Ordinarily, this wouldn’t be much of a project, but the only access to the crawl space containing the furnace and air conditioner was a tiny door, one foot wide and three feet tall. The furnace must have been installed first, and then the interior walls built around it.
I opened up a wall to give the service people a reasonable chance of replacing the system. (That room was next to be remodeled anyway.) We instructed them to give us a week between the removal of the old system and the installation of the new system, so we would have time to re-insulate the space.