16GB + 8GB gives you 16GB of dual-channel.

Yes, it works. Your B450 will boot with 24GB just fine.

The catch: dual-channel only applies to the first 16GB. That’s the 8GB from the old stick plus the first 8GB from the new 16GB stick. The leftover 8GB on the 16GB stick runs single-channel.

In practice, unless you’re hammering memory bandwidth (video editing, heavy simulation), you won’t notice. For gaming or everyday use, 24GB in this config is actually a decent upgrade from 8GB.

Not ideal for maximizing performance, but it’s a cheap way to get more RAM.