In theory, exhaust gasses from our hot water heater and boiler could have started a fire in the attic or escaped into the attic and eventually into the house. The hole had to go. Mr. Man used a wire brush to clean the existing brick and mortar, and then a spray bottle of water to wet the mortar and the brick pieces to be used for patching.
If the brick is not wet, the water from the mortar wicks into the dry brick mortar and weakens the mortar and the bond. BAD! Then he mixed up some mortar and patched the hole using mortar and small pieces of brick. This was his view as he shimmied into the attic crawl space: