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Orphaned water heater vent resizing needed?

Charlie R

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The furnace was changed out and no longer is connected to the large B-vent. The 36,000 BTU water heater with a 3" vent draft hood vent connector is attached (single wall) to the old B vent. I say the B vent is now too large, that the 3 " connection need to go up through the B vent to the end above the roof. If I'm right, can someone help me with some code quotes, but if I'm wrong please slap me soundly. Thanks.

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Bill's right.

But, since People's Gas started getting really stringent on orphaned water heaters (it's the "new" hot button), I've done more calc's than I'd care to. Every time, the flue is too large.

IOW, it's a safe bet this one is too large also.

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To know for certain you need to refer to the vent-sizing lookup tables. You'll find them in the National Fire Protection Association (NFPA) National Fuel Gas Code (www.nfpa.org), in building codes, and in manufacturer handbooks and instructions. The tables are published to ensure flues draw properly and stay as dry inside as possible.

The Journal of Light Construction had an article about ten years ago entitled Venting Gas Appliances written by Bob Dwyer. It's a free article; download it from this link, read it, memorize it, print a couple of extra copies and keep them in your vehicle so you can use the article to back you up when you run into stuff like this.



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From the vent hood the single wall came up about 14", turned almost a 90, went about 4' with a good rise to the connection in the photo, you can see the 90 where the larger B vent goes up to the ceiling, through the attic and extending above the roof was probably 4 to 5 '. Total rise is about 7-8', total run about 8', with the two 90s. I've tried to do the calculations and I can never seem to get it right. The run and lateral are very close to being the same so that's an issue too, I believe. I look at NFPA 54.13.1(a) chart and come up with a 4" maximum. What do you guys come up with?

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