Plugging a Hole in a Chimney

Factual Questions
1

I’ve started refinishing the house I used to live in and am, of course, finding the usual hidden surprises.

The chimney sits in a corner of the kitchen. When I pulled the old wall cabinets yesterday I found an old vent hole in the side of the chimney.

Now I could buy a metal cap and call it good. But my inclination is to fill the hole so as to remove all signs that the hole had ever been there. At present there is a metal “plug” a few inches into the hole (it looks like a lid from a paint can and probably is). My inclination is to fill most of the hole with mortar, then finish off the surface with some plaster to bring the patch even with the existing plaster surface (I’m already obliged to replaster the bottom of the chimney where the original baseboard installation turned the plaster to rubble). (Then the whole thing will get a skim coat of joint compound to erase eighty years’ worth of dings.)

Is this a suitable way to go? Even if there weren’t a metal sleeve (“thimble”) lining the hole, I just can’t see filling in an 8-inch hole with brick as being particularly practicable.

2

Do you know if the chimney is in good shape? Somebody should maybe check it out for condition, because you may be in need of a new liner and repair, besides the patch you’re discussing. They could advise you on the patch, when they look if you need a liner or other work on the chimney.

3

What is the current use of the chimney? Any liner? Overall condition? Any different plans for future use?

4

There is a water heater in the basement (now out of service and to be replaced) and a free-standing heater in the living room that vents into the chimney opposite of the old kitchen vent, though perhaps a bit higher (which means that hole will be capped, too). It will be replaced with a central HVAC in the basement. So far as I can tell the chimney is in good shape. If I remember right there is just a single flue (the chimney is basically square). Never any fireplaces.