Like many many older houses in this town, I have a house that has several apartments (3) in it. The whole house is heated by 1 furnace. Also, not at all unusual, the return air for this 1 furnance come from ONLY 1 of the 3 apartments. In this house, and others, this arrangement certainly is not ideal by today's standards; however, this arreangement has worked for 50+ yrs.--- with no problems. NOW, since natural gas prices have skyrocketed, I want to replace my old inefficient furnance with a new 90+% efficient furnance. Take old furnance out---slip new furnace in---Right?? No, I was told that, IF I choose to upgrade, I would need to get 3 furnances (one for each apt.) and I would need to find a way to get return air to each of the new furnaces from each of the 3 apts. (nearly impossible). If I leave everything as is, it would be considered "grandfathered" situation-- everything would be fine. One installer stated that if I choose to stay with the old furnace (for alot of good reasons), I could consider hooking up the furnance to a central fire alarm system-- in case of a fire the alarm go off and would shut down the furnance (would shut off furnance fan-- and not distrubute products of combustion/fire via fan). Question is, if I can do this with my old furnance to address the problem of products of combustion (in a fire); then, WHY COULDN'T I DO THE SAME WITH ONE NEW REPLACEMENT FURNACE??? (air conditioning in not involved in this situation) Put simply, HOW CAN I REPLACE MY OLD FURNANCE WITH ONE NEW FURNACE (key word "one") and still be within the spirit of any and all codes? PLEASE HELP. Jaes.