Flue Pipe Sizing

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Re: Flue Pipe Sizing

Postby CEO1 » Sat Feb 06, 2016 5:29 pm

If you read the #4 example starting on page 20 (GAMA link), the final paragraph on page 21 suggests the common vent diameter is pretty much determined from total Btu input and vertical height...with 128K input, one fan-one natural, and 20' height, the least you can go is 5" diameter, from tables 3 or 4.
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Re: Flue Pipe Sizing

Postby MITTS » Sat Feb 06, 2016 6:37 pm

I got to the point of reading between the lines. If I was better with the numbers I could figure what the lowest height needed to run 4 inch. Since it goes from the highest appliance and I planned on adding a little extra anyway I am definitely in between 20 and 30 feet that they show on the chart where it shows go no go
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Re: Flue Pipe Sizing

Postby CEO1 » Sat Feb 06, 2016 7:04 pm

MITTS wrote:I got to the point of reading between the lines. If I was better with the numbers I could figure what the lowest height needed to run 4 inch. Since it goes from the highest appliance and I planned on adding a little extra anyway I am definitely in between 20 and 30 feet that they show on the chart where it shows go no go


Yeah, that was my conclusion...4" pipe lost the race by 5K Btu's. :roll:

You can always run whatever size you need, and add a power venter...I suppose.
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Re: Flue Pipe Sizing

Postby MITTS » Sat Feb 06, 2016 8:37 pm

I'll plot the numbers on a graph and see if I can figure the in between numbers.
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Re: Flue Pipe Sizing

Postby CEO1 » Sat Feb 06, 2016 10:48 pm

MITTS wrote:If I was better with the numbers I could figure what the lowest height needed to run 4 inch.


(I misinterpreted what you meant by that. :? )

If you're looking at 123K/20' and 138K/30' and the relationship is "linear", then the additional Btu's per foot is 1500 [(138 - 123)/10]. 4' would get you to 129K capacity...then be sure to buy a good CO alarm. :mrgreen:
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Re: Flue Pipe Sizing

Postby MITTS » Sun Feb 07, 2016 2:04 pm

Like i said I'm not good with math but I plotted the numbers in Google Sheets with the chart numbers with a trend line and it spit out this formula for where we are looking

y=-0.066x^2+4.711x+55.956

I Googled that equation and it gives you the graph with X and y values where ever you select.

The line is not linear but you got it CEO
24' gets me over the 128 btu :)

IMG_20160207_125148.jpg
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Re: Flue Pipe Sizing

Postby 3rdcoast » Mon Feb 08, 2016 11:16 am

You are going to have to add a lot of height to the pipe to use 4" as the common vent. If your connectors are manifolded together and then run horizontally to the main vent you have to deduct 10% from the capacity of the common vent.

"If vent connectors are combined prior to entering
the common vent, the maximum common
vent capacity listed in the common venting
tables must be reduced by 10%" from GAMA note 5 page

So your graph would start at
123-10%(12.3)=110.7 for 20 feet and
138-10%(13.8)=124.2 for 30 feet.
It would have to be 36 feet in height from the GAMA table to use 4 inch. 5 inch would have ample capacity.

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Re: Flue Pipe Sizing

Postby MITTS » Mon Feb 08, 2016 12:13 pm

Thankyou 3C

Another thought I had was I see flexible b- vent that would allow a little jog in this area. Is that OK? The plumbing wall which this runs is the biggest I've seen so this location of the vent is most irritating. The big problem is a roof joist right above the pipe run.

Cost was another factor since I'd have to replace the whole run no matter what to get this med. cab. in. Another thing is our HW tank is a 40 gallon and is borderline with the five of us and finally getting a working bathtub in this house we might be fighting for hot water so then we are looking at a different deal for this flue altogether. I was worried if we replace the HW with some other type and end up not using the 6" for both if it was going to cause issues with the furnace....... Like condensation
I think I'll frame it for the cabinet to be built in but surface mount till something changes lol
I didn't want to have these to appliances from the beginning but the lender made the old homeowner replace them both or they wouldn't give me the mortgage. I too cheap to change them out to high efficiency now :(
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Re: Flue Pipe Sizing

Postby CEO1 » Mon Feb 08, 2016 1:15 pm

3rdcoast wrote:You are going to have to add a lot of height to the pipe to use 4" as the common vent. If your connectors are manifolded together and then run horizontally to the main vent you have to deduct 10% from the capacity of the common vent.

"If vent connectors are combined prior to entering
the common vent, the maximum common
vent capacity listed in the common venting
tables must be reduced by 10%" from GAMA note 5 page

So your graph would start at
123-10%(12.3)=110.7 for 20 feet and
138-10%(13.8)=124.2 for 30 feet.
It would have to be 36 feet in height from the GAMA table to use 4 inch. 5 inch would have ample capacity.


Glad you caught the "woops"... :mrgreen:
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Re: Flue Pipe Sizing

Postby 3rdcoast » Mon Feb 08, 2016 1:26 pm

MITTS wrote:Another thought I had was I see flexible b- vent that would allow a little jog in this area. Is that OK?


Common vents for multiple appliances in the GAMA tables account for up to two 90's. Two 45's is equal to one 90. Any more than that incurs further reduction in size. So the whole common vent has to be considered, if it is straight you could jog up to 4- 45 degree angles with the flex pipe.

I am not sure the flex pipe is approved for the common vent, I know they are approved for the vent connectors.

Best thing to do is a sketch of your existing setup to include any 90's or 45's to calculate from. If you want to get 4 inch in the wall, you will probably have to have vent connectors of 3 or 4 inch in the wall for the furnace and a 3 inch for the water heater ad they would join together above the ceiling to the 6 inch common vent.


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