I know that in the past if I've previously downloaded OS X El Capitan from the App Store I could download it again from the Purchased tab in the App Store, however now that macOS Sierra is out I'm not able to download it as it says INSTALLED (and grayed out) even on a system running OS X Mountain Lion 10.8.5. Pressing any key combination, e.g control, option, etc. doesn't change the button to DOWNLOAD. All other previous versions of OS X can still be downloaded from my Purchased tab in App Store as they still say DOWNLOAD and are not grayed out, thus active.
The image below is from a system running OS X Mountain Lion 10.8.5, does not have OS X El Capitan installed on it or even the Install OS X El Capitan.app on its filesystem, yet it says it's installed and grayed out.
El Capitan Os X Download
Go to the App Store and open the El Capitan page there. Then you must click on the ‘Download; button on the El Capitan Page. If your Mac PC is compatible to perform this upgrade, a file named ‘Install OS X El Capitan’ will download to your Applications folder. The installer will open automatically after finishing the download process. You can El Capitan Download from here on your Mac devices for free. Follow our El capitan guidance to install El Capitan easily. Download El capitan 10.11.