I currently have a tankless hot water heater power vent running through a square 8" x 8" penetration in a hollow cinder block wall. I'm going to be replacing the water heater with another unit which will be vented elsewhere. Once I remove the vent, what's the best way to patch such a large (yet uniform) hole? I have easy access to it inside and outside.
Kind of depends what's on the other side, and whether you need it to be airtight, watertight, smooth finished, capable of bearing a load, and visually indistinct from the rest of the wall.
Easiest is to spray a can or two of foam in there. Looks ugly, but it keeps the wind out.
Best is probably to chisel out the foam and use a combination of mortar and bricks, or a cinder block piece that you've trimmed down a bit with a grinder. Then face it with a thin coat of cement. Wait for a while and paint to match.
A half-block and a bit of mortar. Given a block is 8x16 (nominal, with mortar joints) and a half-block is 8x8, nominally. Rather than cutting down a full block, just buy a half-block in the first place, they are standard items.
Filling with brick and mortar is also common.
Or a full block and knock out the remaining bits of the old block that's been cut away on either side just before inserting it for a more seamless look with more work and slightly more risk. The risk is that you'd be making the hole bigger, briefly. It eyeballs as unlikely to cause collapse from above if you promptly get mortar and the replacement block in there, rather than removing the bits of old block and leaving it for days or weeks before completing the patch.
