Furnace question: 3 rental units in older house-- want to replace furnace with 90+%

[jaes54321]

Title pretty much explains it. --Have older house (90+yrs old)---house has been divided into 3 separate living units---heated by 1 gas furnance in basement (this has been the arrangement for over 1/2 century). Wanted to install new 90+% efficient furnance to replace old one--- was told that I must provide one furnace for each of 3 units. ---Besides being very expensive, for this particular situation, it would be nearly impossible to retrofit such that I can provide 3 furnances. Currently, tenants split gas bill (this has worked exceptionally well for years and year and years); however, since price of natural gas has skyrocketed---I wanted to get most efficient furnance------------GETTING 3 Furnances--- (although each would be efficient) would produce a net increase in gas usage (gas use --with 3 efficient furnaces would be higher than gas use with 1 efficient furnance).
IS THERE ANYWAY that I can use one new furnace--- I totally understand the return-air problem (currently return-air is coming back to furnace from one unit)--- so, smells in that unit would be (has been for 1/2 century) shared with other units ----is there anyway (air exchanger?) that I can deal with this problem (and satisfy code) AND only use one new furnace???
By the way, central air is not involved in this system. Jaes. :confused:

[suemarkp]

While I don't know the code aspects of common heating equipment, it has to be allowed in certain instances such as high rise apartments where there is a central heating source. The answer to that question is probably one that only local officials can answer.

Assuming you can have a central heating source, I'm wondering how your return air works now. Is each living unit on a different floor? Is there any way to get a rather large duct to the basement from the other two living units (perhaps taking a section of 1st floor closet)? Are there any common interior hallways where return air could come under the floor to get to the main return? If no to all of these, then all I can think of would be to build what looks like a modern wood chimney on the side of the house. Inside this false "chimney" would be a large insulated return air duct for each unit (assuming each is on its own floor). Depending on how large this house is, the duct should probably be about 300 square inches (30x10, 12x24, 15x20, etc) and have about 100 - 120 square inches per unit duct.

An air exchanger isn't the right solution to remove odors, some type of filter/conditioner would be needed for that. This house is so old that an air-to-air heat exchanger is not required (you'll get plenty of fresh air intake to the house through cracks in the walls/doors/windows). However, you should make certain that you have a source of fresh combustion air for the furnace, and these can be in rather obscure places.

[mdshunk]

I, too, am puzzled why there's a requirement for 3 furnaces. I've been in plenty of apartment buildings that have only one furnace. Some new, some old. If the goal is to provice "zoning" to each tennant (so that each tennant can control the temp in his/her place), then motorized zone dampers can be used. If the requirement is so that products of combustion from one apartment (during a fire) doesn't get spread next door, then you might be stuck with the 3 furnace option. The other option would be to install one high efficiency hot water furnace or boiler, and put a hot water coil or steam coil in each tennants seperate ductwork. I'd sort of like to know what the real requirement is for 3 furnaces. Doesn't sound like a wise investment to me.

[jaes54321]

In reply to suemarkp--- currently the only return air that comes back to the current furnace--is coming from ONLY one of the 3 apartments. While this certainly is not ideal by today's standards, it has worked nicely for all 3 apartments for 50+ years (so, return air is not/has not been a problem). However, your suggestions as to how to get return air runs from all 3 apartments (e.g., "build what looks like a modern wood chimney on the side of the house") merely points out the fact of how difficult and expensive it is to supply a furnance (and return air) to each of the 3 apartments--- when 1 furnace (with it's less than ideal return air system has been doing fine).
In reply to mdshunk--- I suspect the possiblity of "products of combustion from one apartment (during a fire)" is the reason for the 3 furnance requirement (I am going to discreetly ask someone, locally, about this requirement). The same person that told me that I needed 3 furnances, also told me that I would be required to supply the 3 furnances, ONLY if I went ahead with the upgrade. If I remain with the current, (inefficient) furnance---everything would be ok--- the situation/ the old furnace would be considered a "grandfathered" situation. So, the message is clear, avoid the HUGE cost of 3 new furnaces, all the additional work of supplying return air for each of 3 new furnaces--- and simply stay with the old inefficient furnance! ---And, forget about energy conservation. NOW, one interesting thing was said by the person telling me about the 3 furnance requirement---He stated that if I choose to stay with my old furnance (for a lot of good reasons)--- I could connect the existing old furnance to the central fire alarm system so that when the alarm when off---the furnace would shut down (i.e., the fan would not circulate "products of combustion"). Well, then the question arises, if one can do a set-up like that to take care of the fire concern with the old furnace---WHY COULD'NT ONE DO THE SAME THING WITH A NEW FURNANCE??!! Any further, discussion would be appreciated. Jaes.