Flue Pipe Sizing

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Re: Flue Pipe Sizing

Postby MITTS » Tue Feb 09, 2016 10:45 pm

Goof on bvent everything?
I thought the bvent was for unconditioned cold spaces
Can't say I've seen it done with bvent all the way from start to finish beginning to end.
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Re: Flue Pipe Sizing

Postby CEO1 » Wed Feb 10, 2016 8:02 am

MITTS wrote:Goof on bvent everything?
I thought the bvent was for unconditioned cold spaces


The single wall connectors loose to much heat and that slows down the draft.


Which partly relates to the potential condensing issue...I'm thinking your primary concern (at this point) is just maintaining a continuous draft to get all the vent gases out of the house. Once you get it going, you can measure the flue gas temp and maybe determine if it's above or below dewpoint.
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Re: Flue Pipe Sizing

Postby MITTS » Wed Feb 10, 2016 9:57 am

If I'm moving the HWH a little I could just as easily move it directly under the vertical stack for a straight shot out.

Thinking out loud...
But then that would mess the configuration with the straight through on the tee with the furnace (fan).

Anyway the HWH can be moved. My wife wanted space along the wall for the washer and drier where the HWH is blocking.
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Re: Flue Pipe Sizing

Postby 3rdcoast » Wed Feb 10, 2016 9:59 am

using the GAMA tables to size the common vent you need the vent height from the outlet of the highest appliance to the cap of the vent.
You want to use a 4 inch pipe to clear your medicine cabinet, you have a 40k btu natural draft water heater with a 3 inch outlet and a 88k btu fan assist furnace with a 4 inch outlet.

If you use single wall connectors you would use table 4 from the GAMA table for the common vent. It shows 20 feet is rated for 118k btu and 30 feet is rated for 132k btu. Every foot between 20 and 30 gains 1400 BTU, so 28 feet would be 129 k btu. So if the 28 feet of vertical height measured from the outlet of the highest appliance is something you can meet you would then size the connectors. The tee is considered part of the common vent and should be double wall per the book.

To size a connector you need the vent height which is 28 feet, and the connector rise which is an unknown value in this post. So i will give you some general numbers from the chart.
Using a 28 foot high vent and 4 inch single wall connector for the furnace
connector rise 1 foot fan min would be 77kbtu
connector rise 2 foot fan min would be 81k btu
connector rise 3 foot fan min would be 85 k btu
the fan max are all above 88k btu
so 4 inch single wall should work for the furnace
(my earlier post I was using 80k for the furnace by mistake)

Using a 28 foot high vent and 4 inch single wall connector
connector rise 1 foot nat max would be 56kbtu
connector rise 2 foot nat max would be 66k btu
connector rise 3 foot nat max would be 77k btu
so 4 inch single wall should work for water heater

the connector rise for the furnace fan min is the closet to being out of range, you will have to lower the termination point of your 4 inch vent in the mechanical room to keep the connection tee within 3 feet vertical of the outlet of the furnace and still be above the outlet of the water heater.

Some OEM books say if you are using 2 connectors of the same size, then the common vent has to be one size larger, I do not see that in the GAMA book. If so you would have to investigate to see if 3 inch single wall could be used on water heater, and if does not meet criteria, then perform same calculations using double wall connectors.

All of this is based on tables from the GAMA book, also spread all over the book is the statement "if in doubt go one size bigger".

In my area double wall connectors is all I have ever seen used. In your area it is single wall connectors. Perhaps they like to warm the mechanical room with the waste heat from the single wall connectors, plus save money. Some of the newer codes actually require double wall connectors.

I will give one disclaimer though, I am going out to work on air conditioning today, our high is supposed to be 76 degrees today, so obviously I know nothing about heat!!!!

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Re: Flue Pipe Sizing

Postby CEO1 » Wed Feb 10, 2016 10:19 am

I will give one disclaimer though, I am going out to work on air conditioning today, our high is supposed to be 76 degrees today, so obviouslyI know nothing about heat!!!!


Gas stuff definitely not my forte...I do OK with electrical issues, but sometimes firing/ignition problems leave me dumbfounded...
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Re: Flue Pipe Sizing

Postby 3rdcoast » Wed Feb 10, 2016 11:08 am

MITTS wrote:If I'm moving the HWH a little I could just as easily move it directly under the vertical stack for a straight shot out.

Thinking out loud...
But then that would mess the configuration with the straight through on the tee with the furnace (fan).

Anyway the HWH can be moved. My wife wanted space along the wall for the washer and drier where the HWH is blocking.
Does that qualify as a Valentine's Day Gift :mrgreen:


From the GAMA book
Locate draft hood outlet or flue collar of smallest input appliance closest to or under common vent.

vent.jpg
vent.jpg (9.32 KiB) Viewed 289 times


So your water heater needs to go into the side of the tee unless it is under the common vent

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Re: Flue Pipe Sizing

Postby MITTS » Wed Feb 10, 2016 11:15 am

Thanks guys. The size in the wall is the priority now so I can get the bathroom buttoned up. If I have to mess with exposed connections it will be no problem after the fact.
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Re: Flue Pipe Sizing

Postby MITTS » Wed Feb 10, 2016 4:39 pm

I see the numbers I charted before were if B type connections were used. I redid the chart with the single wall connector chart and looks like I'm still good. I'm going to pipe the HWH directly below the main vent and use a 1.5' piece of 3" into the 4" tee
Was going to 45 the furnace flue over after a vertical from the exhaust so I'll have a little more room to get my take offs into the supply.

Anything seem like I goofed?
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Re: Flue Pipe Sizing

Postby CEO1 » Wed Feb 10, 2016 5:52 pm

MITTS wrote:Thanks guys.


We charge a modest consulting fee...I'll email an invoice. :mrgreen:
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Re: Flue Pipe Sizing

Postby MITTS » Wed Feb 10, 2016 7:47 pm

With the above configuration
No gas or water connected to hot water heater yet.

There will be a mutiny by morning if I don't get something temped in :lol:
Recent-New Measures-7.jpg


Was going to blow the match out and watch the smoke but .........
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