Have a trouble to cut a 5" inch of hole in the trunk. I don't have space to make hole through the side of trunk. the only space is let between joists and, from where its hard to cut a hole of 5" 
Thanks!
How to run a flex duct branch through a unreachable trunk
hvac
Related Solutions
I can't think of any reason not to run the duct through any open path. If the duct goes into any uninsulated space, you'll need to have the duct itself insulated. And on a vertical run between floors, I like to have a fire block between the floors, but that may not be code in a single family dwelling. Be sure to block any drafts, run a vapor barrier, and insulate where the line goes from an interior wall into the attic space.
If the cinder blocks are empty, a standard masonry drill bit in the 3/4 inch (19 mm) range plus a sawzall (which can be inexpensively rented, but you'll have to buy a blade).
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.
Related Topic
- Can 2 – 6“ flex ducts be combined into a single 8” rigid duct
- Should I change the routing of the return air flex duct
- Is the plan for moving return duct a few inches ok
- Flex-duct as a Trunk
- HVAC Ducts – Can 8″ Flex Duct Be Used on 7 Inch Rigid Duct Connection?
- Basement – Should I insulate the duct trunk as I finish the basement
- Append Tee & Boot to Existing Branch ( flexible duct) to Split supply between floors
Best Answer
Some options that come to mind are using a low profile drill to start a hole and then awkwardly cutting it with tin snips or a nibbler if you have enough space. With a little more space you may even be able to fit a hole saw with a low profile drill. Some "Adjustable" hole saws have shorter length than common arbor and cups.
If you really can't fit anything in there, from your picture it looks like you have access to the opposite side, so you can get a 5" holesaw and a 18-24" hole saw extension bit and drill through both sides of the duct. Once you're done, seal the hole you don't want with a metal plate, self tapping screws and that grey duct basher goo. That gets you a hole and just leaves you with needing to screw your flange on, which could still be a huge pain, but again a low profile drill should do the job with only 5-6" clearance. If you don't have 5-6" clearance, you don't have room for your duct anyway, so I assume you have at least that to work with. There may be some adhesive that is considered acceptable, but I've never seen a professional HVAC guy forego screws.
Hex head duct screws are particularly easy to work with with a magnetic bit, but an option for really hard cases is to drill a pilot hole and then put the screw in by hand with a stubby screwdriver.