Need to widen a vent hole in basement / foundation wall.

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Jul 10, 2023
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Friend of mine just got a new place and the original dryer vent hole was cut to an unconventional size. Instead of 4" x 4" it's about 3.25" x 4", as seen here:


We're not going to try a concrete hole saw, since it might travel too much and probably won't, at best, produce a clean whole. At worst, it might get stuck or hit the gas line seen on the left.

Every length of duct we've put through the hole gets squished and then won't connect to the 90-degree elbow we have inside.

Any tips for how we can widen the hole to 4"?
IMO it would be easier to make a sheet metal transition piece than enlarge the hole. That is what I would do.
IMO it would be easier to make a sheet metal transition piece than enlarge the hole. That is what I would do.

If we go that route, how would you recommend we connect that piece to the hood outside and the dryer connector inside, both of which are 4"x4" rigid material?

In all, I'd be down for doing that if it comes down to it. We had to go with a very specific hood+duct all fused into one. The duct on that is just long enough to stick out of the wall inside the basement.


We had to do that because a solid wood soffit outside is right above the hole, so most all vent hoods won't fit because there's no space above the hole between it and the soffit.

Whoever put in this hole when the house was built was really not future-proofing it! :dunno:
If a 4" oval transition would fit the hole you might custom make a hood out of aluminum flashing. You could fashion a prototype out of thin cardboard to use as a template. Then use pop rivets to join. As an alternative you could take the prototype to a sheet metal shop and have a hood fabricated or purchase a hood and rework the round duct to pass through the hole and fit into an oval transition.
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Were you to have reviewed the link, you would have found that the xition reduces from 4" too 3.25", and using them back to back, xitions from 4" back too 4" so all you need is a screened vent hood.

Any of these can be easily adapted to fit; 4" vent hoods
You are right to avoid a hole saw. Since the center of the hole is already gone, it'll skip and jump on the wall. A core drill is also very hard to use in that diameter unless it mounts to the wall. (Expensive if you have to rent one)

A Plan:
If you want to open the hole larger or different shape, draw it on the wall with a bright crayon.
Using a carbide tipped drill bit of about 1/4" diameter, hold the drill perpendicular to the wall and drill a bunch of holes all around the your chalk line. They can be spaced rather far apart, but the closer they are, the nicer the hole edge. One bit diameter apart is my usual. Then tap the core out with a heavy hammer, like a 5 pound baby sledge.

The hammer drill and percussion bits can be rented locally for a few dollars. If you are working indoors, it's good to have someone keep the wall damp while you drill (that's why crayon, not chalk). Or try to vacuum while drilling.

The reason I wrote 1/4" is for a neater hole edge. Larger drill is no problem. I've used 1/2" drills through 6" poured concrete with my smaller SDS many times when it wasn't worth setting up the coring machine.

Hold the drill bit perpendicular to the wall so the hole goes straight through & isn't angled. Angled will make the sleeve hard to install.

Most Important-
Wear Eye, Lung and Hearing protection.

Enjoy Your Project!
I saw this post on another forum. I've never owned a sawzall but it sounds impressive.

I love a sawzall,,,, Never had one until years ago when I extended the garage to take my extended cab pick-up. It went through everything wood, brick, and steel like it was butter. Everyone who watches Wicked Tuna knows that it will cut heads off!!!