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Multiple gas appliances venting into 1 flue pipe

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Several appliances feeding into 1 flue pipe, and they are 2, 4" and 2, 3"pipes, do i understand that i would need a 11" flue or a 9" flue


boiler is a 162,000 btu
furnace is 2 ton
hot water heaters ? standard? one for 1 unit and the other for 2 units

height of flue is adequate ,3' above flat roof of 3 flat bldg.  does this factor in with the diameter?  could a fan connected to the flue resolve the issue?  all appliances have been working fine , but gas co. red tagged it.  they were not specific on the fix.

asked in House Chimney by cristophe (120 points)
edited by darekrudy
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1 Answer

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Hi Cristophe,
This answer is for both of your questions. There are several variables required to properly calculate chimney flue diameter and based on the information you provided I can’t help you with it. You need:
  1. exact BTU’s from each appliance
  2. type of the chimney (masonry, metal)
  3. if masonry; does it have a liner (what size diameter)
  4. height of the chimney from the top of the tallest appliance (assuming they are all in the same room / level, there are different measurements required if appliances installed on different levels) to the top of the chimney above the roof
  5. number and type of elbows on the chimney (45°, 60°, 90°) if any
  6. number and type of elbows on each appliance connector (before they enter the chimney)
  7. length of each connectors before they enter the chimney (vertical and horizontal)
  8. type of the connectors (single wall or double wall)
  9. are all of the appliances natural draft type or they are equipped with an induced draft motor that helps to push exhaust gasses through the chimney
Without all that information you cannot properly calculate chimney’s diameter. You can use Gama tables examples http://0323c7c.netsolhost.com/docs/gama%20venting.pdf (most likely starting from page 20) to better understand calculation process but you will probably need to call a heating or plumbing contractor to resolve this situation, especially since the gas company didn’t specify the reason for the ticket. It could be the size of the common vent, chimney, combustion air requirements, etc. 
Please let me know if you need some help with calculations, if possible, take some pictures of this installation and email it to me  
answered by darekrudy (21,730 points)