This is a pretty odd question but it's something that's bugging me because I love fireplaces. I used to know how to do it in the past but I can't seem to figure it out now.

So, we have those custom fireplaces found in miscellaneous in appliances. I remember placing it over an existing maxis fireplace and chimney using moveobjects on and removing them from the maxis fireplace using the hand. The chimney is supposed to stay and thus the fireplace connecting to the chimney.

It turns out this isn't true and I can't do it anymore. Is there a correct way to do it? Do any of you do it when building?

Try using the sledgehammer tool instead of the hand tool; hover over the chimney components with this tool and you see they highlight individually, so you can delete the fireplace and leave the top stack (or stovepipe if you use the single tile fires).