My house is covered by sheet metal and I currently have a 4 in. vent hole from a previous air exchanger. I'm changing the air excahnger and require a 5 in. hole. So I basically have to make a hole though a sheet of metal and 4 in. wood through the house to enlarge the previous hole. It seems I can't do this with an hole saw. How can I do this on a busget and how profissional actually do this?
Buy a 5" holesaw (and a drill sturdy enough to drive it). Cut a 5" hole in a big chunk of plywood. Screw/nail/clamp the plywood in the place where you want the hole. Drill away.
If you're having a hard time with the drill binding and trying to twist your wrist off, run it in reverse. It's much slower, but you won't have the same problem.
Couple more tips: Once you've started the cut, drill a few 1/4" holes in the cut ring to help clear sawdust. Pull the holesaw out regularly to clear the sawdust that collects on the teeth.
If you can't get a center pilot drill to guide the cut, you can guide it by the outside edge:
Get the 5" holesaw.
Use it to cut a hole out of a board or piece of plywood (or several pieces of plywood stacked and screwed-and-glued together).
Center this 5" hole over the 4" hole, and temporarily anchor it firmly in place somehow (a few nails or screws, perhaps).
Use this as a drilling guide to keep the holesaw in position, at least until it has gone deep enough that its own kerf will guide it the rest of the way.
As the honorable Mr. Defenestrate has pointed out, providing a few holes along the kerf line for sawdust to escape through is a very good idea.