The reasoning behind this is the manufacturer doesn't make a good DH unit. The vinyl DH's need to be better built and stronger than the others. Anytime you have two fully functional sashes you require a more precise installation with more patience. The Single Hungs are just cheaper to make and easier to hang. Almost idiot proof in most cases. They are also more energy efficient from air infiltration becuase a lot of installers don't take the time to shim the DH unit properly. They end up leaking around the sashes, creating that ever so hated call back. With a stationary top sash they don't have to worry about anything leaking around the upper sash. The downfall of the SH is you can't tilt the sash in for cleaning. This can be quite trouble some for the windows off the ground a ways. There are ways around that for cleaning if shown the proper way.

The history of the DH window goes back to the old farm houses that never had AC or Central Air. They would pull the top sash down about six inches and the lower sash up about six inches. This would let the air circulate better in each room by creating the drafting affect (Firefighters call it breathing during a house fire). With todays AC in almost every house the DH windows aren't really required for the drafting situation. You can still get them from almost every dealer. If they are balking at DH's with reasoning like they gave you. I would steer clear of them and start shopping for another supplier. Hope this makes sense to you. God Luck!!!

Thanks for the information. I am thinking of putting the DH windows on the top floor and SH windows on the main floor. With DH on the top floor, like you mentioned, cleaning won't be as much of a problem and we can get better air circulation up there (where it is definitely warmer in the summer - no AC). One of the other suppliers that I have been talking with has no issues with the DH windows, and says that he actually prefers them for the retrofit (I think it has to do with the frame size). I appreciate the advice!