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Can a vent (4") be merged with the furnace vent (5") into a chimney stack (6")?

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The existing vents are 3" from the water heater, 5" from the furnace and a 6" chimney. I would like to upgrade the water heater, but it would have a 4" vent.
asked in Heating and AC by cwleazer (120 points)
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1 Answer

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Hi Cwleazer,
There’s a little more information needed to fully answer your furnace and water heater venting question. Since I don’t know:
  • BTU’s of your water heater and furnace (furnace with natural draft or induced draft motor equipped?)
  • chimney type (masonry / metal)
  • chimney height
  • liner material
  • vent connectors type (single or double wall)
  • their vertical and horizontal lengths for each appliance
  • number of elbows on each connector (approximately how big they are: 45°, 60°, 90°) 

I would suggest to use Gama Tables at the following link - http://icpindexing.toddsit.com/documents/003046/gama%20vent%20tab.pdf

Read the first few pages until you get to the tables where you should scroll down to examples on page 19 and find the closest one to your particular installation (probably second table on page #13 "Common Vent Capacity).
Let me know if you need further assistance with calculations but I will need answers to those questions above. Just click “comment” below.
answered by darekrudy (21,730 points)

The chimney part is a double wall galvanized 6" vertical run of approximately 10', one story. It starts in the ceiling of the basement. All other vent material in the basement is single wall galvanized. There are 3-90degree elbows down to a wye down to 5" for the furnace (135K input BTU, 80% eff.), and currently a 3" tap down to the water heater of 30K BTU. I was wanting to increase the water heater to 40K BTU and a 4" run to the wye. The 3" vent has 3 90 degree elbows and runs 3 feet to the wye. There is a couple of feet between each 6" elbow and has a rise of 2 feet. I hope the picture of the existing installation helps.Furnace and water heater common vent calculations


I believe that the vertical run of your B-vent is (hopefully) more than 10’ (to the top of the chimney).  Even if you have a flat roof on your house the chimney has to terminate at least 1 foot above it, plus the distance from the basement’s ceiling to the top of each appliance. I’m going to use 15’ for our calculations (I slightly altered your picture above)… If the total chimney height is only 10’ you’d have to go one size higher on your B-vent (7”).
I’m using example #3 from Gama Tables on page 20 (all you need to do is replace the values with yours) and tables on page #17. 
Your new water heater – 40 000 BTU and 4” vent pipe. With 4” vent pipe you can vent up to 52K BTU appliance (based on the values you provided) less 10% for the third elbow - only two are permitted without deductions and you have three.
Furnaces 5” connector is OK up to 138K BTU (using: 15 feet vent total height, connector rise - 2’)
Combined BTU’s from both appliances: 135 + 40 = 175K BTU. Using lower table #4 for 15’ vent height, NAT+NAT (shaded column) under 6” diameter connector gives you up to 200K BTU. So the existing 6” diameter common connector will be sufficient, you’d just need a new 4” connector for the new water heater and a 4”x5”x6” WYE. 
Just wanted to thank you for the analysis and reply. I appreciate it. The 15 feet height is probably right, it has to go up through most of the roof pitch as it is near the center of the ridge.

You are welcome. Come back any time and good luck on your project.smiley