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I have 2 forty gal gas fired water heaters with smoke pipe going into unlined chimney along with a gas fired burnahm boiler with 5" flue going into the same chimney and I am wondering if I can add one more small gas fired boiler`s pipe into same chimney. This would be a total of two boilers and two water heaters.If it has to be lined how do you conncect all those pipes to the liner .I would think that would be difficult. Maybe my best option is a powered vent boiler but they are more money and I think the flue for those is expensive as well. I am trying to add a zone of heat for a second floor apartment building so any advice or information regarding this would be appreciated! PS current boiler is not big enough to cover btu`s for zone upstairs! Another option that was mentioned to me was rinnai gas fired heaters but I think those would be difficult to install in an old house and how does the heat get to all the rooms evenly?