Can I roll-back a new iMac to an older OSX?

I am ordering a bunch of new iMacs early next year and I assume they are going to have OS X Sierra and I have an image on an external disk for El Capitan and I am wondering if these new macs will allow me to deploy this image to them since they will come with the new OS X as default.

In the past I have tried using an image with an older OS X on a new iMac and I just get this 'white do not enter' symbol and it seems like the hardware in the new macs prevent me from putting an older OS on it.

Also if it is possible do you know whats the oldest OS X I am allowed to put on any new iMac?

If you need more info let me know.
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Eoin OSullivanConnect With a Mentor ConsultantCommented:
If you order the iMacs that have the same hardware as the current iMacs then it SHOULD be possible to install the version of OSX which was available when those iMacs were released .. BUT it will involve some work.

All NEW Macs come with a hidden recovery partition on the Hard Drive .. the latest version of OSX is installed on that partition so if you buy any iMac from now on this will be OSX Sierra 10.12

But if you have an install image on an external hard drive it will be possible to wipe the HD and install it as long as the Hardware is the same.

The current 21.5 and 27 inch iMacs are largely unchanged for the last 3 years so the last couple of versions of OSX (10.10 & 10.11) should work fine.

WARNING .. there are new iMacs probably due in early 2017 .. there is a chance that apart from a new processor there may be other hardware changes which may well make it impossible to install the older OSX versions so if you wait to next year to buy then bear in mind any hardware changes. If the OSX system downgrade fails you can always re-install OSX Sierra from the recovery partition in any case.
strungConnect With a Mentor Commented:
Generally speaking, you cannot boot a Mac from an operating system older than the one it shipped with.
serialbandConnect With a Mentor Commented:
The older OS X installers will likely not have driver software to support the newer hardware.  You'd likely also have to hack the installer's plist file to allow the install.  It would be messy.
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