We had new furnace and central AC units along all new returns/supplies installed in the basement and lower level of our 95 year old house 2 months ago. The installer ran new supply duct up to the 2nd floor through the chimney to new ceiling vents, using the old inside wall supply ducts as the new returns. He ran into a problem with a narrowing of the chimney and wasn't able to get the size (8"X14"?) that he had planned to install and had to run 3 oval metal ducts instead. We are not seeing the air flow we had hoped for and his suggestion is to dismantle the chimney and run the original sized supply. He has been great to work with and has done a quality install so far. I have volunteered to take down the chimney and he will do the duct work for free as part of the original install. He says it will be easy as the mortar is crumbling. I'm concerned about it being load bearing in some way. Is this common in older homes?