Do I have to remove the old window jambs when installing new nail flange window?
Do I have to remove the old window jambs when installing new nail flange window?


The answer is yes; if you use that nail-on window the way it was designed to be used it nails to the underlying wall studs. If you use a "nail-on" window like you describe, face-nailing it to the old window jambs, it will look like crap and will probably leak.

If you do not intend to purchase a window designed for your application (a block frame or "replacement style" window), an option might be for you to cut off the nail flanges so you end up with a window that has what is essentially a box/block frame. Then drill holes through the frame and screw it to the existing jambs, caulking all the way around and finishing it off with aesthetically pleasing trim of some sort. I have done this with success.



The picture above shows frame types, from left to right:


nail-on frame for new construction, usually purchased in standard sizes.
block frame for various replacement applications, and very suitable for replacing old windows when the jambs and sill are left in place. Usually ordered to specific size needed.
"replacement style" window, also used for various replacement applications and includes exterior trim flange to hide old jambs. Usually ordered to specific size needed.