Converting from single furnace to four - chimney question

4 Replies


I'm looking at a four unit building that has a single furnace with four zones. The chimney is presently power vented through the wall as there is no chimney.

If I add three more furnaces and power vent them like the existing one, I would gas out the first floor apartment.

My question is what kind of venting solution is available short of building a four flue masonry chimney?

There seems to be reference to a setup that will allow multiple furnaces to vent into some kind of specially engineered pre-fab chimney, but it's a bit confusing trying to figure out what's legal, safe and available.

Anyone solved a problem like this? If so, how did you do it?



The easiest on this would be to power vent each individually.  Many state/town building codes won't allow multiple appliances using one power vent.  

@Craig Shrimpton 

By power venting I would assume they are Hi Efficency units??

If so (by code) each unit would need separate venting to the outside so I don't understand the 'chimney' reference as these should all be PVC exhaust/supply. Unless you are going with chimney hookups that would probably cost more for the chimney/liner etc.

Furthermore, If the furnaces all 'reside' in the same mechanical room you would be advised to include intake combustion venting to avoid O2 depletion.

@Craig Shrimpton  what was the btu of the old furnace and what is the btu of the 4 furnaces? You want to include air intake and they should all be vented separately . You will have to look at specs for distance between all the vents. 

Nice to read everyone's input.  I am purchasing a 4 family that has....get ready for it....4 furnaces (natural gas) and 4 water heaters (also natural gas) piped to one unlined flue!  I don't want to replace them as they are functional but need to find a better solution.  I have a second chimney that is not being used and may have to reline it and use it.  Anyother suggestions would be greatly apreciated it