16GB COrsair Vengeance (pro series) + 8GB Corsair Vengeance in one pc

[skywere]

My father recently bought 2 extra 8GB 2133MHZ sticks for his pc to add to the 8GB 2133MHz (2x4) he already had.
When i installed it and booted it up it would show all 24GB as installed but only 16GB could be used.

How can i make it work so all 24GB can be used or is this not possible?
They are installed on a ASUS Sabertooth Z77 Mobo (i7-3770K proccesor)

 

[rgd1101]

https://msdn.microsoft.com/en-us/library/windows/desktop/aa366778%28v=vs.85%29.aspx
what version of windows?

 

[skywere]

https://msdn.microsoft.com/en-us/library/windows/desktop/aa366778%28v=vs.85%29.aspx
what version of windows?

Windows 7 Home Premium, so we found the problem, thank you for the quick answer!