Flue Pipe Sizing

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Flue Pipe Sizing

Postby MITTS » Sat Feb 06, 2016 10:13 am

I think my flue pipe might be oversized and looking to install a recesed med cabinet in my bathroom where a smaller flue would allow clearance.
Read a few things and all the verriables are confusing so though I'd ask.

We have a 88K 80% (draft induced) furnace with 4" connection and 40K (natural draft) hot water tank with 3" connection tee'd together and the horizontal bvent is 6"

Will get lengths measured up. Any other thing needed to size a flue?
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Re: Flue Pipe Sizing

Postby MITTS » Sat Feb 06, 2016 11:10 am

existing 6" has a 20' rise
6" elbow into a 6x3x5 tee
5" reduced to 4 after a 90 into furnace

The 3" has a 3' horizontal length
the 4" has a 3' horizontal length
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Re: Flue Pipe Sizing

Postby CEO1 » Sat Feb 06, 2016 11:13 am

Here are the GAMA tables...Jim Davis, the combustion guru, says you can't get 'em too big... :mrgreen:

https://www.rhs1.com/support/references ... Tables.pdf
One of the MFIC's...

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Re: Flue Pipe Sizing

Postby CEO1 » Sat Feb 06, 2016 11:15 am

Posted tables before your second post...I can look at it later today.
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Re: Flue Pipe Sizing

Postby MITTS » Sat Feb 06, 2016 11:35 am

figure 7 shows my config.
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Re: Flue Pipe Sizing

Postby MITTS » Sat Feb 06, 2016 11:42 am

would i be using table 3 on page 11?
the small chart titled common vent capacity for the 20 foot bvent run?
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Re: Flue Pipe Sizing

Postby 3rdcoast » Sat Feb 06, 2016 11:45 am


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Re: Flue Pipe Sizing

Postby MITTS » Sat Feb 06, 2016 11:58 am

yep was reading that one too

same confusion for me

that one shows a one letter on type of pipe that i cant find in that pdf

looks like the longer the run the more btu it can handle from both papers

that puts me inbetween

was going to get a little extra bvent to 22' actually if i was going to replace with smaller.

5" looks like a sure thing if I'm understanding it properly

4" would get me the clearance I need but doesnt look like it in the 20 foot range
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Re: Flue Pipe Sizing

Postby MITTS » Sat Feb 06, 2016 12:05 pm

I see in the H&C book they say the total height is from the appliance
if thats the case then 26' total but not sure how that plays with having two vents combined :?
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Re: Flue Pipe Sizing

Postby 3rdcoast » Sat Feb 06, 2016 2:45 pm

MITTS wrote:I see in the H&C book they say the total height is from the appliance
if thats the case then 26' total but not sure how that plays with having two vents combined :?


Common vent height is from the higher appliance connection point strictly vertical height (page 15). If you need smaller vertical pipe, you can run two smaller b vent connectors into attic space (one from each appliance) and tee it in the attic to a common vent. See page 19 of hart and cooley book.

It sounds like your common vent has a lateral, so it will reduce the capacity shown in the table by 10% and is limited to 18 inches per vent diameter. That may be where your 6 inch comes into play. Best thing is to do a line diagram of your system and print the book out and work through it. There is usually ways to make things work, by increasing total vent height above roof.
Last edited by 3rdcoast on Tue Feb 09, 2016 8:54 pm, edited 1 time in total.
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