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.

  • 1
    The general approach is to just break up the blocks with a hammer. But this doesn't work if the blocks are filled with cement. Do you know if the blocks in question have been filled?
    – longneck
    Oct 21, 2013 at 12:38
  • What are the ducts coming from? It might not be advisable to combine them. Also, how big are the flex ducts? Oct 22, 2013 at 14:07
  • The flex-ducts, both 6", are coming off the supply main right where it terminates. Both ducts are collapsing in on themselves where they make a 90 degree turn as they go through the window opening, so I have to imagine they should be replaced for this reason alone. The flex-ducts terminate at each of their registers.
    – SBerg413
    Oct 22, 2013 at 14:25
  • My thought was to combine them into a single rigid duct so that I can use an elbow AND only have a single rigid duct going through the wall. Then T them off to each flex duct. Ultimately, I want to cover the window and only use it as entrance to the crawl space.
    – SBerg413
    Oct 22, 2013 at 14:28
  • The general approach is to just break up the blocks with a hammer. But this doesn't work if the blocks are filled with cement. Do you know if the blocks in question have been filled?
    – longneck
    Oct 22, 2013 at 15:53

3 Answers 3


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.)

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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.

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    If you don't have experience with a reciprocating saw, buy multiple blades. They're easy to bend. You don't have to replace them the first time you bend them; if it's minor enough you can usually hammer it straight and keep using it. But a bad enough fold will require a new blade. Oct 22, 2013 at 20:42

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.

  • If this is true, then it's obviously the way to go. (I have no reason to doubt the truth of this answer.) Oct 22, 2013 at 16:58
  • Do you have an online reference for one of these? I wanted to see what it looks like, but couldn't find any after a brief google search. Though admittedly, I'm not sure I used the right search terms to find it.
    – Johnny
    Oct 22, 2013 at 20:27
  • I'm having trouble finding one online, too. But I bought 2 for my previous house and installed them myself.
    – longneck
    Oct 22, 2013 at 21:12

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