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Can a gas water heater and two gas furnaces be safely connected to a single larger flue?

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Our chimney needed a new lining and flues...as part of this task the chimney crew  proceeded to connect our 3 gas appliances (one water heater and two furnaces - all 4 inch pipes) to a single 6 inch flue, via wye connectors, which then connected, thru the wall, into the chimney itself...we were told the total btu for all three appliances is 210,000, whereas the flue itself can accommodate up to 240,000 btu's......does this sound like a safe and proper set up?


asked in House Chimney by tspyun (120 points)
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1 Answer

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Hi Tom,
I cannot answer your question because I need more information to properly calculate common vent pipe (the chimney flue) minimum and max diameter. 
The easiest way to confirm proper setup of your appliances and their venting would be to use Gama Venting Tables - http://xpedio.carrier.com/idc/groups/public/documents/techlit/58dfa-9si.pdf

There are some important definitions at the beginning of the document (first few pages) and than your example would probably be on page 20.

  • Example #3 if all of your appliances have draft hoods
  • Example #4 if they are mixed; draft hood (water heaters) + induced draft motor (maybe your furnace)
If one of the above is your setup (they apply to more than two appliances as well), you’d scroll to page #13 and read the values from “Common Vent Capacity” table. You need to know the total vertical height of the chimney (measuring from the top of the appliance) to use Gama tables. 
Let me know if you have any problems with calculations.
answered by darekrudy (21,730 points)