[CentOS] /tmp full with systemd-private*

CentOS: 7.1.1503

I have a problem with systemd which somehow manages to fill /tmp up with a lot of
files. These files obviously are from the Apache server and don't pose a problem
per se. The problem is that these files don't get removed daily:

du -hs systemd-private-*
7,7G	systemd-private-mpg7rm
0	systemd-private-olXnby
0	systemd-private-qvJJ5o
0	systemd-private-Rs2nBv

It was my understanding that these temp-files should have been removed daily as
it is stated here:

$: grep -v '^#' /usr/lib/systemd/system/systemd-tmpfiles-clean.timer

[Unit]
Description=Daily Cleanup of Temporary Directories
Documentation=man:tmpfiles.d(5) man:systemd-tmpfiles(8)

[Timer]
OnBootSec=15min
OnUnitActiveSec=1d

Am I missing something? Is there a better way with a systemd based systemd to
have these files removed daily?