When an addition was put onto the house that I own, the previous owner just ran the duct work into the crawl space through an old basement window. I want to re-route the duct work by actually cutting a hole through the neighboring cinder block wall. There are 2 flex-ducts being routed through the window. I want to combine these into a larger, rigid duct (maybe 8" to 10") and cut just a single hole. What tool do I need to cut through the cinder block wall and should I be concerned with any pit falls. I'm not positive what size I should use for the rigid duct, but I can ask that in another question.
Trace a duct end onto the blocks where you want the hole: it is best that the edges of the hole be at least an inch (3 cm) away from cinder block edges and mortar but can go through them.
Drill the 3/4 inch pilot hole inside but near the circle's line. Use care to get it square (perpendicular) all the way through and press lightly as it passes through the opposite face. (If you drill the hole right on the line, there will be an unsightly gap to deal with later.)
Place sawzall into the hole and—if you haven't used one before, practice by cutting toward the center of the circle—cutting slowly to avoid fracturing the brittle material. Until you get a feel for how much chipping it does, stay safely away from the line—an inch should be enough at first. (See example video.)
The hole need not be big enough for the full height of the sawzall blade. It can be started in about half its full width.
When cutting through the block edges and mortar, angle the saw as shown. Repeat from other side to finish cutting through.
Chalk a circle the size that you want. Drill holes every quarter of an inch or so. Drill bit only need be thick enough to not break as it passes through. Do this all the way around till you can gently hammer out the hole.
If you go to an HVAC supply warehouse, you can find pre-made ducts the size of a standard cinder block that include structural support to replace the support lost by removing a block. The HVAC supply guys can also advise on the proper spacing/placement for putting in more than one duct, which it sounds like you need.