direct vent/conventional chimney share rooftop chimney box

[vato]

This is my first mech post, so please have patience with my ignorance.
I am looking at a multistory condo, penthouse unit. Owner is replacing the wood fireplace with
a direct vent gas unit. The flue and chases are separate, but 2 flues exit to one existing screened in
chimney exhaust feature. Perhaps this is such an incorrect approach that it is not covered in the code, but
I would appreciate some guidance on how to handle the situation, per the code. ie, distance from chimney
exhaust to direct vent appliance intake perhaps? Or if someone would say don't ever do it, period.
I don't feel good about about it at all.
thanks,

 

[dbill74]

It is not uncommon for a combustion appliance such as gas fired water heater to use a "concentric" flue/combustion air intake fitting.

It sounds like you have multiple chimney flues in a large single chase up to the roof. In this case I would be hesitant to use a concentric fitting due to the proximity of other flue exhausts.

Best bet would be to route flue exhaust in the chimney (in same place as for the wood fireplace; perhaps re-use existing flue stack), and route combustion air intake to a wall or other part of the roof (separation requirements are in fuel gas code). Combustion air intake will not get hot like the flue, so you can use something other than metal and do not need to worry about insulating it from the building structure like you do the flue.

Fuel Gas Code has detailed requirements for gas fired appliances including distance requirements between flue/combustion intake and other parts of a building.

On a side note, there are gas fireplaces that do not require a direct vent, nor is the direct vent actually required by code. Use of one of these types of fireplaces could be much cheaper. They pull combustion air in through the face of the fireplace and usually require the user to crack a window or install a small louver somewhere; check manufacturer's requirements.

Recommend hiring a mechanical engineer to properly design the system.