So it was originally a dutch door?

The way that all lines up doesn't look like it was originally a dutch door. To me it looks like a 6-lite door that was hacked into at some point. I would think an actual dutch door would have a thicker rail on the top and bottom of both the top and bottom doors. A good way to tell would be to strip the paint just where the hinges are and see if there were any patches of where a center hinge was filled in.

I think you could strip it yourself if you really wanted to save the door. It's a messy and painful job (I have BTDT) but it's really more of a craft project than a construction one. Once you get it stripped down you could see about how to treat the gap.

But at this point, it's possible the door has been so badly weathered and then sawed into that it's not worth trying to salvage.

Wow I like the tile work and the colours. I wonder if some previous homeowner sawed through the door so that the upper half could be opened without hitting that little window ledge.

Never seen a dutch door like that and honestly can not even fathom the reasoning behind it. I would replace it for the reason you astate and structurall there is nothing to that upper half.

This door from simpson matches your previous door, if you are entertaining a replacement.

http://www.simpsondoor.com/find-a-door/default.aspx?view=detail&doorType=Exterior&BaseSpecificationID=3237&architecturalGroup=All#DoorDetail

Cut where it was, it never would have been structurally viable.

I love old doors and have been known to expend lots of effort on saving an almost dead door. That said, I would replace this one without thinking twice! That door has been cut and will never be right. A purpose built Dutch door would have all the glass in the upper section and a rail where the upper door closes against the lower section.

If you wish to have an authentic door, check out your local ReStore, other salvage place and Craigslist for possibilities

I think it shows how resourceful some people were in the past. Instead of running out and spending hundreds of dollars and buying a new door to solve their problem (ventilation), they adapted what they had.

I love them and remember more people having them in the past - I think its just a way to get ventilation while still having psychological sense of privacy, without your free range chickens and dogs (or human neighbors as the case may be) feeling entitled to walk in.

Cant imagine theyd be any more of an energy leak than the other edges of the door where it meets the jamb.

Google images shows some being used for interior doors as a variation on the barn door thing. Now those make sense as a stylish baby gate - close bottom to keep baby confined.

They are handy too for keeping your talking horse from wandering around the neighborhood ...

I'm not a carpenter or expert, but why couldn't the metal plates be removed (unless you wanted to keep them there for posterity) and the door be glued and biscuited backtogether in one piece and then refinished?

I have a dutch door circa 1940, and as the others pointed out, it separates below the glass not in the middle! If you want to save it maybe it can moved to another spot in the house or basement where you're not so worried about energy loss? We ended up putting ours going out to the catio from the mudroom (only because it's too wide to fit anywhere else but the front door which is original to the house, c1920.) My Dutch door was on my grandmother's home which was built in 1940-41.

Reduce, reuse, recycle. Waste not, want not. those people were very resourceful and used what they had. Apparently they had 2 doors, neither of which were in good shape. They used those two doors to make a new door.