The Nutone units come with "rods" or brackets that attach to the bottom of the rafters. This is probably great for new construction but for existing ceiling do you need remove the fan units and just nail the housing to the rafter? Can't get to the rafters and didn't consider that when I bought the unit.

That Nutone will fit just fine. You should carefully read the installation booklet. Remove the heater, fan and light units from the box. Pull you 3 sets of cables to the electrical connection on the top of the box and do all you conncetions before pusjing the box up between the joists and screw into the joist throw the side wall of the metal box, discard the brackets. As you have guessed they are for new construction only.

Thanks for the help. The instructions only show the installation using the hanger brackets but I'll install as you described. That's how I installed the old unit 25 years ago...

Thanks!