If you are embarking on a window project which seems to be pointing you into uncharted waters, it would be wise to invest in hiring a professional. I have seen more than my share of do it yourselfer's turn a window project into a natural disaster. You will be saving alot of money{{gwi:807}} by doing so.

This post was edited by mmarse1 on Fri, Nov 1, 13 at 8:09

1. I'd have to agree with marels. This could be a very expensive mistake.
2. This guy at HD clearly does not know anything. Any manufacturer can do a DH/PW/DH far larger than 87" wide. Moreover, manufacturers will provide mull kits for field mulling if that is what you desire to do. It will also come with instructions.

If you intend to move forward on this DIY, I'd recommend talking to some more informed people. Try a local lumberyard. You would be better served to purhase the windows from them as well.

sorry but I may have not made the facts clear.
I am installing Andersen 400 double hung insert windows.
Not to state the obvious but even when I went to Home Depot the guy at the front desk went right to the full frame replacement windows and not the insert. For those that don"t know these are very specific windows that are made only for double hung replacement as well as picture windows up to a certain size. The frame is only 3 1/8 wide. for an interior install the interior stop is removed and the upper and lower sashes are removed while the exterior stop is left in place. The windows are custom sized and just slide in place. they are shimmed and screwed into the jambs there is some exterior weather stripping but the install is easy with the interior and exterior untouched.
Adersen does make an combo unit using the insert double hungs and an insert picture window but there is a size limit. I saw the notice on the computer at home depot that the 112 inches we needed exceed the max width. the andersen literature says the same. The guy at home depot says it may be because the frame of the insert is only 3 1/8 wide and may be unstable at larger sizes. I dont really know the reason. They certainly do make larger combo units but these are full frame windows and much more robust. And as I said i do not want to tear off exterior stucco or go 2 stories.
This is as much as I know. and now for what I dont know.
It seems one could make 3 sepertate openings using 2 mullions. For example, the openings for the double hungs would be built same as if this was an existing singe double hung window. the mullion could have an exteroir stop that is set 3 1/8 deep so that the insert window is set in place. And then the window is finished as with any other double hung insert. the mullion then serves the same purpose as one of the side jambs in an existing double hung window fram. the picture window is screwed into the other side of the mullion. The insert double hung windows are typically screwed in through the side and top jamb and the picture through all 4 sides. what I am not sure of is how secure are the mullions and is there a typical width or material type. It seems the the depth of the mullion going to the outside would just need to match the overall frame depth.
Also does it make sense? If not then why.

I have to admit that I my usage of Andersen products is few and far between, but that said, I have never heard of ANY manufacturer that has a max size on a DH/PW/DH of 87". My thought is that maybe 87" is the largest that they will go with a PW alone, or perhaps that is the largest that they will FACTORY mull, but even that does not sound right. Order 2) 28" DH's and one 56" PW, and tell them that you need a field mull kit to make that into one unit on site.... Something is missing or being lost in translation here.
Your plan above is fine and would work if you want to go that route, I'm just saying that it is not necessary. It will create more work for you and you will lose glass space.