If you are running an Exchange Server for a couple of years you will soon or later notice that the free disk space on the Exchange install disk shrinks after every CU install. At this point, you can take in consideration to remove old OWA versions to free up disk space.
The size of these folders located under the path below can take be up to 2GB for each version:
The question is:
Is it safe to remove old owa versions?
Unfortunately, I never found a satisfying answer to this question from Microsoft. On technet forums you can find answers like You can do it but it is not recommended.
This it is not a statement that makes you remove old OWA versions with a peace of mind.
So I gave it a try in our test environment.
As you can see in the picture below there are some OWA version folders in the /Owa directory. The last modified date matches the date we have installed CU12 on this server. These folders belong to the current OWA version at that time. ( before we installed CU14).
In the /Owa/prem folder on the right side, you see the real OWA version folders with about 1GB in size which is eating up your disk space. After every CU install a new OWA version folder is added and the older versions are kept.
When you need to upgrade a couple of servers one after another a server that already has been updated to the newest CU still needs the OWA version files from the last CU. If the mailbox which is being accessed through OWA resides on a server still running the old CU then an already upgraded server which serves OWA needs to be able to speak the old language as well.
According to this, I have moved all old OWA versions from /Owa/prem that did not have a match in /Owa and were older than 2 years to another drive. It gained me about 6GB free space.
After no problems occurred in our test environment I have used the same procedure in our production environment ( 18 servers ). There were no problems reported as well while having different CUs in place and no problems after all servers have been upgraded.