Mario from Vancouver, B.C. writes: "I want to make a cabinet in the wall of the bathroom and the best area is between the vent stack and the furnace exhaust. They are separated by a piece of drywall inside the wall. Can I remove the drywall in order to make more space?"
Mario sent a photo of a hole he made in the wall. In the photo it appears to me that the warm fireplace chimney would be on the left, spaced away from any combustible material, but he swears that it is on the right. In that case the 2x4 appears to me to be far too close the chimney already, as if someone who put in the heat protection did it backwards. In the second graphic you can see how a chimney must have careful and specific fire spacing around it all the way up through the house, with the spacing depending upon what kind of chimney it is and hence how hot it could get.
I think that the best professional to bring in to check this would be a heating contractor, and they could tell you just how close you can go with that cabinet.