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B Vent Sizing


agstaff
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I've read quite a few of the posts here. I've researched this topic extensively and can't seem to find a definite answer. I've read the tables and performed the calculations. Here is my situation:

80% 66,000 btu fan assisted Bryant furnace, 4' single wall connector, 2 45 elbows, no horizontal runs.

40 gallon water heater, 40,000 btu with natural draft hood, 3' single wall connector, 2 45 elbows, no horizontal runs.

Both the furnace and water heater vent through 4" single wall pipe to a 6x5x4 sheet metal wye. The 5" outlet has a 4" reducer that connects to the furnace. The 6" end of the wye then connects to a 6" b vent that travels straight up, no offsets, through a wall chase and through the roof where it terminates, approximately 17'. This is the height to the tallest appliance, the water heater. Approximately 14' of actual 6" b-vent.

From all the sizing guidelines I have read, it looks like I should be running a 4" b-vent connector from the furnace and can use a 4" double wall for the common vent up through the roof. I currently have 6" b-vent for my common vent. Will this cause problems with condensation and furnace efficiency? If I'm going to fix it I want to do it right!

Thanks for any insite!

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  • 2 weeks later...

The rule of thumb is that the exhaust vent has to be equal to the largest connector plus 50% of all others. So, a 4" connector has an area of 12.56 square inches. The second connector is also a 4" so half of that is 6.28. Add the two together and the minimum allowable flue has to have a cross sectional area of not less than 18.84 square inches.

A 5" vent has an area of 19.62 square inches and a 6" vent has an area of 28.26 square inches; so a 6" vent is literally 50% larger than the smallest allowable flue size. A 5" vent would work fine. You might experience some condensation issues as a result of using that larger vent.

There are other factors that come into play. The installer should have sized the flue according to tables published by the NFPA for sizing gas appliance flues. There is an excellent article that give you insight into this process at the Journal of Light Construction Online that was published in a 2003 issue of JLC. You can find it here. Unless you are a JLCOnline member, it will cost you $2.95 to download it but it will go a long way to helping you to understand how critical gas appliance vent size is.

ONE TEAM - ONE FIGHT!!!

Mike

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you should actually have a 6" cap on the end for cleaning/inspection purposes so there should actuallt be another tee or wye in there

I'm not sure what you're describing. Can you point me toward a reference that requires this? A manufacturer's recommendation or a code section, or something similar?

- Jim Katen,

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I think he's talking about a tee on the bottom of the flue where all of the detritus that accumulates as a result of condensation collects - sort of a like a drip leg on a gas pipe.

Those seem to be used a lot for oversized flues and they eventually rust out. However, most of the condensate ends up in that cap area instead of draining back down to the furnace, so the furnace or boiler isn't damaged a lot and an oversized flue can continue to be used.

If it's what I think it is, I'd call it a jackleg practice unless someone could show me where it's required by code or recommended by a manufacturer.

ONE TEAM - ONE FIGHT!!!

Mike

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