 2004.

If you chair has no arms rests, you can easily add them by removing the 4x hex bolts (5mm head) securing the seat back and then attaching the arms to the channels in the frame. If the chair has been fitted with fixed positioned arm rests, you can adjust the height of these using the same method after discarding the plastic filler - the head of the bolt securing the arm rest to the chair frame requires a T-27 screw bit.

^{(c) Herman Miller}

Additionally, if you find that the arms are loose even when you have tightened the thumbnail locks fully, there is a simple solution. The arms are held to the chair via the long bolt: to ensure a tighter fit, remove the chair back and with the thumbwheel loose, support the arm and start to tighten the bolt stopping when the arm is no longer slipping/sagging. Ensure that the arm can still slide up and down relatively easily. At this point, replace the seat back and now when you tighten the thumbwheel lock, the arms should provide a tighter grip.

One important note: the new style flip lock arm rests are not intended to be removable and appear to be thread locked although that doesn't stop some good folks trying, so be careful when looking for spare arm rest parts for Frankenstein'ing your chair.

New adjustable flip lock vinyl arm rests are ~180 GBP: thumbwheel lock style arms are no longer produced.

Seat Back

This is the simplest part of replace/remove as its held to the frame with 4x hex bolts (5mm head). The same comments regarding smells/rips to the mesh frabric apply here too.

New replacement seat back are ~90GBP.

Lumbar Support

The original/basic lumbar support tends to be hard wearing and is usable even when it is split. Herman Miller introduced in 2002 their Posture Fit (looks like an X on the back of the chair) support that can be an option for new chairs or can be retrofitted on existing chairs: most chairs that include the Posture Fit will be factory fitted.

^{(c) Herman Miller}
New replacement lumbar supports are ~60GBP but many ebay items are ~20GBP.

Gas Lift

^{2022 Gas lift replacement experience}
Aside from the seat and back mesh, this is the most likely failure point, with sticking or self lowering gas cylinders being the most common problems. The extending gas cylinder is also the main reason for the chair to wobble.

Unfortunately, replacing the gas lift is reported to be the most difficult refurbishment task. Herman Miller sell a "lift off tool" but this is relatively expensive (~100GBP) and hard to find.

^{(c) Herman Miller}
A lot people recommend using a pipe wrench to twist off the cylinder but others who have been unsuccessful with this have also tried knocking out the cylinder from within the tilt mechanism using an 11/16th socket. The HM service manual details the same procedure as a means to retain the gas cylinder for swapping out the tilt mechanism, although they use a service tool for the same purpose.

^{(c) Herman Miller}
New OEM replacement gas lift are ~ 65GBP but any universal 28mm/50mm gas lift will work.

Final thoughts

Hopefully this helps outline potential pitfalls for anyone looking at second hand Aerons. The main areas to consider is the age of the chair (looking at the arm rest thumbnail vs flip locks and hip bolt styles) due to the lacking of support in the mesh seat and issues of the gas lift. Whilst the chairs do appear to last - in our office there are many Aerons that appear to be from the late 1990s (one guy having had the same chair from new since 1999) - it is still worth considering the additional costs and potential trouble (see the seat pan bolts/gas lift removal) of having to replace the chair parts when sizing up any bargain you've thought you found.

If you have a selection of chairs of similar price, certainly look for chairs with flip lock arms to at least limit the age of the chair to late 2004, instead of potentially a decade earlier.